A BrachyPhantom for verification of dose calculation of HDR brachytherapy planning system C. Austerlitz and C. A. T. Campos Citation: Medical Physics 40, 112103 (2013); doi: 10.1118/1.4826170 View online: http://dx.doi.org/10.1118/1.4826170 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/40/11?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Dosimetric characterization and output verification for conical brachytherapy surface applicators. Part II. High dose rate 192Ir sources Med. Phys. 41, 022104 (2014); 10.1118/1.4862506 Measurement of absorbed dose-to-water for an HDR 192Ir source with ionization chambers in a sandwich setup Med. Phys. 40, 092101 (2013); 10.1118/1.4816673 Calculated organ doses using Monte Carlo simulations in a reference male phantom undergoing HDR brachytherapy applied to localized prostate carcinoma Med. Phys. 40, 033901 (2013); 10.1118/1.4791647 Verification of TG-61 dose for synchrotron-produced monochromatic x-ray beams using fluence-normalized MCNP5 calculations Med. Phys. 39, 7462 (2012); 10.1118/1.4761870 Comparison of dose calculation algorithms in slab phantoms with cortical bone equivalent heterogeneities Med. Phys. 34, 3323 (2007); 10.1118/1.2750972
A BrachyPhantom for verification of dose calculation of HDR brachytherapy planning system C. Austerlitza) Clinica Diana Campos, Recife, PE 52020-030, Brazil
C. A. T. Campos Pontifícia Universidade Católica do Rio de Janeiro, RJ 22451-900, Brazil
(Received 30 August 2013; revised 27 September 2013; accepted for publication 6 October 2013; published 22 October 2013) Purpose: To develop a calibration phantom for 192 Ir high dose rate (HDR) brachytherapy units that renders possible the direct measurement of absorbed dose to water and verification of treatment planning system. Methods: A phantom, herein designated BrachyPhantom, consists of a Solid WaterTM 8-cm high cylinder with a diameter of 14 cm cavity in its axis that allows the positioning of an A1SL ionization chamber with its reference measuring point at the midheight of the cylinder’s axis. Inside the BrachyPhantom, at a 3-cm radial distance from the chamber’s reference measuring point, there is a circular channel connected to a cylindrical-guide cavity that allows the insertion of a 6-French flexible plastic catheter from the BrachyPhantom surface. The PENELOPE Monte Carlo code was used to calculate lw , to correct the reading of the ionization chamber to a full scatter condition in liquid a factor, Psw water. The verification of dose calculation of a HDR brachytherapy treatment planning system was performed by inserting a catheter with a dummy source in the phantom channel and scanning it with a CT. The CT scan was then transferred to the HDR computer program in which a multiple treatment plan was programmed to deliver a total dose of 150 cGy to the ionization chamber. The instrument lw factor. reading was then converted to absorbed dose to water using the Ngas formalism and the Psw Likewise, the absorbed dose to water was calculated using the source strength, Sk , values provided by 15 institutions visited in this work. lw . The expanded uncertainty in the abResults: A value of 1.020 (0.09%, k = 2) was found for Psw sorbed dose assessed with the BrachyPhantom was found to be 2.12% (k = 1). To an associated Sk of 27.8 cGy m2 h−1 , the total irradiation time to deliver 150 cGy to the ionization chamber point of reference was 161.0 s. The deviation between the absorbed doses to water assessed with the BrachyPhantom and those calculated by the treatment plans and using the Sk values did not exceed ±3% and ±1.6%, respectively. Conclusions: The BrachyPhantom may be conveniently used for quality assurance and/or verification of HDR planning system with a priori threshold level to spot problems of 2% and ±3%, respectively, and in the long run save time for the medical physicist. © 2013 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4826170] Key words: BrachyPhantom, HDR, end-to-end testing, absolute dosimetry, N-gas 1. INTRODUCTION The importance of proper quality assurance (QA) in the field of high dose rate (HDR) brachytherapy has been extensively stated in the scientific literature1–4 and recommended by several organizations.5–8 An abundant number of tools and/or detectors have been used to perform QA in HDR brachytherapy treatment planning, such as MOSFET detectors,9 PMMA phantoms with TLD,10 PMMA with ionization chamber,11 water phantoms and Fricke dosimetry,12 radiographic films,13 and well-type chambers.12 An important QA procedure in HDR remote afterloading brachytherapy is the verification of the consistency of the dose delivered to the patient by direct test with a dosimetric medium prior to the patient’s first treatment.14 When the detectors as mentioned above are used to determine the absorbed dose, they may rely on the source-to-detector distance 112103-1
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determination and/or verification. This is a time consuming task for the medical physicist and one of the largest sources of uncertainty for most detectors,15 particularly when the HDR equipment has a dwell position that renders it limited to a relatively large stepwise movement making it hard to position the source with respect to the detector. Consequently, a large uncertainty may result if the detector is too close to the radioactive source. The well-type ionization chamber has been the preferred detector for HDR brachytherapy. Such detectors are calibrated against a reference 192 Ir source in terms of air kerma, which is a major contributor to the uncertainty in its calibration factor.16 Recent published data have shown that through the use of the Ngas calibration factor formalism, the direct measurement of the absorbed dose to water in HDR 192 Ir brachytherapy may be achieved with an ionization chamber immersed in water with a lower uncertainty than that obtained
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measuring point at a radial distance of 3 cm. The cavity in the cartridge is designed to fit an A1SL ionization chamber with its reference measuring point in a fixed position which corresponds to the same plan at which the BrachyPhantom’s channel is located. 2.B. Determination of the absorbed dose to water with the BrachyPhantom
The determination of the absorbed dose to water for clinical HDR 192 Ir radioactive sources, DwIr , measured in this work with the BrachyPhantom and an A1SL chamber was calculated by Eq. (1): Co Ir lw Mraw Pion Pelec Ptp Ppol PCo Psw , DwIr = ND,w
F IG . 1. Cross-section view of the BrachyPhantom showing the cover, the base, the cartridge, the stopper, and the channel where a 6-French plastic flexible catheter is inserted.
with a well-type chamber.15 However, such quantity was assessed with a sophisticated calibration setup which may not be convenient in routine work. This work has had the aim to project and test a calibration phantom for quality assurance and the verification of the treatment plan system in HDR 192 Ir brachytherapy that does not require the determination of the position of the radioactive source in relation to the detector and allows the direct measurement of the absorbed dose to water using the Ngas formalism.
(1)
Co is the ionization chamber absorbed dose-based where ND,w calibration factor for 60 Co, Mraw is the instrument’s raw reading, Pion is the recombination correction factor, Pelec is the electrometer calibration factor, Ptp is the temperature– pressure correction factor, and Ppol is the polarity correcCo Ir is a factor to convert ND,w to the tion factor, whereas PCo Ir calibration factor for ND,w . The quantities Mraw , Pion , Pelec , Ptp , and Ppol are defined in the AAPM’s TG-51 protocol.17 Ir PCo is a factor based on the cavity gas calibration factor 60Co (Ngas ) formalism18 to convert ND,w to ionization chamber absorbed dose-based calibration factor for 192 Ir. A Monte Carlo Ir from data calculated (MC) derived factor of 1.0007 for PCo by Sarfehnia et al.15 for a source–detector distance of 3.49 lw refers to a cm was used in this work to determine DwIr . Psw PENELOPE MC calculated factor corresponding to the ratio of the absorbed dose to water calculation in a 0.5-cm diameter sphere of water located at the center of a 40-cm high and 40-cm diameter cylinder of liquid water (lw) and the absorbed dose to water calculated in a 0.5-cm diameter sphere of water centered in a cylinder of Solid WaterTM with the same dilw yields to the mensions of the BrachyPhantom. The factor Psw conversion of the ionization chamber reading measured in the
2. METHODOLOGY 2.A. Phantom description
The phantom pictured in Fig. 1, manufactured by the TecGraf of Rio de Janeiro, Brazil, and henceforth designated BrachyPhantom, is made of four pieces of Solid WaterTM described as follows: a 4-cm high and 14-cm wide cylindrical cover with a 4.01-cm circular hole in its axis on top of another 4-cm high and 14-cm wide cylinder with a 4.01-cm wide and 2-cm deep well in its axis, an 8-cm high and 4-cm wide cylindrical cartridge with a cavity in its axis to allow an A1SL ionization chamber to be inserted, and a 2-cm high and 1-cm wide cylindrical stopper. The cover and its base have a semicircular trench with a radius of 3 cm connected to a semicircular cylindrical guide cavity on their outer surface. When the cover, the base, the stopper, the cartridge, and the A1SL are properly assembled, it is possible to insert a 6-French flexible catheter that surrounds the ionization chamber’s reference Medical Physics, Vol. 40, No. 11, November 2013
F IG . 2. A cropped cross-section view of the 40-cm high and 40-cm diameter cylinder of liquid water with the sphere of water and the 192 Ir Nucletron Model mHDR-v2 source drawn by the Gview2D PENELOPE package.
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BrachyPhantom to that measured in liquid water in full scatter condition. Figure 2 shows a cropped cross-section view of the 40-cm high and 40-cm diameter cylinder of liquid water with the sphere of water and the 192 Ir Nucletron Model mHDRv2 source drawn by the Gview2D PENELOPE package. The 192 Ir source was positioned at a 3-cm radial distance from the reference measuring point of the A1SL ionization chamber. The data related to the Nucletron Model mHDR-v2 source, i.e., materials, dimensions, source encapsulation, and cable were taken from data published by Benhabib.19 The section “Tally Energy Deposition” from the PENELOPE code was used to score the dose in the water detector. The photon energy cutoff was set to 10 keV and simulations were run until about 0.1% statistical uncertainty was reached. 2.C. Verification of the chamber’s “hot spot”
Because the BrachyPhantom was constructed based on the assumption that the effective measuring point of the A1SL ionization chamber coincides with the geometric center of its sensitive volume, this had to be checked. Thus, the verification of the “hot spot” of the ionization chamber inside the BrachyPhantom along the BrachyPhantom axis was performed by replacing the original cartridge with a shorter one and using a set of five 0.027-mm thick and 39-mm diameter disks made of radiochromic film, which is tissue-equivalent. The depth of the cavity for the A1SL chamber in the shorter cartridge was the same as that manufactured for the BrachyPhantom, so that, when the A1SL ionization chamber was inserted in the BrachyPhantom using this shorter cartridge, its reference measuring point was underneath the midpoint of the BrachyPhantom’s ring channel. A set of three measurements were performed with the A1SL ionization chamber with this short cartridge inserted in the BrachyPhantom loaded with an 192 Ir radioactive source. The radiochromic disks, one at time, were placed at the bottom of the 4-cm cavity of the base of the BrachyPhantom pictured in Fig. 1, and the measurements were repeated. The highest instrument reading yielded the “hot spot position.” 2.D. Verification of the treatment plan system
The verification of the treatment plan system computer file generated by the treatment planning software to control the HDR afterloading was replaced as follows. Both a 6French catheter together with a metallic dummy sources and the A1SL were inserted in the BrachyPhantom and the phantom underwent CT scanning. In order to reduce image artifact effects caused by the dummy sources in the image data acquired by the CT imaging system, the BrachyPhantom was positioned on the CT couch with its axis tilted 45◦ with respect to the CT rotation axis. The CT scan was then transferred to the HDR computer treatment plan. The treatment plan was performed, through digitized points of the catheter’s reconstruction, so that a total dose of 150 cGy is delivered in a multidwell position plan to the chamber’s reference measuring point. To do that, the source was stopped at 16 equally spaced points in the BrachyPhantom’s circular channel. The Medical Physics, Vol. 40, No. 11, November 2013
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absorbed dose to water measured with the A1SL was compared against those predicted by the treatment plan and calculated by the AAPM TG-43 (Ref. 20) formalism using the source strength values provided by 15 institutions visited in this work. 2.E. Calculation of the absorbed dose based on the source strength value
The absorbed dose D(r, θ) , to water, at a radial distance of r = 3 cm and polar angle of θ = 90◦ when the 192 Ir source was inside the ring channel of the BrachyPhantom during a period of time t(h), based on the source strength, Sk , value was calculated using TG-43 formalism20 by Eqs. (2) and (3): Gr,θ g(r)F (r, θ ), D(r,θ) = Sk (2) Gr0 ,θ0 D(3 cm,90◦ ) (cGy) = Sk (cGy cm2 h−1 ) × 0.1232 (cm−2 ) × t (h), (3) where Sk is the source air-kerma strength of the source; is the dose-rate constant; θ is the angle subtended by the central axis of the source and the line connecting the center of the source and the dose point; r0 and θ 0 are reference parameters (taken to be 1 cm and 90◦ , respectively); G(r, θ ) is the source geometry factor that accounts for the geometric falloff of the photon fluence with distance from the source and depends on the distribution of radioactive materials; and g(r) is the radial dose function that accounts for radial dependence of photon absorption and scattering in the medium along the radioactive source transverse axis (θ = π /2). F(r, θ ) is the anisotropy function that accounts for the effect of angular dependence (anisotropy of dose distribution around the source) of photon absorption and scattering in the medium. In this work the ratio [(Gr=3 cm,θ=90◦ )/(Gr0 =1 cm,θ=90◦ )] and the factor F(3 cm, 90◦ ), taken from AAPM TG-43 protocol,20 were 0.981 and 1.000, respectively. A value of 1.108 (±0.18%), in units cGyh−1 U−1 R 192 for the MicroSelectron Ir source used in this work, calculated by Daskalov et al.,21 was adopted for . The values of Sk were provided by the participating institutions and used to calculate the absorbed dose by mean of Eq. (3). 3. RESULTS AND DISCUSSION 3.A. Influence of source–detector distance on absorbed dose to water determined with the BrachyPhantom
Neglecting the average dose effect, the influence of the source–detector distance on the absorbed dose to water determined with the BrachyPhantom results from two different influence quantities. The first influence quantity is due to the uncertainty in the radial distance along the transverse axis of the radioactive source (θ = π /2) between the 192 Ir source and the ionization chamber. The second effect is caused by the anisotropy of dose distribution around the source and the uncertainty in the angle θ . The average dose effect is not an
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air as it was done in the simulations. The influence on the instrument reading caused by such amount of scatter radiation was not measured. However, because in the measurements performed in this work the BrachyPhantom was always positioned on the surface of such low-Z materials (carbon fiber, plastic, and wood) and its dimension may shield low-energy backscatter radiation, this effect was neglected. Ir 3.C. The Ngas and PCo factors
F IG . 3. Instrument reading as a function of the position of the reference measuring point of the A1SL ionization chamber in relation to the axis of the 192 Ir radioactive source.
objective of this work. Therefore, it is not included in the uncertainty analysis described in Sec. 3.D. The uncertainties caused by the source–detector distances are discussed in Sec. 3.D. The effect of the radioactive source anisotropy on the ionization chamber reading is pictured in Fig. 3. The “zero” position in the x-axis of Fig. 3 corresponds to the centroid of the collecting volume or the reference measuring point of the A1SL ionization chamber and the midpoint of the BrachyPhantom’s ring channel as well. This may be seen in Fig. 3, the hot spot position corresponds to the reference measuring point of the A1SL ionization chamber. When the cartridge with the ionization chamber was shifted up and down from −0.054 to 0.055 cm in relation to the “0” position, it led to a deviation of ±0.15% in the value of the absorbed dose. The uncertainty caused by the source anisotropy in the A1SL ionization chamber reading is discussed in Sec. 3.D. 3.B. Size and material of the BrachyPhantom: lw The Psw factor
The MC calculated energy deposited in the 0.5 cm diameter sphere of liquid water centered inside a cylinder of Solid WaterTM with physical dimensions equivalent to that of the BrachyPhantom (using the configuration shown in Fig. 2) was 6.374 eV (0.064%, k = 2). In the case of the 40-cm high and 40-cm diameter cylinder of liquid water, the calculated energy deposited in this sphere was 6.503 eV (0.064%, k = 2). This yields to a P lw sw value of 1.020 (0.09%, k = 2). Even though this factor has been calculated with a relatively low uncertainty, it might not reflect the overall amount of scatter radiation as seen by the BrachyPhantom in routine work. The simulations were performed in free air, while in the measurements performed in this work the BrachyPhantom was positioned on the surface of the patient couches, wood tables, and plastic tables. The contributions of scatter radiation to the ionization chamber from all these different setups were not taken into account in the P lw sw factor calculation. Therefore, the measured absorbed dose determined with such setups may differ from the one calculated with the BrachyPhantom positioned in free Medical Physics, Vol. 40, No. 11, November 2013
The Ngas factor is unique to each ionization chamber and does not depend on the composition of the dosimetry Ir (Eq. (3)) is a MC calculation factor, based on phantom.18 PCo the Ngas formalism to convert the ionization chamber absorbed dose-based calibration factor for the 60 Co to 192 Ir. A factor of 0.9027 (0.09%, k = 1) and another of 0.9048 (1k = 0.11%) for Ir were calculated by Sarfehnia et al.15 for an A1SL ionizaPCo tion chamber immersed in liquid water at radial distances of 3.49 and 5.15 cm from a 192 Irradioactive source, respectively. The ring channel of the BrachyPhantom, where an 192 Ir source may be positioned, is located at a radial distance of 3 cm from the reference measuring point of the ionization chamber. UsIr calculated by Sarfehnia, a linear extraping the values of PCo Ir olation of PCo values for a radial distance of 3 cm leads to a Ir . A linear extrapolated value of 0.9027 value of 0.9021 for PCo with an expanded calculated uncertainty of 0.2% (k = 1) was kept in this work to calculate the absorbed dose to water with the BrachyPhantom. 3.D. Uncertainty analysis
Table I shows the uncertainty analysis for the absorbed dose determination with the A1SL Exradin ionization chamber measurements performed with the BrachyPhantom. The uncertainties in the calibration factors (for the A1SL ionization chamber and electrometer) and in the thermometer and TABLE I. Uncertainty evaluation for the absorbed dose to water assessed with the A1SL Exradin ionization and the BrachyPhantom. Uncertainty Components
Type A
Type B
60
ND,wCo (A1SL calibration factor) Electrometer (calibration factor) Charge measurement Temperature and pressure Leakage Radial distance Anisotropy lw (phantom material and size correction Psw factor) Ir (MC factor to convert N Co to PCo D,w ionization chamber absorbed dose-based calibration factor for 192 Ir) Ir (difference between MC SSD and PCo BrachyPhantom SSD) Expanded uncertainty (k = 1)
1.4 0.1 0.25 0.2 0.01 1.53 0.01 0.05 0.22
0.2 2.12%
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barometer were based on the calibration certificates provided by national laboratories. The uncertainty in the charge measurement was based on the standard deviation measured during short periods. The uncertainty in the leakage was based on the mean values of the current leakage current measured during the clinical HDRs dose assessment. The uncertainties in the source–chamber distances (radial distance and source anisotropy) were based in the assumption that there is a gap of 0.006 cm between the radioactive source and the catheter hole and another 0.005-cm gap between the catheter and the BrachyPhantom’s channel. The uncertainties in the phantom dimensions were based on the data provided by Tecgraph, Brazil,22 where the BrachyPhantom was designed and manIr were based on the MC ufactured. The uncertainties in PCo calculation uncertainties data published by Sarfehnia et al.15 for source–detector distances of 3.49 and 5.15 cm. The uncertainty in P lw sw (phantom material and size) was taken from the MC calculation performed in this work. The uncertainties in the absorbed dose, calculated by the treatment plan system and with the TG-51 formalism, which depend on the treatment plan system and the value of the source strength provided by the users, respectively, were not among the objectives of this work. However, a brief comment about the uncertainty in the assessment of the absorbed dose calculated by the treatment plan caused by the catheter reconstruction process is included in Sec. 3.E. 3.E. Image artifacts effect
The effect of the position of the BrachyPhantom in relation to the CT rotation axis is shown in Figs. 4(a) and 4(b) (rotated to the 4A-plane by the Oncentra treatment planning system). In the first, the BrachyPhantom’s axis is vertical. In the second, the axis is horizontal and rotated in 45◦ in relation to the CT rotation axis, which resulted in a much better image. A better image may minimize the error in the treatment plan process, since the digitized points in the catheter reconstruction process may depend on the quality of the CT image. The
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placement of the catheter in an incorrect location will lead to computer errors during the treatment planning preparation process and a deviation in the dose delivered to the ionization chamber positioned in the BrachyPhantom. This is illustrated in Fig. 4(b) by the points represented by the letters “a ” and “b .” The point “a ” looks a little bit away from the ionization chamber’s reference measuring point, whereas the point “b ” is somehow closer to it. This is an effect of the digitalization procedure which may be exacerbated if the image has a poor quality. 3.F. Intercomparison of absorbed dose
Table II shows the results of absorbed dose to water from 15 clinical HDRs brachytherapy equipment calculated by the Ocentra Master Treatment Planning system (TP, fourth column), assessed with the BrachyPhantom (BP, fifth column), and based on the source strength values provided by 15 institutions visited in this work (mGy m2 h−1 , third column and Sk , sixth column). The last three columns in the table show the relative percent deviation between absorbed doses determined with the BrachyPhantom and treatment plan, source strength and treatment plan, and BrachyPhantom and source strength. Except for the measurements performed with the 192 Ir channel times of 199.3 and 166.4 s, the relative percent deviation between the absorbed dose assessed with the BrachyPhantom and the treatment plan (BP/TP, seventh column) lies within ±1.35%. The relative percent deviation for 199.3 s (institution # 7) and 166.4 s (institution # 15) are 2.91% and −2.53%, respectively. A comparison between the relative percent deviation of absorbed doses calculated using the source strength values and those calculated by the treatment plan for these channel times (Sk /TP, eighth column) has shown that such deviations follow those measured with the BrachyPhantom and that the deviation does not exceed ±1.7% (institution #7). It means that the radioactive source channel time as seen by the A1SL ion chamber confirm those given in 7th column which only depends on the values of the source strength stated in
F IG . 4. CT imaging of the BrachyPhantom positioned on the CT couch (a) with its central axis vertical; (b) with its central axis horizontal and rotated in 45◦ in relation to the CT rotation axis. Medical Physics, Vol. 40, No. 11, November 2013
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TABLE II. Comparison of absorbed doses to water assessed with the BrachyPhantom, calculated by the HDR brachytherapy treatment planning systems and the source strength values. Absorbed dose to water (cGy) Institution 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Relative deviation (%)
Channel time (s)
Air-kerma rate (mGy m2 h−1 )
Treatment plan
BrachyPhantom
Source strength (Sk )
BP/TP
Sk /TP
BP/Sk
203.20 168.00 168.00 122.40 168.00 141.60 199.30 122.40 154.45 139.20 178.20 131.30 184.00 216.00 166.40
21.36 25.86 26.04 35.71 25.86 30.80 22.38 35.71 28.76 31.26 24.26 32.60 23.76 20.31 26.06
150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 Mean % Deviation
148.90 151.10 148.70 148.90 150.00 149.10 154.50 149.30 150.45 149.60 149.30 148.40 148.00 150.20 146.30 149.52 1.09
148.57 148.72 149.75 149.64 148.72 149.29 152.67 149.64 152.07 148.97 147.97 146.52 149.68 150.21 148.45 149.39 0.99
− 0.74 0.73 − 0.87 − 0.74 0.00 − 0.60 2.91 − 0.47 0.30 − 0.27 − 0.47 − 1.08 − 1.35 0.13 − 2.53 − 0.34 0.73
− 0.96 − 0.84 − 0.17 − 0.24 − 0.85 − 0.47 1.73 − 0.24 1.37 − 0.69 − 1.36 − 2.34 − 0.22 0.14 − 1.06 − 0.41 0.66
0.22 1.57 − 0.70 − 0.50 0.85 − 0.13 1.19 − 0.23 − 1.07 0.42 0.89 1.26 − 1.13 0.00 − 1.47 0.08 0.73
the treatment plan report. Consequently, these relatively large deviations have been attributed to errors caused by the catheter reconstruction process.23 The dose rate constant, , in units (cGyh−1 U−1 ), calculated by Benhabib19 according to the recommendations of the TG-43 data for the Nucletron Model mHDR-v2 and using the PENELOPE code was found to be 1.108 (0.1%, k = 1) cGyh−1 U−1 . The value of this quantity calculated by Daskalov et al.24 is 1.108 (0.13%). Therefore, the modeling of the Nucletron Model mHDR-v2 written by Benhabibi has been used in this work to calculate P lw sw . To avoid any undesirable energy dependence effect, a sphere of water has been used in the MC simulations to calculate the P lw sw factor. High dose rate remote afterloading intracavitary brachytherapy requires an independent verification of a treatment plan system in the quality assurance program. Such verification may include the accurate determination of the absorbed dose at the prescription point, which depends on the source–detector distance, dwell time and catheter reconstruction. A radial distance of 3 cm was preferred for the radioactive source channel in the BrachyPhantom. This radial distance serves not only to check the catheter reconstruction when a single source position is assigned for the treatment plan but also to perform measurements in short periods. Any deviation of ±0.02 cm in the radial distance may lead instrument readings by ±3%. At higher radial distances, the above-mentioned deviation in the radial distance may be greatly minimized. However, the effect of the catheter reconstruction from the CT scanner transferred to HDR unit on dose at the prescription point is also greatly minimized and the measurements require long periods of time. Therefore, a 3-cm radial distance was chosen to turn possible the use of the BrachyPhantom for fast measurements Medical Physics, Vol. 40, No. 11, November 2013
and to estimate the error in the catheter’s reconstruction procedure when a single-dwell position plan is used to deliver the total dose. For an associated source strength of 27.8 cGy m2 h−1 (mean value of the source strengths used in this work), the total irradiation time to deliver 150 cGy to the ionization chamber point of reference was 161.0 s. Because the absorbed dose measured with the BrachyPhantom does not depend on the position of the source with respect to the detector and is based on the Ngas formalism, a fast and independent assessment of absorbed doses may be achieved with such a device. The model of the BrachyPhantom presented in this work was not designed to check source dwell position. However, a possible way to determine the dwell position may be achieved by redesigning the phantom to make it possible for the positioning of a radiograph film sandwiched between the top and the base of the phantom. Also, by replacing the ionization chamber with a dummy chamber made of tissue equivalent material filled with, e.g., powder TLD, such phantom may be used for postal measurement purposes. Such possibilities are being studied by the authors. The reason and extent to which by tilting the BrachyPhantom of 45◦ scan reduced the image artifact and its effect on the HU numbers, respectively, does not make part of this study. 4. CONCLUSIONS A calibration BrachyPhantom was developed and tested for verification of dose calculation by HDR brachytherapy treatment planning. The BrachyPhantom does not require the determination of the hot spot position and therefore allows a fast and independent determination of the absorbed dose to water with an uncertainty of 2.12% (k = 1). Measurements performed in 15 clinical HDR 192 Ir radioactive sources have
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shown that discrepancies between the absorbed doses to water assessed with the BrachyPhantom and those calculated by the treatment plans did not exceed ±2.91%. The maximum deviation between the absorbed doses measured with the BrachyPhantom and with the TG43 formalism, using the source strength values provided by 15 institutions visited in this work, was ±1.57%. Overall, the BrachyPhantom may be used for QA and/or verification of HDR planning system with, a priori, a threshold level of about 2% and 3% to spot problems, respectively. For its convenience and ease of use, it is a device that, in the long run, may save time for the medical physicist. ACKNOWLEDGMENTS The authors are very grateful for useful advices, suggestions, and support from Dr. C. Sibata, Director Physics Quality Control Section of the 21st Century Oncology, USA. The authors express their gratitude toward the physicists from the different Offices of the 21st Century Oncology visited in this work, A. Organista, A. Shamaoun, B. Pomij, C. Chipley, G. McNerney, H. Gotts, J. Miranda, J. Moralos, K. Noda, M. Soldano, P. Pomije, P. Shendero, R. Ducan, R. Gotts, R. Richardson, S. Chakrabon, S. Darwish, S. Olivera, S. Peter, W. Neeranjah, and X. Chen, for their help to perform the experimental part of this work. Moreover, the authors acknowledge Dr. S. Benhabibi from the East Carolina University for providing inputs in the MC calculation.
a) Author
to whom correspondence should be addressed. Electronic mail: [email protected]; Telephone: (941) 894-0546. 1 W. M. Butler and G. S. Merrick, “Clinical practice and quality assurance challenges in modern brachytherapy sources and dosimetry,” Int. J. Radiat. Oncol., Biol., Phys. 71, S142–S146 (2008). 2 L. A. DeWerd, P. Jursinic, R. Kitchen, and B. R. Thomadsen, “Quality assurance tool for high dose rate brachytherapy,” Med. Phys. 22, 435–440 (1995). 3 P. Kohr and F. A. Siebert, “Quality assurance of brachytherapy afterloaders using a multi-slit phantom,” Phys. Med. Biol. 52, N387–N391 (2007). 4 C. A. Mangold, A. Rijnders, D. Georg, E. Van Limbergen, and D. Huyskens, “Quality control in interstitial brachytherapy of the breast using pulsed dose rate: Treatment planning and dose delivery with an Ir192 afterloading system,” Radiother. Oncol. 58, 43–51 (2001). 5 H. D. Kubo, G. P. Glasgow, T. D. Pethel, B. R. Thomadsen, and J. F. Williamson, “High dose-rate brachytherapy treatment delivery: Report of the AAPM Radiation Therapy Committee Task Group No. 59,” Med. Phys. 25, 375–403 (1998). 6 S. Nag, B. Erickson, S. Parikh, N. Gupta, M. Varia, and G. Glasgow, “The American Brachytherapy Society recommendations for high-doserate brachytherapy for carcinoma of the endometrium,” Int. J. Radiat. Oncol., Biol., Phys. 48, 779–790 (2000).
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Nag, M. Dally, M. de la Torre, H. Tatsuzaki, N. Kizilbash, S. Kurusun, L. Pinillos, B. Pokrajac, R. Sur, and V. Levin, “Recommendations for implementation of high dose rate 192 Ir brachytherapy in developing countries by the Advisory Group of International Atomic Energy Agency,” Radiother. Oncol. 64, 297–308 (2002). 8 T. Eudaldo, H. Huizenga, I. L. Lamm, A. McKenzie, F. Milano, W. Schlegel, D. Thwaites, and G. Heeren, “Guidelines for education and training of medical physicists in radiotherapy: Recommendations from an ESTRO/EFOMP working group,” Radiother. Oncol. 70, 125–135 (2004). 9 Z. Y. Qi, X. W. Deng, S. M. Huang, J. Lu, M. Lerch, D. Cutajar, and A. Rosenfeld, “Verification of the plan dosimetry for high dose rate brachytherapy using metal oxide semiconductor field effect transistor detectors,” Med. Phys. 34, 2007–2013 (2007). 10 R. Ochoa, I. H. Ferreira, F. Gutt, and C. E. de Almeida, “Design of a phantom for the quality control of high dose rate 192 Ir source used in brachytherapy,” Radiother. Oncol. 82, 222–228 (2007) 11 PTW, Dosimetry in Afterloading Brachytherapy, D432.208.0/1 ed. (PTWFreiburg, Freiburg, Germany, 2012). 12 C. Austerlitz, H. Mota, C. E. Almeida, R. Allison, and C. Sibata, “Quality assurance of HDR Ir sources using a Fricke dosimeter,” Med. Phys. 34, 1348–1353 (2007). 13 M. D. C. Evans, S. Devic, and E. B. Podgorsak, “High dose-rate brachytherapy source position quality assurance using radiochromic film,” Med. Dosim. 32, 13–15 (2007). 14 K. D. Steidley, “Use of radiochromic dosimetry film for HDR brachytherapy quality assurance,” Med. Dosim. 23, 37–38 (1998). 15 A. Sarfehnia, I. Kawrakow, and J. Seuntjens, “Direct measurement of absorbed dose to water in HDR Ir brachytherapy: Water calorimetry, ionization chamber, Gafchromic film, and TG-43,” Med. Dosim. 37(4), 1924– 1931 (2010). 16 R. J. M. Elfrink, I. K. K. Kolkman-Deurloo, H. J. van Kleffens, A. Rijnders, B. Schaeken, T. H. L. Aalbers, W. J. F. Dries, and J. L. M. Venselaar, “Quality control of brachytherapy equipment in the Netherlands and Belgium: Current practice and minimum requirements,” Radiother. Oncol. 62, 95–102 (2002). 17 P. R. Almond, P. J. Biggs, B. Coursey, W. Hanson, M. S. Huq, R. Nath, and D. Rogers, “AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams,” Med. Phys. 26, 1847–1870 (1999). 18 AAPM, “A protocol for the determination of absorbed dose from highenergy photon and electron beams Task Group 21,” Med. Phys. 10, 741– 771 (1983). 19 S. Benhabib, “A proposed methodology to assess HDR 192 Ir radial dose with Fricke dosimetry,” Ph.D. thesis, East Carolina University, 2011. 20 M. J. Rivard, B. M. Coursey, L. A. DeWerd, W. F. Hanson, M. S. Huq, G. S. Ibbott, M. G. Mitch, R. Nath, and J. F. Williamson, “Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculations,” Med. Phys. 31, 633–674 (2004). 21 G. M. Daskalov and J. F. Williamson, “Monte Carlo-aided dosimetry of a new high dose-rate brachytherapy source,” Med. Phys. 25, 2200–2208 (1998). 22 C. A. T. Campos, The Design of the BrachyPhantom (Universidade Católoca do Rio de Janeiro, RJ, Brazil, 2012). 23 R. Nath, L. L. Anderson, G. Luxton, K. A. Weaver, J. F. Williamson, and A. S. Meigooni, “Dosimetry of interstitial brachytherapy sources: Recommendations of the AAPM Radiation Therapy Committee Task Group No. 43,” Energy 1, 209–234 (1995). 24 G. M. Daskalov, R. S. Baker, D. W. Rogers, and J. F. Williamson, “Dosimetric modeling of the microselectron high-dose rate 192 Ir source by the multigroup discrete ordinates method,” Med. Phys. 27(10), 2307– 2319.(2000).
A BrachyPhantom for verification of dose calculation of HDR brachytherapy planning system C. Austerlitza) Clinica Diana Campos, Recife, PE 52020-030, Brazil
C. A. T. Campos Pontifícia Universidade Católica do Rio de Janeiro, RJ 22451-900, Brazil
(Received 30 August 2013; revised 27 September 2013; accepted for publication 6 October 2013; published 22 October 2013) Purpose: To develop a calibration phantom for 192 Ir high dose rate (HDR) brachytherapy units that renders possible the direct measurement of absorbed dose to water and verification of treatment planning system. Methods: A phantom, herein designated BrachyPhantom, consists of a Solid WaterTM 8-cm high cylinder with a diameter of 14 cm cavity in its axis that allows the positioning of an A1SL ionization chamber with its reference measuring point at the midheight of the cylinder’s axis. Inside the BrachyPhantom, at a 3-cm radial distance from the chamber’s reference measuring point, there is a circular channel connected to a cylindrical-guide cavity that allows the insertion of a 6-French flexible plastic catheter from the BrachyPhantom surface. The PENELOPE Monte Carlo code was used to calculate lw , to correct the reading of the ionization chamber to a full scatter condition in liquid a factor, Psw water. The verification of dose calculation of a HDR brachytherapy treatment planning system was performed by inserting a catheter with a dummy source in the phantom channel and scanning it with a CT. The CT scan was then transferred to the HDR computer program in which a multiple treatment plan was programmed to deliver a total dose of 150 cGy to the ionization chamber. The instrument lw factor. reading was then converted to absorbed dose to water using the Ngas formalism and the Psw Likewise, the absorbed dose to water was calculated using the source strength, Sk , values provided by 15 institutions visited in this work. lw . The expanded uncertainty in the abResults: A value of 1.020 (0.09%, k = 2) was found for Psw sorbed dose assessed with the BrachyPhantom was found to be 2.12% (k = 1). To an associated Sk of 27.8 cGy m2 h−1 , the total irradiation time to deliver 150 cGy to the ionization chamber point of reference was 161.0 s. The deviation between the absorbed doses to water assessed with the BrachyPhantom and those calculated by the treatment plans and using the Sk values did not exceed ±3% and ±1.6%, respectively. Conclusions: The BrachyPhantom may be conveniently used for quality assurance and/or verification of HDR planning system with a priori threshold level to spot problems of 2% and ±3%, respectively, and in the long run save time for the medical physicist. © 2013 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4826170] Key words: BrachyPhantom, HDR, end-to-end testing, absolute dosimetry, N-gas 1. INTRODUCTION The importance of proper quality assurance (QA) in the field of high dose rate (HDR) brachytherapy has been extensively stated in the scientific literature1–4 and recommended by several organizations.5–8 An abundant number of tools and/or detectors have been used to perform QA in HDR brachytherapy treatment planning, such as MOSFET detectors,9 PMMA phantoms with TLD,10 PMMA with ionization chamber,11 water phantoms and Fricke dosimetry,12 radiographic films,13 and well-type chambers.12 An important QA procedure in HDR remote afterloading brachytherapy is the verification of the consistency of the dose delivered to the patient by direct test with a dosimetric medium prior to the patient’s first treatment.14 When the detectors as mentioned above are used to determine the absorbed dose, they may rely on the source-to-detector distance 112103-1
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determination and/or verification. This is a time consuming task for the medical physicist and one of the largest sources of uncertainty for most detectors,15 particularly when the HDR equipment has a dwell position that renders it limited to a relatively large stepwise movement making it hard to position the source with respect to the detector. Consequently, a large uncertainty may result if the detector is too close to the radioactive source. The well-type ionization chamber has been the preferred detector for HDR brachytherapy. Such detectors are calibrated against a reference 192 Ir source in terms of air kerma, which is a major contributor to the uncertainty in its calibration factor.16 Recent published data have shown that through the use of the Ngas calibration factor formalism, the direct measurement of the absorbed dose to water in HDR 192 Ir brachytherapy may be achieved with an ionization chamber immersed in water with a lower uncertainty than that obtained
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measuring point at a radial distance of 3 cm. The cavity in the cartridge is designed to fit an A1SL ionization chamber with its reference measuring point in a fixed position which corresponds to the same plan at which the BrachyPhantom’s channel is located. 2.B. Determination of the absorbed dose to water with the BrachyPhantom
The determination of the absorbed dose to water for clinical HDR 192 Ir radioactive sources, DwIr , measured in this work with the BrachyPhantom and an A1SL chamber was calculated by Eq. (1): Co Ir lw Mraw Pion Pelec Ptp Ppol PCo Psw , DwIr = ND,w
F IG . 1. Cross-section view of the BrachyPhantom showing the cover, the base, the cartridge, the stopper, and the channel where a 6-French plastic flexible catheter is inserted.
with a well-type chamber.15 However, such quantity was assessed with a sophisticated calibration setup which may not be convenient in routine work. This work has had the aim to project and test a calibration phantom for quality assurance and the verification of the treatment plan system in HDR 192 Ir brachytherapy that does not require the determination of the position of the radioactive source in relation to the detector and allows the direct measurement of the absorbed dose to water using the Ngas formalism.
(1)
Co is the ionization chamber absorbed dose-based where ND,w calibration factor for 60 Co, Mraw is the instrument’s raw reading, Pion is the recombination correction factor, Pelec is the electrometer calibration factor, Ptp is the temperature– pressure correction factor, and Ppol is the polarity correcCo Ir is a factor to convert ND,w to the tion factor, whereas PCo Ir calibration factor for ND,w . The quantities Mraw , Pion , Pelec , Ptp , and Ppol are defined in the AAPM’s TG-51 protocol.17 Ir PCo is a factor based on the cavity gas calibration factor 60Co (Ngas ) formalism18 to convert ND,w to ionization chamber absorbed dose-based calibration factor for 192 Ir. A Monte Carlo Ir from data calculated (MC) derived factor of 1.0007 for PCo by Sarfehnia et al.15 for a source–detector distance of 3.49 lw refers to a cm was used in this work to determine DwIr . Psw PENELOPE MC calculated factor corresponding to the ratio of the absorbed dose to water calculation in a 0.5-cm diameter sphere of water located at the center of a 40-cm high and 40-cm diameter cylinder of liquid water (lw) and the absorbed dose to water calculated in a 0.5-cm diameter sphere of water centered in a cylinder of Solid WaterTM with the same dilw yields to the mensions of the BrachyPhantom. The factor Psw conversion of the ionization chamber reading measured in the
2. METHODOLOGY 2.A. Phantom description
The phantom pictured in Fig. 1, manufactured by the TecGraf of Rio de Janeiro, Brazil, and henceforth designated BrachyPhantom, is made of four pieces of Solid WaterTM described as follows: a 4-cm high and 14-cm wide cylindrical cover with a 4.01-cm circular hole in its axis on top of another 4-cm high and 14-cm wide cylinder with a 4.01-cm wide and 2-cm deep well in its axis, an 8-cm high and 4-cm wide cylindrical cartridge with a cavity in its axis to allow an A1SL ionization chamber to be inserted, and a 2-cm high and 1-cm wide cylindrical stopper. The cover and its base have a semicircular trench with a radius of 3 cm connected to a semicircular cylindrical guide cavity on their outer surface. When the cover, the base, the stopper, the cartridge, and the A1SL are properly assembled, it is possible to insert a 6-French flexible catheter that surrounds the ionization chamber’s reference Medical Physics, Vol. 40, No. 11, November 2013
F IG . 2. A cropped cross-section view of the 40-cm high and 40-cm diameter cylinder of liquid water with the sphere of water and the 192 Ir Nucletron Model mHDR-v2 source drawn by the Gview2D PENELOPE package.
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BrachyPhantom to that measured in liquid water in full scatter condition. Figure 2 shows a cropped cross-section view of the 40-cm high and 40-cm diameter cylinder of liquid water with the sphere of water and the 192 Ir Nucletron Model mHDRv2 source drawn by the Gview2D PENELOPE package. The 192 Ir source was positioned at a 3-cm radial distance from the reference measuring point of the A1SL ionization chamber. The data related to the Nucletron Model mHDR-v2 source, i.e., materials, dimensions, source encapsulation, and cable were taken from data published by Benhabib.19 The section “Tally Energy Deposition” from the PENELOPE code was used to score the dose in the water detector. The photon energy cutoff was set to 10 keV and simulations were run until about 0.1% statistical uncertainty was reached. 2.C. Verification of the chamber’s “hot spot”
Because the BrachyPhantom was constructed based on the assumption that the effective measuring point of the A1SL ionization chamber coincides with the geometric center of its sensitive volume, this had to be checked. Thus, the verification of the “hot spot” of the ionization chamber inside the BrachyPhantom along the BrachyPhantom axis was performed by replacing the original cartridge with a shorter one and using a set of five 0.027-mm thick and 39-mm diameter disks made of radiochromic film, which is tissue-equivalent. The depth of the cavity for the A1SL chamber in the shorter cartridge was the same as that manufactured for the BrachyPhantom, so that, when the A1SL ionization chamber was inserted in the BrachyPhantom using this shorter cartridge, its reference measuring point was underneath the midpoint of the BrachyPhantom’s ring channel. A set of three measurements were performed with the A1SL ionization chamber with this short cartridge inserted in the BrachyPhantom loaded with an 192 Ir radioactive source. The radiochromic disks, one at time, were placed at the bottom of the 4-cm cavity of the base of the BrachyPhantom pictured in Fig. 1, and the measurements were repeated. The highest instrument reading yielded the “hot spot position.” 2.D. Verification of the treatment plan system
The verification of the treatment plan system computer file generated by the treatment planning software to control the HDR afterloading was replaced as follows. Both a 6French catheter together with a metallic dummy sources and the A1SL were inserted in the BrachyPhantom and the phantom underwent CT scanning. In order to reduce image artifact effects caused by the dummy sources in the image data acquired by the CT imaging system, the BrachyPhantom was positioned on the CT couch with its axis tilted 45◦ with respect to the CT rotation axis. The CT scan was then transferred to the HDR computer treatment plan. The treatment plan was performed, through digitized points of the catheter’s reconstruction, so that a total dose of 150 cGy is delivered in a multidwell position plan to the chamber’s reference measuring point. To do that, the source was stopped at 16 equally spaced points in the BrachyPhantom’s circular channel. The Medical Physics, Vol. 40, No. 11, November 2013
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absorbed dose to water measured with the A1SL was compared against those predicted by the treatment plan and calculated by the AAPM TG-43 (Ref. 20) formalism using the source strength values provided by 15 institutions visited in this work. 2.E. Calculation of the absorbed dose based on the source strength value
The absorbed dose D(r, θ) , to water, at a radial distance of r = 3 cm and polar angle of θ = 90◦ when the 192 Ir source was inside the ring channel of the BrachyPhantom during a period of time t(h), based on the source strength, Sk , value was calculated using TG-43 formalism20 by Eqs. (2) and (3): Gr,θ g(r)F (r, θ ), D(r,θ) = Sk (2) Gr0 ,θ0 D(3 cm,90◦ ) (cGy) = Sk (cGy cm2 h−1 ) × 0.1232 (cm−2 ) × t (h), (3) where Sk is the source air-kerma strength of the source; is the dose-rate constant; θ is the angle subtended by the central axis of the source and the line connecting the center of the source and the dose point; r0 and θ 0 are reference parameters (taken to be 1 cm and 90◦ , respectively); G(r, θ ) is the source geometry factor that accounts for the geometric falloff of the photon fluence with distance from the source and depends on the distribution of radioactive materials; and g(r) is the radial dose function that accounts for radial dependence of photon absorption and scattering in the medium along the radioactive source transverse axis (θ = π /2). F(r, θ ) is the anisotropy function that accounts for the effect of angular dependence (anisotropy of dose distribution around the source) of photon absorption and scattering in the medium. In this work the ratio [(Gr=3 cm,θ=90◦ )/(Gr0 =1 cm,θ=90◦ )] and the factor F(3 cm, 90◦ ), taken from AAPM TG-43 protocol,20 were 0.981 and 1.000, respectively. A value of 1.108 (±0.18%), in units cGyh−1 U−1 R 192 for the MicroSelectron Ir source used in this work, calculated by Daskalov et al.,21 was adopted for . The values of Sk were provided by the participating institutions and used to calculate the absorbed dose by mean of Eq. (3). 3. RESULTS AND DISCUSSION 3.A. Influence of source–detector distance on absorbed dose to water determined with the BrachyPhantom
Neglecting the average dose effect, the influence of the source–detector distance on the absorbed dose to water determined with the BrachyPhantom results from two different influence quantities. The first influence quantity is due to the uncertainty in the radial distance along the transverse axis of the radioactive source (θ = π /2) between the 192 Ir source and the ionization chamber. The second effect is caused by the anisotropy of dose distribution around the source and the uncertainty in the angle θ . The average dose effect is not an
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air as it was done in the simulations. The influence on the instrument reading caused by such amount of scatter radiation was not measured. However, because in the measurements performed in this work the BrachyPhantom was always positioned on the surface of such low-Z materials (carbon fiber, plastic, and wood) and its dimension may shield low-energy backscatter radiation, this effect was neglected. Ir 3.C. The Ngas and PCo factors
F IG . 3. Instrument reading as a function of the position of the reference measuring point of the A1SL ionization chamber in relation to the axis of the 192 Ir radioactive source.
objective of this work. Therefore, it is not included in the uncertainty analysis described in Sec. 3.D. The uncertainties caused by the source–detector distances are discussed in Sec. 3.D. The effect of the radioactive source anisotropy on the ionization chamber reading is pictured in Fig. 3. The “zero” position in the x-axis of Fig. 3 corresponds to the centroid of the collecting volume or the reference measuring point of the A1SL ionization chamber and the midpoint of the BrachyPhantom’s ring channel as well. This may be seen in Fig. 3, the hot spot position corresponds to the reference measuring point of the A1SL ionization chamber. When the cartridge with the ionization chamber was shifted up and down from −0.054 to 0.055 cm in relation to the “0” position, it led to a deviation of ±0.15% in the value of the absorbed dose. The uncertainty caused by the source anisotropy in the A1SL ionization chamber reading is discussed in Sec. 3.D. 3.B. Size and material of the BrachyPhantom: lw The Psw factor
The MC calculated energy deposited in the 0.5 cm diameter sphere of liquid water centered inside a cylinder of Solid WaterTM with physical dimensions equivalent to that of the BrachyPhantom (using the configuration shown in Fig. 2) was 6.374 eV (0.064%, k = 2). In the case of the 40-cm high and 40-cm diameter cylinder of liquid water, the calculated energy deposited in this sphere was 6.503 eV (0.064%, k = 2). This yields to a P lw sw value of 1.020 (0.09%, k = 2). Even though this factor has been calculated with a relatively low uncertainty, it might not reflect the overall amount of scatter radiation as seen by the BrachyPhantom in routine work. The simulations were performed in free air, while in the measurements performed in this work the BrachyPhantom was positioned on the surface of the patient couches, wood tables, and plastic tables. The contributions of scatter radiation to the ionization chamber from all these different setups were not taken into account in the P lw sw factor calculation. Therefore, the measured absorbed dose determined with such setups may differ from the one calculated with the BrachyPhantom positioned in free Medical Physics, Vol. 40, No. 11, November 2013
The Ngas factor is unique to each ionization chamber and does not depend on the composition of the dosimetry Ir (Eq. (3)) is a MC calculation factor, based on phantom.18 PCo the Ngas formalism to convert the ionization chamber absorbed dose-based calibration factor for the 60 Co to 192 Ir. A factor of 0.9027 (0.09%, k = 1) and another of 0.9048 (1k = 0.11%) for Ir were calculated by Sarfehnia et al.15 for an A1SL ionizaPCo tion chamber immersed in liquid water at radial distances of 3.49 and 5.15 cm from a 192 Irradioactive source, respectively. The ring channel of the BrachyPhantom, where an 192 Ir source may be positioned, is located at a radial distance of 3 cm from the reference measuring point of the ionization chamber. UsIr calculated by Sarfehnia, a linear extraping the values of PCo Ir olation of PCo values for a radial distance of 3 cm leads to a Ir . A linear extrapolated value of 0.9027 value of 0.9021 for PCo with an expanded calculated uncertainty of 0.2% (k = 1) was kept in this work to calculate the absorbed dose to water with the BrachyPhantom. 3.D. Uncertainty analysis
Table I shows the uncertainty analysis for the absorbed dose determination with the A1SL Exradin ionization chamber measurements performed with the BrachyPhantom. The uncertainties in the calibration factors (for the A1SL ionization chamber and electrometer) and in the thermometer and TABLE I. Uncertainty evaluation for the absorbed dose to water assessed with the A1SL Exradin ionization and the BrachyPhantom. Uncertainty Components
Type A
Type B
60
ND,wCo (A1SL calibration factor) Electrometer (calibration factor) Charge measurement Temperature and pressure Leakage Radial distance Anisotropy lw (phantom material and size correction Psw factor) Ir (MC factor to convert N Co to PCo D,w ionization chamber absorbed dose-based calibration factor for 192 Ir) Ir (difference between MC SSD and PCo BrachyPhantom SSD) Expanded uncertainty (k = 1)
1.4 0.1 0.25 0.2 0.01 1.53 0.01 0.05 0.22
0.2 2.12%
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barometer were based on the calibration certificates provided by national laboratories. The uncertainty in the charge measurement was based on the standard deviation measured during short periods. The uncertainty in the leakage was based on the mean values of the current leakage current measured during the clinical HDRs dose assessment. The uncertainties in the source–chamber distances (radial distance and source anisotropy) were based in the assumption that there is a gap of 0.006 cm between the radioactive source and the catheter hole and another 0.005-cm gap between the catheter and the BrachyPhantom’s channel. The uncertainties in the phantom dimensions were based on the data provided by Tecgraph, Brazil,22 where the BrachyPhantom was designed and manIr were based on the MC ufactured. The uncertainties in PCo calculation uncertainties data published by Sarfehnia et al.15 for source–detector distances of 3.49 and 5.15 cm. The uncertainty in P lw sw (phantom material and size) was taken from the MC calculation performed in this work. The uncertainties in the absorbed dose, calculated by the treatment plan system and with the TG-51 formalism, which depend on the treatment plan system and the value of the source strength provided by the users, respectively, were not among the objectives of this work. However, a brief comment about the uncertainty in the assessment of the absorbed dose calculated by the treatment plan caused by the catheter reconstruction process is included in Sec. 3.E. 3.E. Image artifacts effect
The effect of the position of the BrachyPhantom in relation to the CT rotation axis is shown in Figs. 4(a) and 4(b) (rotated to the 4A-plane by the Oncentra treatment planning system). In the first, the BrachyPhantom’s axis is vertical. In the second, the axis is horizontal and rotated in 45◦ in relation to the CT rotation axis, which resulted in a much better image. A better image may minimize the error in the treatment plan process, since the digitized points in the catheter reconstruction process may depend on the quality of the CT image. The
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placement of the catheter in an incorrect location will lead to computer errors during the treatment planning preparation process and a deviation in the dose delivered to the ionization chamber positioned in the BrachyPhantom. This is illustrated in Fig. 4(b) by the points represented by the letters “a ” and “b .” The point “a ” looks a little bit away from the ionization chamber’s reference measuring point, whereas the point “b ” is somehow closer to it. This is an effect of the digitalization procedure which may be exacerbated if the image has a poor quality. 3.F. Intercomparison of absorbed dose
Table II shows the results of absorbed dose to water from 15 clinical HDRs brachytherapy equipment calculated by the Ocentra Master Treatment Planning system (TP, fourth column), assessed with the BrachyPhantom (BP, fifth column), and based on the source strength values provided by 15 institutions visited in this work (mGy m2 h−1 , third column and Sk , sixth column). The last three columns in the table show the relative percent deviation between absorbed doses determined with the BrachyPhantom and treatment plan, source strength and treatment plan, and BrachyPhantom and source strength. Except for the measurements performed with the 192 Ir channel times of 199.3 and 166.4 s, the relative percent deviation between the absorbed dose assessed with the BrachyPhantom and the treatment plan (BP/TP, seventh column) lies within ±1.35%. The relative percent deviation for 199.3 s (institution # 7) and 166.4 s (institution # 15) are 2.91% and −2.53%, respectively. A comparison between the relative percent deviation of absorbed doses calculated using the source strength values and those calculated by the treatment plan for these channel times (Sk /TP, eighth column) has shown that such deviations follow those measured with the BrachyPhantom and that the deviation does not exceed ±1.7% (institution #7). It means that the radioactive source channel time as seen by the A1SL ion chamber confirm those given in 7th column which only depends on the values of the source strength stated in
F IG . 4. CT imaging of the BrachyPhantom positioned on the CT couch (a) with its central axis vertical; (b) with its central axis horizontal and rotated in 45◦ in relation to the CT rotation axis. Medical Physics, Vol. 40, No. 11, November 2013
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TABLE II. Comparison of absorbed doses to water assessed with the BrachyPhantom, calculated by the HDR brachytherapy treatment planning systems and the source strength values. Absorbed dose to water (cGy) Institution 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Relative deviation (%)
Channel time (s)
Air-kerma rate (mGy m2 h−1 )
Treatment plan
BrachyPhantom
Source strength (Sk )
BP/TP
Sk /TP
BP/Sk
203.20 168.00 168.00 122.40 168.00 141.60 199.30 122.40 154.45 139.20 178.20 131.30 184.00 216.00 166.40
21.36 25.86 26.04 35.71 25.86 30.80 22.38 35.71 28.76 31.26 24.26 32.60 23.76 20.31 26.06
150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 150.00 Mean % Deviation
148.90 151.10 148.70 148.90 150.00 149.10 154.50 149.30 150.45 149.60 149.30 148.40 148.00 150.20 146.30 149.52 1.09
148.57 148.72 149.75 149.64 148.72 149.29 152.67 149.64 152.07 148.97 147.97 146.52 149.68 150.21 148.45 149.39 0.99
− 0.74 0.73 − 0.87 − 0.74 0.00 − 0.60 2.91 − 0.47 0.30 − 0.27 − 0.47 − 1.08 − 1.35 0.13 − 2.53 − 0.34 0.73
− 0.96 − 0.84 − 0.17 − 0.24 − 0.85 − 0.47 1.73 − 0.24 1.37 − 0.69 − 1.36 − 2.34 − 0.22 0.14 − 1.06 − 0.41 0.66
0.22 1.57 − 0.70 − 0.50 0.85 − 0.13 1.19 − 0.23 − 1.07 0.42 0.89 1.26 − 1.13 0.00 − 1.47 0.08 0.73
the treatment plan report. Consequently, these relatively large deviations have been attributed to errors caused by the catheter reconstruction process.23 The dose rate constant, , in units (cGyh−1 U−1 ), calculated by Benhabib19 according to the recommendations of the TG-43 data for the Nucletron Model mHDR-v2 and using the PENELOPE code was found to be 1.108 (0.1%, k = 1) cGyh−1 U−1 . The value of this quantity calculated by Daskalov et al.24 is 1.108 (0.13%). Therefore, the modeling of the Nucletron Model mHDR-v2 written by Benhabibi has been used in this work to calculate P lw sw . To avoid any undesirable energy dependence effect, a sphere of water has been used in the MC simulations to calculate the P lw sw factor. High dose rate remote afterloading intracavitary brachytherapy requires an independent verification of a treatment plan system in the quality assurance program. Such verification may include the accurate determination of the absorbed dose at the prescription point, which depends on the source–detector distance, dwell time and catheter reconstruction. A radial distance of 3 cm was preferred for the radioactive source channel in the BrachyPhantom. This radial distance serves not only to check the catheter reconstruction when a single source position is assigned for the treatment plan but also to perform measurements in short periods. Any deviation of ±0.02 cm in the radial distance may lead instrument readings by ±3%. At higher radial distances, the above-mentioned deviation in the radial distance may be greatly minimized. However, the effect of the catheter reconstruction from the CT scanner transferred to HDR unit on dose at the prescription point is also greatly minimized and the measurements require long periods of time. Therefore, a 3-cm radial distance was chosen to turn possible the use of the BrachyPhantom for fast measurements Medical Physics, Vol. 40, No. 11, November 2013
and to estimate the error in the catheter’s reconstruction procedure when a single-dwell position plan is used to deliver the total dose. For an associated source strength of 27.8 cGy m2 h−1 (mean value of the source strengths used in this work), the total irradiation time to deliver 150 cGy to the ionization chamber point of reference was 161.0 s. Because the absorbed dose measured with the BrachyPhantom does not depend on the position of the source with respect to the detector and is based on the Ngas formalism, a fast and independent assessment of absorbed doses may be achieved with such a device. The model of the BrachyPhantom presented in this work was not designed to check source dwell position. However, a possible way to determine the dwell position may be achieved by redesigning the phantom to make it possible for the positioning of a radiograph film sandwiched between the top and the base of the phantom. Also, by replacing the ionization chamber with a dummy chamber made of tissue equivalent material filled with, e.g., powder TLD, such phantom may be used for postal measurement purposes. Such possibilities are being studied by the authors. The reason and extent to which by tilting the BrachyPhantom of 45◦ scan reduced the image artifact and its effect on the HU numbers, respectively, does not make part of this study. 4. CONCLUSIONS A calibration BrachyPhantom was developed and tested for verification of dose calculation by HDR brachytherapy treatment planning. The BrachyPhantom does not require the determination of the hot spot position and therefore allows a fast and independent determination of the absorbed dose to water with an uncertainty of 2.12% (k = 1). Measurements performed in 15 clinical HDR 192 Ir radioactive sources have
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C. Austerlitz and C. A. T. Campos: A BrachyPhantom for HDR brachytherapy
shown that discrepancies between the absorbed doses to water assessed with the BrachyPhantom and those calculated by the treatment plans did not exceed ±2.91%. The maximum deviation between the absorbed doses measured with the BrachyPhantom and with the TG43 formalism, using the source strength values provided by 15 institutions visited in this work, was ±1.57%. Overall, the BrachyPhantom may be used for QA and/or verification of HDR planning system with, a priori, a threshold level of about 2% and 3% to spot problems, respectively. For its convenience and ease of use, it is a device that, in the long run, may save time for the medical physicist. ACKNOWLEDGMENTS The authors are very grateful for useful advices, suggestions, and support from Dr. C. Sibata, Director Physics Quality Control Section of the 21st Century Oncology, USA. The authors express their gratitude toward the physicists from the different Offices of the 21st Century Oncology visited in this work, A. Organista, A. Shamaoun, B. Pomij, C. Chipley, G. McNerney, H. Gotts, J. Miranda, J. Moralos, K. Noda, M. Soldano, P. Pomije, P. Shendero, R. Ducan, R. Gotts, R. Richardson, S. Chakrabon, S. Darwish, S. Olivera, S. Peter, W. Neeranjah, and X. Chen, for their help to perform the experimental part of this work. Moreover, the authors acknowledge Dr. S. Benhabibi from the East Carolina University for providing inputs in the MC calculation.
a) Author
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