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Measurements of the neutron dose equivalent for various radiation qualities, treatment machines and delivery techniques in radiation therapy
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Institute of Physics and Engineering in Medicine Phys. Med. Biol. 59 (2014) 2457–2468
Physics in Medicine and Biology
doi:10.1088/0031-9155/59/10/2457
Measurements of the neutron dose equivalent for various radiation qualities, treatment machines and delivery techniques in radiation therapy ¨ 1 , J Besserer 1 , M Boschung 2 , S Mayer 2 , R A Halg A J Lomax 3 and U Schneider 1,4 1
Radiotherapy Hirslanden, Medical Physics, CH-8032 Zurich, Switzerland Paul Scherrer Institut, Division for Radiation Safety and Security, CH-5232 Villigen, Switzerland 3 Paul Scherrer Institut, Center for Proton Therapy, CH-5232 Villigen, Switzerland 4 University of Zurich, Science Faculty, CH-8057 Zurich, Switzerland 2
E-mail: [email protected] Received 23 October 2013, revised 14 February 2014 Accepted for publication 18 March 2014 Published 28 April 2014 Abstract
In radiation therapy, high energy photon and proton beams cause the production of secondary neutrons. This leads to an unwanted dose contribution, which can be considerable for tissues outside of the target volume regarding the long term health of cancer patients. Due to the high biological effectiveness of neutrons in regards to cancer induction, small neutron doses can be important. This study quantified the neutron doses for different radiation therapy modalities. Most of the reports in the literature used neutron dose measurements free in air or on the surface of phantoms to estimate the amount of neutron dose to the patient. In this study, dose measurements were performed in terms of neutron dose equivalent inside an anthropomorphic phantom. The neutron dose equivalent was determined using track etch detectors as a function of the distance to the isocenter, as well as for radiation sensitive organs. The dose distributions were compared with respect to treatment techniques (3Dconformal, volumetric modulated arc therapy and intensity-modulated radiation therapy for photons; spot scanning and passive scattering for protons), therapy machines (Varian, Elekta and Siemens linear accelerators) and radiation quality (photons and protons). The neutron dose equivalent varied between 0.002 and 3 mSv per treatment gray over all measurements. Only small differences were found when comparing treatment techniques, but substantial differences were observed between the linear accelerator models. The neutron dose equivalent for proton therapy was higher than for photons in general and in particular for 0031-9155/14/102457+12$33.00
© 2014 Institute of Physics and Engineering in Medicine Printed in the UK & the USA 2457
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double-scattered protons. The overall neutron dose equivalent measured in this study was an order of magnitude lower than the stray dose of a treatment using 6 MV photons, suggesting that the contribution of the secondary neutron dose equivalent to the integral dose of a radiotherapy patient is small. Keywords: secondary neutron, stray dose, measurement, track etch
1. Introduction It is assumed that with the application of modern radiation treatment techniques, such as intensity-modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT) and proton therapy, the cancer cure rates are increased and simultaneously unwanted side effects are reduced (Mohan et al 1994, Burman et al 1997, Bentzen et al 2010). There are clear therapeutic advantages to using these techniques, for example the improved conformity of the dose to the target volumes. However, there are also concerns about an increase in radiation-induced malignancies with the application of these techniques (Followill et al 1997, Hall and Wuu 2003, Newhauser and Durante 2011, Paganetti et al 2012, Paganetti 2012), in particular, for younger patients (Rowland et al 2004). For intensity modulation techniques using photons, there is an increase in the beam-on time to deliver the same dose to the target volume compared to conventional treatment techniques. In the case of therapy modalities using high nominal photon energies (>10 MeV) or protons, neutrons are unintentionally produced. These secondary neutrons are produced in the beam delivery system and in the patient. Since most of the neutrons from high energy photon beams are produced in the gantry head (Zanini et al 2004), the amount of neutron production in the patient is only of concern for the case of protons or other heavy particles. The absorbed dose from secondary neutrons is small compared to the target dose, but due to their high biological effectiveness in regards to cancer induction (Little 1997, International Commission on Radiological Protection 2007), even small neutron doses outside of the target volume can be important regarding the long term health of radiation therapy patients. Therefore, the influence of the primary dose distribution, stray dose and imaging dose, as well as the secondary neutron radiation on secondary cancer incidence should be investigated further. A comprehensive list of studies evaluating neutron dose contributions for treatments using high energy photons, protons and other techniques or radiation qualities can be found in a review article by Xu et al (2008). In a more recent article, the current status of neutron dose studies for photon radiotherapy is reviewed by Takam et al (2011). According to them, most of the reports in the literature used neutron dose measurements free in air, on the surface of phantoms or inside geometrical phantoms. Organ neutron doses were determined using direct measurements or calculations, either with Monte Carlo simulations or dose profiles and estimated distances to the organs. The aim of this study was to measure neutron dose equivalent inside an anthropomorphic phantom, associated with the irradiation of a radiation therapy patient during a typical treatment. The measured neutron (organ) doses were compared with respect to treatment technique, therapy machines and radiation quality. 2. Methods and materials The clinical setup of the study was the curative irradiation of a rhabdomyosarcoma of the prostate for an adolescent patient, who was represented by an Alderson–Rando phantom. The planning CT of the phantom and the contouring of the target structures, as well as that of 2458
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Table 1. Comparison of the beam energies used and monitor units (MU) per plan.
Nominal energy TPR20,10 3DCRT VMAT IMRT
Varian
Elekta
Siemens
15 MeV 0.760 234 MU 485 MU 498 MU
15 MeV 0.757
18 MeV 0.773
527 MU
436 MU
the radiation sensitive organs (done by a physician), were performed at one institution. The dose prescription for the irradiation was 26 × 2.0 Gy for the well established 3D-conformal treatment and 23 × 2.2 Gy for the other treatment modalities. For the proton treatments, the dose prescription is given in cobalt gray equivalent and is therefore already weighted with a relative biological effectiveness factor of 1.1, which is the standard procedure for proton irradiations worldwide (Paganetti et al 2002). The fractionation schemes have been matched to represent the same biological target dose using the linear–quadratic model and an α/β ratio of 2.8 (Timmerman and Mendonca 2002). This information was sent to all the institutions participating in this study along with planning guidelines containing the CT calibration curves, dose prescription, planning structures, dose constraints and the location of the isocenter. All measurements and detector readouts were performed by the same people. This procedure ensured the comparability of the treatment plans, as well as of the irradiations and measurements at the different sites. The series of measurements included photon and proton irradiations. The photon beams were delivered by treatment machines from the manufacturers Varian, Elekta and Siemens. On the Varian linear accelerator, the following irradiation techniques were measured: 3Dconformal (3DCRT), VMAT and IMRT. IMRT treatments were investigated for linear accelerators from all three manufacturers. The proton irradiations were performed using spot scanning and double scattering. Measurements were performed at Radiotherapy Hirslanden, Aarau Switzerland (Varian Clinac 21 iX), Canton Hospital St. Gallen, St. Gallen Switzerland (Elekta Synergy), Canton Hospital Aarau, Aarau Switzerland (Siemens Oncor Avant-Garde), Center for Proton Therapy, Paul Scherrer Institut (PSI), Villigen Switzerland (proton spot scanning, Gantry 1) and Francis H Burr Proton Therapy Center, Massachusetts General Hospital, Boston, USA (proton double scattering, Gantry 2). The different photon beams had nominal energies according to BJR-17 (British Institute of Radiology 1983) of 15 MV (Varian), 15 MV (Elekta) and 18 MV (Siemens), which corresponded to measured tissue– phantom ratios TPR20,10 of 0.760, 0.757 and 0.773, respectively. The energies of the spotscanned proton fields were 160 and 177 MeV. For the double scattering beamline, energies in the ranges of 135 to 140 MeV and 175 to 180 MeV were used. A comparison of the beam energies used and monitor units (MU) per plan for the photon treatments is given in table 1. In order to evaluate the neutron dose for a patient, an anthropomorphic Alderson–Rando phantom (RSD Long Beach, USA) was used in this study to account for the patient anatomy. It is not a priori clear that neutron dose measurements inside a phantom represent the neutron dose a patient would receive from the same irradiation. Because neutron production cross sections are dependent on the chemical composition of the material, different phantoms can lead to different measured neutron doses. Monte Carlo simulations showed that these differences could be up to 30% (H¨alg et al 2011) and 50% (Dowdell et al 2009), depending on the phantom material. The Alderson soft tissue material, present in the irradiated phantom of this work, is suitable for neutron dosimetry within an average of 5% (H¨alg et al 2011), which is enough for the achievable measurement accuracy. The dose measurements were performed 2459
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Figure 1. A dedicated cavity inside the Alderson–Rando phantom with a PADC detector and two radiators.
using poly(allyl diglycol carbonate) (PADC) track etch detectors provided by the Radiation Metrology Section of the Division for Radiation Safety and Security at Paul Scherrer Institut in Villigen, Switzerland (Fiechtner et al 1997, Fiechtner and Wernli 1999). This detector material has virtually no photon sensitivity (Gilvin et al 1987, 2001) and is thus suitable for measuring neutron dose in a mixed field of photons and neutrons. The sensitivity of the PADC track etch detectors used to thermal and fast neutrons was investigated at PSI and included in the calibration procedure (Fiechtner and Wernli 1999). For this measurement series, no additional signal from thermal neutrons was detected. The dosimeters were placed inside and on the surface of the phantom, facilitating the determination of a three-dimensional dose distribution and the evaluation of the dose to radiation sensitive organs according to the ICRP recommendations (International Commission on Radiological Protection 2007). For the detector positions inside the phantom, dedicated cavities were milled. An example of a PADC detector with two radiators inside the Alderson–Rando phantom is shown in figure 1. A subset of the detectors were positioned on the medial patient axis. This allowed the determination of a one-dimensional representation of the neutron dose from the target volume in the prostate to the head. For the neutron dose measurements at the proton facilities, the detector positions inside the treated volume were shifted dorsally outside of the target volume in order to minimize the influence of primary protons on the measured neutron dose. A total of 23 detector positions and two additional reference detectors were used per measurement. In figure 2, the positions of the PADC detectors inside the phantom are shown in a frontal scout view of the Alderson– Rando phantom together with the isocenter in the prostate and the medial patient axis. All measurements were performed twice and the average readout of the two detectors per position was used to report the results. Since the sensitivity of the PADC track etch detectors is best in a limited dose range (about 1 to 100 mSv), the detectors at the different positions in the phantom were irradiated with different multiples of the treatment plans. This applied total dose was determined in a set of preparing measurements and chosen according to the position of the detector relative to the primary radiation field, treatment modality and radiation quality. The readout of the PADC detectors was initially calibrated in personal dose equivalent Hp (10) using the radiation field of the 241 Am–Be neutron source at PSI and an ISO water slab phantom (Schuhmacher 2004, Hoedlmoser et al 2011, ISO 2460
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Figure 2. Frontal scout view of the Alderson–Rando phantom with the PADC detector
positions inside the phantom and the medial patient axis with the isocenter in the prostate.
4037-1:1996 1996, ISO 4037-2:1997 1997, ISO 4037-3:1999 1999). This was done for every batch of detectors used for the series of measurements, allowing the individual sensitivity of the batches to be accounted for. In addition, subsets of background and transport detectors were used for each batch to subtract possible unwanted signals from the readout. A further calibration procedure was applied to account for the changes of the irradiation setup for the measurements compared to the calibration setup. This included calibration factors for the neutron spectrum of the radiation treatment beam quality and the changes of the neutron fluence inside the phantom. A detailed description of this procedure can be found in the work by H¨alg et al (2012). 2461
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Neutron dose equivalent H/mSv/whole treatment
102 Varian 3DCRT Varian VMAT Varian IMRT
101
100
10−1 −10
0
10 20 30 40 50 60 Distance along patient axis x/cm
70
80
Figure 3. Neutron dose equivalent along the medial patient axis. The isocenter
corresponds to x = 0 cm and the vertical lines represent the border of the target volume along this axis. Total course of treatment (26 × 2.0 Gy (3DCRT) or 23 × 2.2 Gy) for different techniques on the Varian linear accelerator. The error bars represent a standard uncertainty of 28%.
The neutron doses were determined in terms of neutron dose equivalent H per treatment gray and for the whole course of the treatment. In addition, the effective neutron dose was calculated using the summation of the organ specific weighted neutron dose equivalent according to the definition of effective dose in the ICRP recommendations (International Commission on Radiological Protection 2007). 3. Results The neutron dose equivalent was measured on the medial patient axis with 13 detectors from the target in the prostate to the head. In figure 3, the measured neutron dose equivalent along this axis is shown for the different treatment techniques on the Varian linear accelerator. The dose was scaled to neutron dose equivalent per total treatment dose of the whole course of treatment to account for the different fractionation schemes of the different treatment techniques (26 × 2.0 Gy (3DCRT) or 23 × 2.2 Gy). A comparison of the neutron dose equivalent for IMRT treatments on the different linear accelerators along the medial patient axis per treatment gray can be found in figure 4. The same dose distribution for the proton treatments is depicted in figure 5. The dose from the IMRT treatment on the Varian linear accelerator is added for comparison. The distance 0 cm on the x-axis in the figures corresponds to the isocenter in the prostate and the vertical lines are the borders of the target volume along the selected direction (x = −4.5 cm and x = 4.0 cm). The remaining PADC detectors were positioned inside and on the surface of the phantom to measure organ specific neutron dose equivalent. The resulting neutron dose equivalent per treatment gray for the selected organs is given in table 2. The organ doses listed in table 2 were used to calculate effective neutron doses. For the complete course of treatment they were calculated to be 2.9 mSv, 2.6 mSv, 2.3 mSv, 0.54 mSv, 1.9 mSv, 18 mSv and 24 mSv for Varian 3DCRT, Varian VMAT, Varian IMRT, Elekta IMRT, Siemens IMRT, spot scanning protons and passively scattered protons, respectively. A detailed listing of all the effective neutron doses, calculated for different primary doses, can be found in table 3. 2462
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Neutron dose equivalent H/mSv/treatment Gy
100 Varian IMRT TPR20,10 = 0.760 Elekta IMRT TPR20,10 = 0.757 Siemens IMRT TPR20,10 = 0.773
10−1
10−2
10−3 −10
0
10 20 30 40 50 60 Distance along patient axis x/cm
70
80
Figure 4. Neutron dose equivalent along the medial patient axis. The isocenter
corresponds to x = 0 cm and the vertical lines represent the border of the target volume along this axis. IMRT treatments on different linear accelerators. The error bars represent a standard uncertainty of 28%.
Neutron dose equivalent H/mSv/treatment Gy
101 Active protons PSI Passive protons MGH Varian IMRT
100
10−1
10−2
10−3 −10
0
10 20 30 40 50 60 Distance along patient axis x/cm
70
80
Figure 5. Neutron dose equivalent along the medial patient axis. The isocenter
corresponds to x = 0 cm and the vertical lines represent the border of the target volume along this axis. Active and passive proton therapy compared to photon IMRT on the Varian linear accelerator. The error bars represent a standard uncertainty of 28%.
4. Discussion The observed differences in neutron dose equivalent between the different treatment techniques on the Varian linear accelerator were within the measurement uncertainty for most of the measurement points. The 3DCRT treatment tends to have lower neutron dose equivalent outside of the primary field close to the field border but higher dose within the treatment field. This finding is consistent with another study by H¨alg et al where the neutron dose equivalents from open and intensity-modulated fields in a geometrical phantom were compared (H¨alg et al 2012). For high energy photon beams, the neutrons are mostly produced in the gantry head (McCall et al 1984). The amount of neutrons produced in the patient (materials with low atomic number Z) is about a factor of 10 (Ongaro et al 2000) to 50 (Zanini et al 2004) lower 2463
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Table 2. Organ neutron dose equivalents H in μSv per treatment Gy for various treatment modalities with a standard uncertainty of 28%.
ICRP organ
Varian 3DCRT
Varian VMAT
Varian IMRT
Elekta IMRT
Siemens IMRT
Active protons
Passive protons
15 11 34 7 86 64 218 8 17 9 15 11 32 161
183 861 17 24 17 701 3152 17 61 14 183 3 5 34
423 851 346 232 328 785 1913 199 485 141 423 104 210 647
μSv/treatment Gy Bone marrow Colon Lung Stomach Breast Remainder Bladder Oesophagus Liver Thyroid Bone surface Brain Salivary glands Skin
17 9 41 6 71 127 485 11 13 13 17 18 45 159
21 14 56 7 106 87 278 10 25 9 21 29 77 166
21 14 43 12 83 83 257 10 17 11 21 17 48 148
6 3 16 3 39 6 12 4 5 5 6 7 18 54
Table 3. Effective neutron doses of the different treatment modalities in mSv per treatment Gy, per whole course of treatment (total treatment dose of 52.0 Gy for 3DCRT and 50.6 Gy for the other treatments) and per whole course of treatment outside of the target volume.
Treatment
Effective neutron dose per treatment Gy
Effective neutron dose per whole treatment
Effective neutron dose outside target
Varian 3DCRT Varian VMAT Varian IMRT Elekta IMRT Siemens IMRT Active protons Passive protons
0.056 0.051 0.045 0.011 0.038 0.35 0.48
2.9 2.6 2.3 0.54 1.9 18 24
1.2 1.6 1.3 0.50 1.2 8.8 19
than in the beam delivery system (high Z materials). Therefore differences in the neutron production and shielding between different linear accelerators are directly observed in the phantom measurements. The individual components of the beam delivery system have varying influences on the neutron production. In the gantry head of a Varian 2100 linear accelerator for instance, most of the neutrons are produced by interactions of the high energy photons with the primary collimator, followed by the jaw collimators, the bremsstrahlung target and a possible flattening filter, whereas the order is dependent on the field size settings (Kry et al 2008). When comparing the IMRT treatment technique to different linear accelerators, it was found that Elekta treatment machines produce considerably lower neutron dose than Varian and Siemens machines. This was observed for all measurement positions. The neutron dose equivalent from the Siemens machine is slightly lower than that from the Varian machine in general. No restriction was given in the planning guidelines for the number of monitor units for the IMRT plans. Instead, dose constraints for the planning volumes were given and the institutions were asked to perform the treatment planning as they would do for a regular patient. Therefore, there were differences in the number of monitor units used, as shown in table 1. 2464
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103 Varian 3DCRT 15X neutron Passive protons MGH neutron Varian 3DCRT 6X photon
Neutron dose equivalent H/mSv/treatment Gy
102
102
101
101
100
100
10−1
10−1
10−2
10−2
10−3
0
20 40 60 Distance along patient axis x/cm
80
Absorbed dose D/mGy/treatment Gy
103
10−3
Figure 6. Comparison of the photon stray dose along the medial patient axis (3DCRT on a Varian Clinac 21 iX with 6 MV photons) to the neutron dose equivalent from irradiations with high energy photons (3DCRT on a Varian Clinac 21 iX with 15 MV photons) and with protons (double-scattered proton irradiation at Massachusetts General Hospital) for the same treatment intention and using the same phantom. The axis on the left-hand side shows the neutron dose equivalent, the axis on the right-hand side the photon stray dose. The photon dose distribution was taken from a study by H¨alg et al (2012a).
For proton treatments, neutron dose equivalent is higher in general as compared to the photon treatments investigated and apparently higher for scattered compared to spot-scanned protons. Inside and close to the primary radiation field, the measured dose was higher for active protons than for passive protons. For distances far away from the treatment field, the neutron dose equivalent from spot scanning protons is in the same order as or even lower than that from the Varian IMRT irradiation. An important aspect for the evaluation of a treatment irradiation is the integral dose of a patient. It is therefore interesting to compare the neutron dose equivalent determined in this work to the photon stray dose from a treatment irradiation using photons. The comparison of the photon stray dose for a 3D-conformal irradiation using a 6 MV photon beam (H¨alg et al 2012a) to the neutron stray dose for the 3D-conformal irradiation using 15 MV photons and for the treatment with double-scattered protons along the medial patient axis is shown in figure 6. It is clearly shown that the neutron dose equivalent is well below the photon dose, even far away from the treatment field. When comparing the photon stray dose from the 6 MV 3DCRT irradiation to the neutron dose equivalent from the 15 MV 3DCRT irradiation, the neutron dose equivalent was everywhere at least one order of magnitude lower than the photon dose. Even the treatment with the highest neutron dose equivalent in this study, the double-scattered proton irradiation, resulted in a lower neutron dose equivalent than the photon stray dose from the 3DCRT irradiation. This finding contradicts the article by Hall (2006), who hypothesized that the neutron dose from passive proton therapy is larger than the stray dose from photon IMRT. Thus, the conclusions based on this assumption, which were drawn by Hall (2006), are wrong. The values of the effective neutron doses are dominated by the contribution of the neutron dose equivalent measured in the bladder. This dose value is the highest because of the partial overlap of the target volume with the bladder. The stated neutron dose equivalent has to be examined in relation to the much larger dose of the primary treatment field for organs in or close to the target volume. The dose deposition in the tumour volume can be viewed as 2465
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‘non-avoidable’; therefore the effective neutron doses including the organs outside of the treatment field were added in table 3. The listed neutron effective doses make it a possibility to estimate the integral dose of a patient, which is of concern for the long term health of radiation therapy patients. As pointed out in the above section, the photon stray dose of radiation therapy treatments using photons is higher than from neutrons. In terms of the resulting effective dose, regarding only the organs outside of the high dose region, the photon stray dose can be a factor of 100 larger than the stray dose from neutrons (H¨alg et al 2012a). On the other hand, when comparing the effective doses from neutrons with effective doses from computed tomography imaging procedures involved in radiation therapy, one can see that these are of the same order of magnitude (H¨alg et al 2012b). The initial absolute calibration factor for the PADC detectors in terms of Hp (10) for the detectors used in this work varied between 10.21 × 10−3 mSv cm2 ±13% and 12.55 × 10−3 mSv cm2 ±7%. These values represent the sensitivity of the different detector batches used for the different measurements in terms of the signal from the 241 Am–Be neutron source calibration irradiation. As the absolute signal of the PADC detectors is dependent on the size of the readout region, which is itself dependent on the track density, the calibration factor is normalized per unit area. The uncertainty was determined by one standard deviation of the readouts of the subset of calibration and background detectors. For the whole calibration procedure, including the calibration factors for the spectral changes in the neutron fluence, the standard uncertainty was determined to be 40% (H¨alg et al 2012). This uncertainty already includes the aforementioned uncertainty of the readout of the calibration detectors, which is about 30% for determining the track density on the detector. As every measurement was done twice, this leads to a standard uncertainty of 28% for every measurement point, which is illustrated in figures 3, 4, and 5.
5. Conclusions Measurements of neutron dose equivalent were performed to assess the neutron dose from typical treatments in radiation therapy for different treatment techniques, therapy machines and radiation qualities. Only small differences in the neutron dose equivalent were found when comparing 3D-conformal, intensity-modulated and volumetric modulated arc therapy treatment techniques on the Varian linear accelerator. On the other hand, substantial differences in neutron dose equivalent were measured for different linear accelerator models using IMRT. The neutron dose from the Elekta linear accelerator investigated was much lower than those from the linear accelerators from Siemens and Varian. For proton therapy, the neutron dose equivalent was higher than for photon irradiation therapy in general, and in particular, higher for the double scattering proton beam compared to the spot scanning beam investigated in this study. The overall dose, additional to the therapeutic dose induced by secondary neutrons, was of the same order of magnitude as the dose from a wide range of contemporary imaging modalities used for image guided radiotherapy (H¨alg et al 2012b) and an order of magnitude lower than the stray dose of a corresponding treatment with a 6 MV photon beam (H¨alg et al 2012a). Considering this, the stray dose contributions from passive proton therapy (highest neutron stray dose in this study) were smaller than those from photon therapy when the neutron and the photon stray dose were combined. On the other hand, active proton therapy corresponded to the lowest out-of-field dose. The results of this study contradict the assumptions made by Hall (2006). Therefore, both active and passive proton therapy result in a smaller effective dose for the patient than IMRT. This suggests that the secondary neutron dose may have 2466
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been overestimated in the past. Therefore the usability of treatment techniques which produce secondary neutrons, in particular high energy photon IMRT, should be considered in the same way as the usage of additional imaging modalities for the benefit of the patient. Acknowledgments This work was funded in part by the KIRO grant from the Federal Office of Public Health (FOPH), Switzerland. The authors would like to express their gratitude to M Gantert, S Khan and G Lutters from Canton Hospital Aarau, S Peters and H Schiefer from Canton Hospital St. Gallen, P Pemler from City Hospital Triemli, M Engelsman and J Flanz from MGH Boston and A Lehde and C Algranati from Paul Scherrer Institut for helping with and giving us the opportunity to perform the measurements at their institutions. References Bentzen S M, Constine L S, Deasy J O, Eisbruch A, Jackson A, Marks L B, Ten Haken R K and Yorke E D 2010 Quantitative analyses of normal tissue effects in the clinic (QUANTEC): an introduction to the scientific issues Int. J. Radiat. Oncol. Biol. Phys. 76 (Suppl. 3) S3–9 British Institute of Radiology 1983 Central axis depth dose data for use in radiotherapy: a survey of depth doses and related data measured in water or equivalent media Br. J. Radiol. Suppl. 17 1–147 (PMID: 6600113) Burman C et al 1997 Planning, delivery, and quality assurance of intensity-modulated radiotherapy using dynamic multileaf collimator: a strategy for large-scale implementation for the treatment of carcinoma of the prostate Int. J. Radiat. Oncol. Biol. Phys. 39 863–73 Dowdell S, Clasie B, Wroe A, Guatelli S, Metcalfe P, Schulte R and Rosenfeld A 2009 Tissue equivalency of phantom materials for neutron dosimetry in proton therapy Med. Phys. 36 5412–9 Fiechtner A, Gm¨ur K and Wernli C 1997 A personal neutron dosimetry system based on etched track and automatic readout by Autoscan 60 Radiat. Prot. Dosim. 70 157–60 Fiechtner A and Wernli C 1999 Individual neutron monitoring with CR-39 detectors at an accelerator centre Radiat. Prot. Dosim. 85 35–8 Followill D, Geis P and Boyer A 1997 Estimates of whole-body dose equivalent produced by beam intensity modulated conformal therapy Int. J. Radiat. Oncol. Biol. Phys. 38 667–72 Gilvin P, Bartlett D, Shaw P, Steele J and Tanner R 2001 The NRPB PADC neutron personal dosimetry service Radiat. Prot. Dosim. 96 191–5 Gilvin P J, Bartlett D T and Steele J D 1987 The NRPB PADC neutron personal dosimetry service Radiat. Prot. Dosim. 20 99–102 (http://rpd.oxfordjournals.org/content/20/1-2/99.short) H¨alg R A, Besserer J, Boschung M, Mayer S, Clasie B, Kry S F and Schneider U 2012 Field calibration of PADC track etch detectors for local neutron dosimetry in man using different radiation qualities Nucl. Instrum. Methods Phys. Res. A 694 205–10 H¨alg R A, Besserer J, Boschung M, Mayer S and Schneider U 2012 Monitor units are not predictive of neutron dose for high-energy IMRT Radiat. Oncol. 7 138 H¨alg R A, Besserer J and Schneider U 2011 Comparative simulations of neutron dose in soft tissue and phantom materials for proton and carbon ion therapy with actively scanned beams Med. Phys. 38 3149–56 H¨alg R A, Besserer J and Schneider U 2012a Systematic measurements of whole-body dose distributions for various treatment machines and delivery techniques in radiation therapy Med. Phys. 39 7662–76 H¨alg R A, Besserer J and Schneider U 2012b Systematic measurements of whole-body imaging dose distributions in image-guided radiation therapy Med. Phys. 39 7650–61 Hall E J 2006 Intensity-modulated radiation therapy, protons, and the risk of second cancers Int. J. Radiat. Oncol. Biol. Phys. 65 1–7 Hall E J and Wuu C S 2003 Radiation-induced second cancers: the impact of 3D-CRT and IMRT Int. J. Radiat. Oncol. Biol. Phys. 56 83–8 Hoedlmoser H, Schuler C, Butterweck G, Ch´etelat N and Mayer S 2011 Characterization of the 241 Am-Be neutron source of the PSI Calibration Laboratory Paul Scherrer Institut(PSI) Villigen, Switzerland Laboratory Technical Report TM-96-11-02 (dx.doi.org/10.1063/1.3665339) 2467
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International Commission on Radiological Protection 2007 The 2007 Recommendations of the International Commission on Radiological Protection ICRP Publication 103 Ann. ICRP 37 1–332 (PMID: 18082557) ISO 4037-1:1996 1996 X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy – Part 1: radiation characteristics and production methods (Geneva, Switzerland: International Organization for Standardization) ISO 4037-2:1997 1997 X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy – part 2: dosimetry for radiation protection over the energy ranges from 8 keV to 1, 3 MeV and 4 MeV to 9 MeV International Organization for Standardization (Geneva, Switzerland: International Organization for Standardization) ISO 4037-3:19991999 X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy – part 3: calibration of area and personal dosemeters and the measurement of their response as a function of energy and angle of incidence (Geneva, Switzerland: International Organization for Standardization) Kry S F, Howell R M, Titt U, Salehpour M, Mohan R and Vassiliev O N 2008 Energy spectra, sources, and shielding considerations for neutrons generated by a flattening filter-free Clinac Med. Phys. 35 1906–11 Little M P 1997 Estimates of neutron relative biological effectiveness derived from the Japanese atomic bomb survivors Int. J. Radiat. Biol. 72 715–26 McCall R C, Almond P R, Holeman G R, Devanney I A, Lanzl L H and Fuller E G 1984 Neutron contamination from medical electron accelerators NCRP Rep. 79 1–132 Mohan R, Wang X H, Jackson A, Bortfeld T, Boyer A L, Kutcher G J, Leibel S A, Fuks Z and Ling C C 1994 The potential and limitations of the inverse radiotherapy technique Radiother. Oncol. 32 232–48 Newhauser W D and Durante M 2011 Assessing the risk of second malignancies after modern radiotherapy Nature Rev. Cancer 11 438–48 Ongaro C, Zanini A, Nastasi U, Rodenas J, Ottaviano G, Mafredotti C and Burn K W 2000 Analysis of photoneutron spectra produced in medical accelerators Phys. Med. Biol. 45 L55–61 Paganetti H 2012 Assessment of the risk for developing a second malignancy from scattered and secondary radiation in radiation therapy Health Phys. 103 652–61 Paganetti H, Athar B S, Moteabbed M, A Adams J, Schneider U and Yock T I 2012 Assessment of radiation-induced second cancer risks in proton therapy and IMRT for organs inside the primary radiation field Phys. Med. Biol. 57 6047–61 Paganetti H, Niemierko A, Ancukiewicz M, Gerweck L E, Goitein M, Loeffler J S and Suit H D 2002 Relative biological effectiveness (RBE) values for proton beam therapy Int. J. Radiat. Oncol. Biol. Phys. 53 407–21 Rowland J, Mariotto A, Aziz N, Tesauro G, Feuer E J, Blackman D, Thompson P and Pollack L A 2004 Cancer survivorship—United States, 1971-2001 MMWR Morb. Mortal. Wkly. Rep. 53 526–9 Schuhmacher H 2004 Neutron calibration facilities Radiat. Prot. Dosim. 110 33–42 Takam R, Bezak E, Marcu L G and Yeoh E 2011 Out-of-field neutron and leakage photon exposures and the associated risk of second cancers in high-energy photon radiotherapy: current status Radiat. Res. 176 508–20 Timmerman R D and Mendonca M 2002 In regard to Donaldson et al: results from the IRS-IV randomized trial of hyperfractionated radiotherapy in children with rhabdomyosarcoma—a report from the IRSG. IJROBP 2001;51 : 718-728 Int. J. Radiat. Oncol. Biol. Phys. 54 1579–80 (PMID: 12459396) Xu X G, Bednarz B and Paganetti H 2008 A review of dosimetry studies on external-beam radiation treatment with respect to second cancer induction Phys. Med. Biol. 53 R193–241 Zanini A et al 2004 Monte Carlo simulation of the photoneutron field in linac radiotherapy treatments with different collimation systems Phys. Med. Biol. 49 571–82
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Measurements of the neutron dose equivalent for various radiation qualities, treatment machines and delivery techniques in radiation therapy
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Institute of Physics and Engineering in Medicine Phys. Med. Biol. 59 (2014) 2457–2468
Physics in Medicine and Biology
doi:10.1088/0031-9155/59/10/2457
Measurements of the neutron dose equivalent for various radiation qualities, treatment machines and delivery techniques in radiation therapy ¨ 1 , J Besserer 1 , M Boschung 2 , S Mayer 2 , R A Halg A J Lomax 3 and U Schneider 1,4 1
Radiotherapy Hirslanden, Medical Physics, CH-8032 Zurich, Switzerland Paul Scherrer Institut, Division for Radiation Safety and Security, CH-5232 Villigen, Switzerland 3 Paul Scherrer Institut, Center for Proton Therapy, CH-5232 Villigen, Switzerland 4 University of Zurich, Science Faculty, CH-8057 Zurich, Switzerland 2
E-mail: [email protected] Received 23 October 2013, revised 14 February 2014 Accepted for publication 18 March 2014 Published 28 April 2014 Abstract
In radiation therapy, high energy photon and proton beams cause the production of secondary neutrons. This leads to an unwanted dose contribution, which can be considerable for tissues outside of the target volume regarding the long term health of cancer patients. Due to the high biological effectiveness of neutrons in regards to cancer induction, small neutron doses can be important. This study quantified the neutron doses for different radiation therapy modalities. Most of the reports in the literature used neutron dose measurements free in air or on the surface of phantoms to estimate the amount of neutron dose to the patient. In this study, dose measurements were performed in terms of neutron dose equivalent inside an anthropomorphic phantom. The neutron dose equivalent was determined using track etch detectors as a function of the distance to the isocenter, as well as for radiation sensitive organs. The dose distributions were compared with respect to treatment techniques (3Dconformal, volumetric modulated arc therapy and intensity-modulated radiation therapy for photons; spot scanning and passive scattering for protons), therapy machines (Varian, Elekta and Siemens linear accelerators) and radiation quality (photons and protons). The neutron dose equivalent varied between 0.002 and 3 mSv per treatment gray over all measurements. Only small differences were found when comparing treatment techniques, but substantial differences were observed between the linear accelerator models. The neutron dose equivalent for proton therapy was higher than for photons in general and in particular for 0031-9155/14/102457+12$33.00
© 2014 Institute of Physics and Engineering in Medicine Printed in the UK & the USA 2457
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double-scattered protons. The overall neutron dose equivalent measured in this study was an order of magnitude lower than the stray dose of a treatment using 6 MV photons, suggesting that the contribution of the secondary neutron dose equivalent to the integral dose of a radiotherapy patient is small. Keywords: secondary neutron, stray dose, measurement, track etch
1. Introduction It is assumed that with the application of modern radiation treatment techniques, such as intensity-modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT) and proton therapy, the cancer cure rates are increased and simultaneously unwanted side effects are reduced (Mohan et al 1994, Burman et al 1997, Bentzen et al 2010). There are clear therapeutic advantages to using these techniques, for example the improved conformity of the dose to the target volumes. However, there are also concerns about an increase in radiation-induced malignancies with the application of these techniques (Followill et al 1997, Hall and Wuu 2003, Newhauser and Durante 2011, Paganetti et al 2012, Paganetti 2012), in particular, for younger patients (Rowland et al 2004). For intensity modulation techniques using photons, there is an increase in the beam-on time to deliver the same dose to the target volume compared to conventional treatment techniques. In the case of therapy modalities using high nominal photon energies (>10 MeV) or protons, neutrons are unintentionally produced. These secondary neutrons are produced in the beam delivery system and in the patient. Since most of the neutrons from high energy photon beams are produced in the gantry head (Zanini et al 2004), the amount of neutron production in the patient is only of concern for the case of protons or other heavy particles. The absorbed dose from secondary neutrons is small compared to the target dose, but due to their high biological effectiveness in regards to cancer induction (Little 1997, International Commission on Radiological Protection 2007), even small neutron doses outside of the target volume can be important regarding the long term health of radiation therapy patients. Therefore, the influence of the primary dose distribution, stray dose and imaging dose, as well as the secondary neutron radiation on secondary cancer incidence should be investigated further. A comprehensive list of studies evaluating neutron dose contributions for treatments using high energy photons, protons and other techniques or radiation qualities can be found in a review article by Xu et al (2008). In a more recent article, the current status of neutron dose studies for photon radiotherapy is reviewed by Takam et al (2011). According to them, most of the reports in the literature used neutron dose measurements free in air, on the surface of phantoms or inside geometrical phantoms. Organ neutron doses were determined using direct measurements or calculations, either with Monte Carlo simulations or dose profiles and estimated distances to the organs. The aim of this study was to measure neutron dose equivalent inside an anthropomorphic phantom, associated with the irradiation of a radiation therapy patient during a typical treatment. The measured neutron (organ) doses were compared with respect to treatment technique, therapy machines and radiation quality. 2. Methods and materials The clinical setup of the study was the curative irradiation of a rhabdomyosarcoma of the prostate for an adolescent patient, who was represented by an Alderson–Rando phantom. The planning CT of the phantom and the contouring of the target structures, as well as that of 2458
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Table 1. Comparison of the beam energies used and monitor units (MU) per plan.
Nominal energy TPR20,10 3DCRT VMAT IMRT
Varian
Elekta
Siemens
15 MeV 0.760 234 MU 485 MU 498 MU
15 MeV 0.757
18 MeV 0.773
527 MU
436 MU
the radiation sensitive organs (done by a physician), were performed at one institution. The dose prescription for the irradiation was 26 × 2.0 Gy for the well established 3D-conformal treatment and 23 × 2.2 Gy for the other treatment modalities. For the proton treatments, the dose prescription is given in cobalt gray equivalent and is therefore already weighted with a relative biological effectiveness factor of 1.1, which is the standard procedure for proton irradiations worldwide (Paganetti et al 2002). The fractionation schemes have been matched to represent the same biological target dose using the linear–quadratic model and an α/β ratio of 2.8 (Timmerman and Mendonca 2002). This information was sent to all the institutions participating in this study along with planning guidelines containing the CT calibration curves, dose prescription, planning structures, dose constraints and the location of the isocenter. All measurements and detector readouts were performed by the same people. This procedure ensured the comparability of the treatment plans, as well as of the irradiations and measurements at the different sites. The series of measurements included photon and proton irradiations. The photon beams were delivered by treatment machines from the manufacturers Varian, Elekta and Siemens. On the Varian linear accelerator, the following irradiation techniques were measured: 3Dconformal (3DCRT), VMAT and IMRT. IMRT treatments were investigated for linear accelerators from all three manufacturers. The proton irradiations were performed using spot scanning and double scattering. Measurements were performed at Radiotherapy Hirslanden, Aarau Switzerland (Varian Clinac 21 iX), Canton Hospital St. Gallen, St. Gallen Switzerland (Elekta Synergy), Canton Hospital Aarau, Aarau Switzerland (Siemens Oncor Avant-Garde), Center for Proton Therapy, Paul Scherrer Institut (PSI), Villigen Switzerland (proton spot scanning, Gantry 1) and Francis H Burr Proton Therapy Center, Massachusetts General Hospital, Boston, USA (proton double scattering, Gantry 2). The different photon beams had nominal energies according to BJR-17 (British Institute of Radiology 1983) of 15 MV (Varian), 15 MV (Elekta) and 18 MV (Siemens), which corresponded to measured tissue– phantom ratios TPR20,10 of 0.760, 0.757 and 0.773, respectively. The energies of the spotscanned proton fields were 160 and 177 MeV. For the double scattering beamline, energies in the ranges of 135 to 140 MeV and 175 to 180 MeV were used. A comparison of the beam energies used and monitor units (MU) per plan for the photon treatments is given in table 1. In order to evaluate the neutron dose for a patient, an anthropomorphic Alderson–Rando phantom (RSD Long Beach, USA) was used in this study to account for the patient anatomy. It is not a priori clear that neutron dose measurements inside a phantom represent the neutron dose a patient would receive from the same irradiation. Because neutron production cross sections are dependent on the chemical composition of the material, different phantoms can lead to different measured neutron doses. Monte Carlo simulations showed that these differences could be up to 30% (H¨alg et al 2011) and 50% (Dowdell et al 2009), depending on the phantom material. The Alderson soft tissue material, present in the irradiated phantom of this work, is suitable for neutron dosimetry within an average of 5% (H¨alg et al 2011), which is enough for the achievable measurement accuracy. The dose measurements were performed 2459
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Figure 1. A dedicated cavity inside the Alderson–Rando phantom with a PADC detector and two radiators.
using poly(allyl diglycol carbonate) (PADC) track etch detectors provided by the Radiation Metrology Section of the Division for Radiation Safety and Security at Paul Scherrer Institut in Villigen, Switzerland (Fiechtner et al 1997, Fiechtner and Wernli 1999). This detector material has virtually no photon sensitivity (Gilvin et al 1987, 2001) and is thus suitable for measuring neutron dose in a mixed field of photons and neutrons. The sensitivity of the PADC track etch detectors used to thermal and fast neutrons was investigated at PSI and included in the calibration procedure (Fiechtner and Wernli 1999). For this measurement series, no additional signal from thermal neutrons was detected. The dosimeters were placed inside and on the surface of the phantom, facilitating the determination of a three-dimensional dose distribution and the evaluation of the dose to radiation sensitive organs according to the ICRP recommendations (International Commission on Radiological Protection 2007). For the detector positions inside the phantom, dedicated cavities were milled. An example of a PADC detector with two radiators inside the Alderson–Rando phantom is shown in figure 1. A subset of the detectors were positioned on the medial patient axis. This allowed the determination of a one-dimensional representation of the neutron dose from the target volume in the prostate to the head. For the neutron dose measurements at the proton facilities, the detector positions inside the treated volume were shifted dorsally outside of the target volume in order to minimize the influence of primary protons on the measured neutron dose. A total of 23 detector positions and two additional reference detectors were used per measurement. In figure 2, the positions of the PADC detectors inside the phantom are shown in a frontal scout view of the Alderson– Rando phantom together with the isocenter in the prostate and the medial patient axis. All measurements were performed twice and the average readout of the two detectors per position was used to report the results. Since the sensitivity of the PADC track etch detectors is best in a limited dose range (about 1 to 100 mSv), the detectors at the different positions in the phantom were irradiated with different multiples of the treatment plans. This applied total dose was determined in a set of preparing measurements and chosen according to the position of the detector relative to the primary radiation field, treatment modality and radiation quality. The readout of the PADC detectors was initially calibrated in personal dose equivalent Hp (10) using the radiation field of the 241 Am–Be neutron source at PSI and an ISO water slab phantom (Schuhmacher 2004, Hoedlmoser et al 2011, ISO 2460
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Figure 2. Frontal scout view of the Alderson–Rando phantom with the PADC detector
positions inside the phantom and the medial patient axis with the isocenter in the prostate.
4037-1:1996 1996, ISO 4037-2:1997 1997, ISO 4037-3:1999 1999). This was done for every batch of detectors used for the series of measurements, allowing the individual sensitivity of the batches to be accounted for. In addition, subsets of background and transport detectors were used for each batch to subtract possible unwanted signals from the readout. A further calibration procedure was applied to account for the changes of the irradiation setup for the measurements compared to the calibration setup. This included calibration factors for the neutron spectrum of the radiation treatment beam quality and the changes of the neutron fluence inside the phantom. A detailed description of this procedure can be found in the work by H¨alg et al (2012). 2461
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Neutron dose equivalent H/mSv/whole treatment
102 Varian 3DCRT Varian VMAT Varian IMRT
101
100
10−1 −10
0
10 20 30 40 50 60 Distance along patient axis x/cm
70
80
Figure 3. Neutron dose equivalent along the medial patient axis. The isocenter
corresponds to x = 0 cm and the vertical lines represent the border of the target volume along this axis. Total course of treatment (26 × 2.0 Gy (3DCRT) or 23 × 2.2 Gy) for different techniques on the Varian linear accelerator. The error bars represent a standard uncertainty of 28%.
The neutron doses were determined in terms of neutron dose equivalent H per treatment gray and for the whole course of the treatment. In addition, the effective neutron dose was calculated using the summation of the organ specific weighted neutron dose equivalent according to the definition of effective dose in the ICRP recommendations (International Commission on Radiological Protection 2007). 3. Results The neutron dose equivalent was measured on the medial patient axis with 13 detectors from the target in the prostate to the head. In figure 3, the measured neutron dose equivalent along this axis is shown for the different treatment techniques on the Varian linear accelerator. The dose was scaled to neutron dose equivalent per total treatment dose of the whole course of treatment to account for the different fractionation schemes of the different treatment techniques (26 × 2.0 Gy (3DCRT) or 23 × 2.2 Gy). A comparison of the neutron dose equivalent for IMRT treatments on the different linear accelerators along the medial patient axis per treatment gray can be found in figure 4. The same dose distribution for the proton treatments is depicted in figure 5. The dose from the IMRT treatment on the Varian linear accelerator is added for comparison. The distance 0 cm on the x-axis in the figures corresponds to the isocenter in the prostate and the vertical lines are the borders of the target volume along the selected direction (x = −4.5 cm and x = 4.0 cm). The remaining PADC detectors were positioned inside and on the surface of the phantom to measure organ specific neutron dose equivalent. The resulting neutron dose equivalent per treatment gray for the selected organs is given in table 2. The organ doses listed in table 2 were used to calculate effective neutron doses. For the complete course of treatment they were calculated to be 2.9 mSv, 2.6 mSv, 2.3 mSv, 0.54 mSv, 1.9 mSv, 18 mSv and 24 mSv for Varian 3DCRT, Varian VMAT, Varian IMRT, Elekta IMRT, Siemens IMRT, spot scanning protons and passively scattered protons, respectively. A detailed listing of all the effective neutron doses, calculated for different primary doses, can be found in table 3. 2462
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Neutron dose equivalent H/mSv/treatment Gy
100 Varian IMRT TPR20,10 = 0.760 Elekta IMRT TPR20,10 = 0.757 Siemens IMRT TPR20,10 = 0.773
10−1
10−2
10−3 −10
0
10 20 30 40 50 60 Distance along patient axis x/cm
70
80
Figure 4. Neutron dose equivalent along the medial patient axis. The isocenter
corresponds to x = 0 cm and the vertical lines represent the border of the target volume along this axis. IMRT treatments on different linear accelerators. The error bars represent a standard uncertainty of 28%.
Neutron dose equivalent H/mSv/treatment Gy
101 Active protons PSI Passive protons MGH Varian IMRT
100
10−1
10−2
10−3 −10
0
10 20 30 40 50 60 Distance along patient axis x/cm
70
80
Figure 5. Neutron dose equivalent along the medial patient axis. The isocenter
corresponds to x = 0 cm and the vertical lines represent the border of the target volume along this axis. Active and passive proton therapy compared to photon IMRT on the Varian linear accelerator. The error bars represent a standard uncertainty of 28%.
4. Discussion The observed differences in neutron dose equivalent between the different treatment techniques on the Varian linear accelerator were within the measurement uncertainty for most of the measurement points. The 3DCRT treatment tends to have lower neutron dose equivalent outside of the primary field close to the field border but higher dose within the treatment field. This finding is consistent with another study by H¨alg et al where the neutron dose equivalents from open and intensity-modulated fields in a geometrical phantom were compared (H¨alg et al 2012). For high energy photon beams, the neutrons are mostly produced in the gantry head (McCall et al 1984). The amount of neutrons produced in the patient (materials with low atomic number Z) is about a factor of 10 (Ongaro et al 2000) to 50 (Zanini et al 2004) lower 2463
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Table 2. Organ neutron dose equivalents H in μSv per treatment Gy for various treatment modalities with a standard uncertainty of 28%.
ICRP organ
Varian 3DCRT
Varian VMAT
Varian IMRT
Elekta IMRT
Siemens IMRT
Active protons
Passive protons
15 11 34 7 86 64 218 8 17 9 15 11 32 161
183 861 17 24 17 701 3152 17 61 14 183 3 5 34
423 851 346 232 328 785 1913 199 485 141 423 104 210 647
μSv/treatment Gy Bone marrow Colon Lung Stomach Breast Remainder Bladder Oesophagus Liver Thyroid Bone surface Brain Salivary glands Skin
17 9 41 6 71 127 485 11 13 13 17 18 45 159
21 14 56 7 106 87 278 10 25 9 21 29 77 166
21 14 43 12 83 83 257 10 17 11 21 17 48 148
6 3 16 3 39 6 12 4 5 5 6 7 18 54
Table 3. Effective neutron doses of the different treatment modalities in mSv per treatment Gy, per whole course of treatment (total treatment dose of 52.0 Gy for 3DCRT and 50.6 Gy for the other treatments) and per whole course of treatment outside of the target volume.
Treatment
Effective neutron dose per treatment Gy
Effective neutron dose per whole treatment
Effective neutron dose outside target
Varian 3DCRT Varian VMAT Varian IMRT Elekta IMRT Siemens IMRT Active protons Passive protons
0.056 0.051 0.045 0.011 0.038 0.35 0.48
2.9 2.6 2.3 0.54 1.9 18 24
1.2 1.6 1.3 0.50 1.2 8.8 19
than in the beam delivery system (high Z materials). Therefore differences in the neutron production and shielding between different linear accelerators are directly observed in the phantom measurements. The individual components of the beam delivery system have varying influences on the neutron production. In the gantry head of a Varian 2100 linear accelerator for instance, most of the neutrons are produced by interactions of the high energy photons with the primary collimator, followed by the jaw collimators, the bremsstrahlung target and a possible flattening filter, whereas the order is dependent on the field size settings (Kry et al 2008). When comparing the IMRT treatment technique to different linear accelerators, it was found that Elekta treatment machines produce considerably lower neutron dose than Varian and Siemens machines. This was observed for all measurement positions. The neutron dose equivalent from the Siemens machine is slightly lower than that from the Varian machine in general. No restriction was given in the planning guidelines for the number of monitor units for the IMRT plans. Instead, dose constraints for the planning volumes were given and the institutions were asked to perform the treatment planning as they would do for a regular patient. Therefore, there were differences in the number of monitor units used, as shown in table 1. 2464
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103 Varian 3DCRT 15X neutron Passive protons MGH neutron Varian 3DCRT 6X photon
Neutron dose equivalent H/mSv/treatment Gy
102
102
101
101
100
100
10−1
10−1
10−2
10−2
10−3
0
20 40 60 Distance along patient axis x/cm
80
Absorbed dose D/mGy/treatment Gy
103
10−3
Figure 6. Comparison of the photon stray dose along the medial patient axis (3DCRT on a Varian Clinac 21 iX with 6 MV photons) to the neutron dose equivalent from irradiations with high energy photons (3DCRT on a Varian Clinac 21 iX with 15 MV photons) and with protons (double-scattered proton irradiation at Massachusetts General Hospital) for the same treatment intention and using the same phantom. The axis on the left-hand side shows the neutron dose equivalent, the axis on the right-hand side the photon stray dose. The photon dose distribution was taken from a study by H¨alg et al (2012a).
For proton treatments, neutron dose equivalent is higher in general as compared to the photon treatments investigated and apparently higher for scattered compared to spot-scanned protons. Inside and close to the primary radiation field, the measured dose was higher for active protons than for passive protons. For distances far away from the treatment field, the neutron dose equivalent from spot scanning protons is in the same order as or even lower than that from the Varian IMRT irradiation. An important aspect for the evaluation of a treatment irradiation is the integral dose of a patient. It is therefore interesting to compare the neutron dose equivalent determined in this work to the photon stray dose from a treatment irradiation using photons. The comparison of the photon stray dose for a 3D-conformal irradiation using a 6 MV photon beam (H¨alg et al 2012a) to the neutron stray dose for the 3D-conformal irradiation using 15 MV photons and for the treatment with double-scattered protons along the medial patient axis is shown in figure 6. It is clearly shown that the neutron dose equivalent is well below the photon dose, even far away from the treatment field. When comparing the photon stray dose from the 6 MV 3DCRT irradiation to the neutron dose equivalent from the 15 MV 3DCRT irradiation, the neutron dose equivalent was everywhere at least one order of magnitude lower than the photon dose. Even the treatment with the highest neutron dose equivalent in this study, the double-scattered proton irradiation, resulted in a lower neutron dose equivalent than the photon stray dose from the 3DCRT irradiation. This finding contradicts the article by Hall (2006), who hypothesized that the neutron dose from passive proton therapy is larger than the stray dose from photon IMRT. Thus, the conclusions based on this assumption, which were drawn by Hall (2006), are wrong. The values of the effective neutron doses are dominated by the contribution of the neutron dose equivalent measured in the bladder. This dose value is the highest because of the partial overlap of the target volume with the bladder. The stated neutron dose equivalent has to be examined in relation to the much larger dose of the primary treatment field for organs in or close to the target volume. The dose deposition in the tumour volume can be viewed as 2465
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‘non-avoidable’; therefore the effective neutron doses including the organs outside of the treatment field were added in table 3. The listed neutron effective doses make it a possibility to estimate the integral dose of a patient, which is of concern for the long term health of radiation therapy patients. As pointed out in the above section, the photon stray dose of radiation therapy treatments using photons is higher than from neutrons. In terms of the resulting effective dose, regarding only the organs outside of the high dose region, the photon stray dose can be a factor of 100 larger than the stray dose from neutrons (H¨alg et al 2012a). On the other hand, when comparing the effective doses from neutrons with effective doses from computed tomography imaging procedures involved in radiation therapy, one can see that these are of the same order of magnitude (H¨alg et al 2012b). The initial absolute calibration factor for the PADC detectors in terms of Hp (10) for the detectors used in this work varied between 10.21 × 10−3 mSv cm2 ±13% and 12.55 × 10−3 mSv cm2 ±7%. These values represent the sensitivity of the different detector batches used for the different measurements in terms of the signal from the 241 Am–Be neutron source calibration irradiation. As the absolute signal of the PADC detectors is dependent on the size of the readout region, which is itself dependent on the track density, the calibration factor is normalized per unit area. The uncertainty was determined by one standard deviation of the readouts of the subset of calibration and background detectors. For the whole calibration procedure, including the calibration factors for the spectral changes in the neutron fluence, the standard uncertainty was determined to be 40% (H¨alg et al 2012). This uncertainty already includes the aforementioned uncertainty of the readout of the calibration detectors, which is about 30% for determining the track density on the detector. As every measurement was done twice, this leads to a standard uncertainty of 28% for every measurement point, which is illustrated in figures 3, 4, and 5.
5. Conclusions Measurements of neutron dose equivalent were performed to assess the neutron dose from typical treatments in radiation therapy for different treatment techniques, therapy machines and radiation qualities. Only small differences in the neutron dose equivalent were found when comparing 3D-conformal, intensity-modulated and volumetric modulated arc therapy treatment techniques on the Varian linear accelerator. On the other hand, substantial differences in neutron dose equivalent were measured for different linear accelerator models using IMRT. The neutron dose from the Elekta linear accelerator investigated was much lower than those from the linear accelerators from Siemens and Varian. For proton therapy, the neutron dose equivalent was higher than for photon irradiation therapy in general, and in particular, higher for the double scattering proton beam compared to the spot scanning beam investigated in this study. The overall dose, additional to the therapeutic dose induced by secondary neutrons, was of the same order of magnitude as the dose from a wide range of contemporary imaging modalities used for image guided radiotherapy (H¨alg et al 2012b) and an order of magnitude lower than the stray dose of a corresponding treatment with a 6 MV photon beam (H¨alg et al 2012a). Considering this, the stray dose contributions from passive proton therapy (highest neutron stray dose in this study) were smaller than those from photon therapy when the neutron and the photon stray dose were combined. On the other hand, active proton therapy corresponded to the lowest out-of-field dose. The results of this study contradict the assumptions made by Hall (2006). Therefore, both active and passive proton therapy result in a smaller effective dose for the patient than IMRT. This suggests that the secondary neutron dose may have 2466
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been overestimated in the past. Therefore the usability of treatment techniques which produce secondary neutrons, in particular high energy photon IMRT, should be considered in the same way as the usage of additional imaging modalities for the benefit of the patient. Acknowledgments This work was funded in part by the KIRO grant from the Federal Office of Public Health (FOPH), Switzerland. The authors would like to express their gratitude to M Gantert, S Khan and G Lutters from Canton Hospital Aarau, S Peters and H Schiefer from Canton Hospital St. Gallen, P Pemler from City Hospital Triemli, M Engelsman and J Flanz from MGH Boston and A Lehde and C Algranati from Paul Scherrer Institut for helping with and giving us the opportunity to perform the measurements at their institutions. References Bentzen S M, Constine L S, Deasy J O, Eisbruch A, Jackson A, Marks L B, Ten Haken R K and Yorke E D 2010 Quantitative analyses of normal tissue effects in the clinic (QUANTEC): an introduction to the scientific issues Int. J. Radiat. Oncol. Biol. Phys. 76 (Suppl. 3) S3–9 British Institute of Radiology 1983 Central axis depth dose data for use in radiotherapy: a survey of depth doses and related data measured in water or equivalent media Br. J. Radiol. Suppl. 17 1–147 (PMID: 6600113) Burman C et al 1997 Planning, delivery, and quality assurance of intensity-modulated radiotherapy using dynamic multileaf collimator: a strategy for large-scale implementation for the treatment of carcinoma of the prostate Int. J. Radiat. Oncol. Biol. Phys. 39 863–73 Dowdell S, Clasie B, Wroe A, Guatelli S, Metcalfe P, Schulte R and Rosenfeld A 2009 Tissue equivalency of phantom materials for neutron dosimetry in proton therapy Med. Phys. 36 5412–9 Fiechtner A, Gm¨ur K and Wernli C 1997 A personal neutron dosimetry system based on etched track and automatic readout by Autoscan 60 Radiat. Prot. Dosim. 70 157–60 Fiechtner A and Wernli C 1999 Individual neutron monitoring with CR-39 detectors at an accelerator centre Radiat. Prot. Dosim. 85 35–8 Followill D, Geis P and Boyer A 1997 Estimates of whole-body dose equivalent produced by beam intensity modulated conformal therapy Int. J. Radiat. Oncol. Biol. Phys. 38 667–72 Gilvin P, Bartlett D, Shaw P, Steele J and Tanner R 2001 The NRPB PADC neutron personal dosimetry service Radiat. Prot. Dosim. 96 191–5 Gilvin P J, Bartlett D T and Steele J D 1987 The NRPB PADC neutron personal dosimetry service Radiat. Prot. Dosim. 20 99–102 (http://rpd.oxfordjournals.org/content/20/1-2/99.short) H¨alg R A, Besserer J, Boschung M, Mayer S, Clasie B, Kry S F and Schneider U 2012 Field calibration of PADC track etch detectors for local neutron dosimetry in man using different radiation qualities Nucl. Instrum. Methods Phys. Res. A 694 205–10 H¨alg R A, Besserer J, Boschung M, Mayer S and Schneider U 2012 Monitor units are not predictive of neutron dose for high-energy IMRT Radiat. Oncol. 7 138 H¨alg R A, Besserer J and Schneider U 2011 Comparative simulations of neutron dose in soft tissue and phantom materials for proton and carbon ion therapy with actively scanned beams Med. Phys. 38 3149–56 H¨alg R A, Besserer J and Schneider U 2012a Systematic measurements of whole-body dose distributions for various treatment machines and delivery techniques in radiation therapy Med. Phys. 39 7662–76 H¨alg R A, Besserer J and Schneider U 2012b Systematic measurements of whole-body imaging dose distributions in image-guided radiation therapy Med. Phys. 39 7650–61 Hall E J 2006 Intensity-modulated radiation therapy, protons, and the risk of second cancers Int. J. Radiat. Oncol. Biol. Phys. 65 1–7 Hall E J and Wuu C S 2003 Radiation-induced second cancers: the impact of 3D-CRT and IMRT Int. J. Radiat. Oncol. Biol. Phys. 56 83–8 Hoedlmoser H, Schuler C, Butterweck G, Ch´etelat N and Mayer S 2011 Characterization of the 241 Am-Be neutron source of the PSI Calibration Laboratory Paul Scherrer Institut(PSI) Villigen, Switzerland Laboratory Technical Report TM-96-11-02 (dx.doi.org/10.1063/1.3665339) 2467
¨ et al R A Halg
Phys. Med. Biol. 59 (2014) 2457
International Commission on Radiological Protection 2007 The 2007 Recommendations of the International Commission on Radiological Protection ICRP Publication 103 Ann. ICRP 37 1–332 (PMID: 18082557) ISO 4037-1:1996 1996 X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy – Part 1: radiation characteristics and production methods (Geneva, Switzerland: International Organization for Standardization) ISO 4037-2:1997 1997 X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy – part 2: dosimetry for radiation protection over the energy ranges from 8 keV to 1, 3 MeV and 4 MeV to 9 MeV International Organization for Standardization (Geneva, Switzerland: International Organization for Standardization) ISO 4037-3:19991999 X and gamma reference radiation for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy – part 3: calibration of area and personal dosemeters and the measurement of their response as a function of energy and angle of incidence (Geneva, Switzerland: International Organization for Standardization) Kry S F, Howell R M, Titt U, Salehpour M, Mohan R and Vassiliev O N 2008 Energy spectra, sources, and shielding considerations for neutrons generated by a flattening filter-free Clinac Med. Phys. 35 1906–11 Little M P 1997 Estimates of neutron relative biological effectiveness derived from the Japanese atomic bomb survivors Int. J. Radiat. Biol. 72 715–26 McCall R C, Almond P R, Holeman G R, Devanney I A, Lanzl L H and Fuller E G 1984 Neutron contamination from medical electron accelerators NCRP Rep. 79 1–132 Mohan R, Wang X H, Jackson A, Bortfeld T, Boyer A L, Kutcher G J, Leibel S A, Fuks Z and Ling C C 1994 The potential and limitations of the inverse radiotherapy technique Radiother. Oncol. 32 232–48 Newhauser W D and Durante M 2011 Assessing the risk of second malignancies after modern radiotherapy Nature Rev. Cancer 11 438–48 Ongaro C, Zanini A, Nastasi U, Rodenas J, Ottaviano G, Mafredotti C and Burn K W 2000 Analysis of photoneutron spectra produced in medical accelerators Phys. Med. Biol. 45 L55–61 Paganetti H 2012 Assessment of the risk for developing a second malignancy from scattered and secondary radiation in radiation therapy Health Phys. 103 652–61 Paganetti H, Athar B S, Moteabbed M, A Adams J, Schneider U and Yock T I 2012 Assessment of radiation-induced second cancer risks in proton therapy and IMRT for organs inside the primary radiation field Phys. Med. Biol. 57 6047–61 Paganetti H, Niemierko A, Ancukiewicz M, Gerweck L E, Goitein M, Loeffler J S and Suit H D 2002 Relative biological effectiveness (RBE) values for proton beam therapy Int. J. Radiat. Oncol. Biol. Phys. 53 407–21 Rowland J, Mariotto A, Aziz N, Tesauro G, Feuer E J, Blackman D, Thompson P and Pollack L A 2004 Cancer survivorship—United States, 1971-2001 MMWR Morb. Mortal. Wkly. Rep. 53 526–9 Schuhmacher H 2004 Neutron calibration facilities Radiat. Prot. Dosim. 110 33–42 Takam R, Bezak E, Marcu L G and Yeoh E 2011 Out-of-field neutron and leakage photon exposures and the associated risk of second cancers in high-energy photon radiotherapy: current status Radiat. Res. 176 508–20 Timmerman R D and Mendonca M 2002 In regard to Donaldson et al: results from the IRS-IV randomized trial of hyperfractionated radiotherapy in children with rhabdomyosarcoma—a report from the IRSG. IJROBP 2001;51 : 718-728 Int. J. Radiat. Oncol. Biol. Phys. 54 1579–80 (PMID: 12459396) Xu X G, Bednarz B and Paganetti H 2008 A review of dosimetry studies on external-beam radiation treatment with respect to second cancer induction Phys. Med. Biol. 53 R193–241 Zanini A et al 2004 Monte Carlo simulation of the photoneutron field in linac radiotherapy treatments with different collimation systems Phys. Med. Biol. 49 571–82
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