Improvement of dose distribution in breast radiotherapy using a reversible transverse magnetic field Linac-MR unit A. D. Esmaeeli, S. R. Mahdavi, M. Pouladian, A. S. Monfared, and S. Bagheri Citation: Medical Physics 41, 011709 (2014); doi: 10.1118/1.4845175 View online: http://dx.doi.org/10.1118/1.4845175 View Table of Contents: http://scitation.aip.org/content/aapm/journal/medphys/41/1?ver=pdfcov Published by the American Association of Physicists in Medicine Articles you may be interested in Tracking the dynamic seroma cavity using fiducial markers in patients treated with accelerated partial breast irradiation using 3D conformal radiotherapy Med. Phys. 40, 021717 (2013); 10.1118/1.4788644 Absorbed doses behind bones with MR image-based dose calculations for radiotherapy treatment planning Med. Phys. 40, 011701 (2013); 10.1118/1.4769407 Monte Carlo characterization of skin doses in 6 MV transverse field MRI-linac systems: Effect of field size, surface orientation, magnetic field strength, and exit bolus Med. Phys. 37, 5208 (2010); 10.1118/1.3488980 Lung dosimetry in a linac-MRI radiotherapy unit with a longitudinal magnetic field Med. Phys. 37, 4722 (2010); 10.1118/1.3475942 Investigation of the effects of treatment planning variables in small animal radiotherapy dose distributions Med. Phys. 37, 590 (2010); 10.1118/1.3276738
Improvement of dose distribution in breast radiotherapy using a reversible transverse magnetic field Linac-MR unit A. D. Esmaeelia) Department of Physics, Rasht Branch, Islamic Azad University, Rasht, Iran 41476-54919
S. R. Mahdavi Department of Medical Physics, Tehran University of Medical Sciences, Tehran, Iran 14174
M. Pouladian Department of Medical Radiation Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran 14778-93855
A. S. Monfared Department of Medical Physics, Babol University of Medical Sciences, Babol, Iran 47148-71167
S. Bagheri Department of Medical Radiation Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran 14778-93855
(Received 23 October 2013; revised 22 November 2013; accepted for publication 26 November 2013; published 19 December 2013) Purpose: To investigate the improvement in dose distribution in tangential breast radiotherapy using a reversible transverse magnetic field that maintains the same direction of Lorentz force between two fields. The investigation has a potential application in future Linac-MR units. Methods: Computed tomography images of four patients and magnetic fields of 0.25–1.5 Tesla (T) were used for Monte Carlo simulation. Two patients had intact breast while the other two had mastectomy. Simulations of planning and chest wall irradiation were similar to the actual clinical process. The direction of superior-inferior magnetic field for the medial treatment beam was reversed for the lateral beam. Results: For the ipsilateral lung and heart mean doses were reduced by a mean (range) of 45.8% (27.6%–58.6%) and 26.0% (20.2%–38.9%), respectively, depending on various treatment plan setups. The mean V20 for ipsilateral lung was reduced by 55.0% (43.6%–77.3%). In addition acceptable results were shown after simulation of 0.25 T magnetic field demonstrated in dose-volume reductions of the heart, ipsilateral lung, and noninvolved skin. Conclusions: Applying a reversible magnetic field during breast radiotherapy, not only reduces the dose to the lung and heart but also produces a sharp drop dose volume histogram for planning target volume, because of bending of the path of secondary charged particles toward the chest wall by the Lorentz force. The simulations have shown that use of the magnetic field at 1.5 T is not feasible for clinical applications due to the increase of ipsilateral chest wall skin dose in comparison to the conventional planning while 0.25 T is suitable for all patients due to dose reduction to the chest wall skin. © 2014 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4845175] Key words: electron return effect, magnetic field, breast radiotherapy, GEANT4 1. INTRODUCTION Optimization of therapeutic benefit depends on maximizing the dose to the planning target volume (PTV) while minimizing the dose to normal tissues. The techniques available for modern radiotherapy enable the dose distribution to be tailored to the three-dimensional (3D) geometry of a tumor and normal tissues.1 However, while adequate tumor dose coverage can be generally achieved, the potential of these techniques is limited by normal tissue tolerance. Breast is the most common site of cancer in women and, with the widespread use of mammography, more than twothirds of breast cancers are diagnosed at an early stage.2, 3 For the patients diagnosed with breast cancer, it is essential to decrease the probability of radiation induced complications. 011709-1
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The main types of chest wall radiotherapy complications are cardiomyopathy, pneumonitis, and secondary cancers.4–6 Although their occurrence is also influenced by lifestyle and/or a predisposing genetic condition,7, 8 it is primarily related to the amount of dose deposited in specific organs.8, 9 So, the most efficient way to prevent from sequelae of treatment is to reduce the amount of dose scattered to internal organs; for example, a radiation technique such as breast IMRT, 3D-CRT partial breast radiotherapy and HDR brachytherapy can be chosen or a platform-based breast shield can be used to minimize the exposure of internal organs.7, 9–13 The effect of an external magnetic field on electron beam dose distributions was first reported more than 60 years ago.14 Monte Carlo simulations15–17 and experiments18 demonstrated that application of a strong magnetic field can provide
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a substantial improvement and control of the dose profile of clinical electron and photon radiotherapy beams. Several institutes and researchers have proposed designs to develop a radiotherapy treatment system with online MR imaging (MRI) modality because online imaging has become a useful tool for margin reduction and, thereby, treatment improvement.19–28 The various proposed designs differ in magnetic field strength and geometry. The UMC Utrecht prototyped a system in which a 6 MV linear accelerator was mounted on a 1.5 T MRI scanner.27, 29, 30 This system became feasible by adapting the Linac and the MRI system to avoid mutual disturbance by the magnetic fields. Also, the Cross Cancer Institute in Edmonton prototyped a system in which the accelerator was mounted on a 0.2 T MRI scanner.31 The effect of magnetic field on the dose distribution in lung radiotherapy has been recently studied by Kirkby et al.22, 24 A similar work for a breast case was performed by Esmaeeli et al.32 The aim of this work is to simulate the effect of magnetic field on the dose distribution in breast radiotherapy in comparison to the conventional radiotherapy process. The emphasis of this work is on the radiobiological preference of the magnetic field application and its direction to be applied on the treatment region. The directions of the magnetic field were in the cranial-caudal and vice versa for medial and lateral tangential beams, respectively. The GEANT4 Monte Carlo (MC) code and DICOM computed tomography (CT) image data files were used for simulation of the treatment planning process.
2. METHODS 2.A. Monte Carlo simulations in the presence of a magnetic field
For all simulations, the MC toolkit GEANT4 (version 9.3) has been used to characterize the dose distribution in a reconstructed patient-derived phantom in the presence of a magnetic field. As discussed in previous work,32 GEANT4 is a freely available code and open source program, created by the GEANT4 Collaboration.33 All regular physical processes for medical applications were actuated from the Low EPhysics Processes package. The most common variance reduction method is energy cutoff. The MC simulation stops transportation of the particle if its energy falls below the cutoff energy and it was set to 0.4 mm for all particles. This means that secondary particles are generated only if their energy is high enough to travel at least 0.4 mm through the medium in which they are generated. Differential equation solver (classical Runge–Kutta method33 ) was used for calculation of the trajectory of charged particles in a magnetic field. This code was shown to provide accurate results for gamma ray dose distributions in the presence of a magnetic field.34 In simulations, statistical uncertainty in the dose distribution was less than 1% (1 SD) for all voxels with the dose of more than 1% of maximum dose. The mathematical phantoms were reconstructed from the DICOM images of the patients by means of GEANT4 DIMedical Physics, Vol. 41, No. 1, January 2014
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COM toolkit. The human body tissues such as lung, liver, chest wall, and bones are modeled according to the International Commission on Radiation Units and measurements (ICRU) report 46. In our previous work,35 the head of a clinical linear accelerator (Siemens Primus) was simulated based on the manufacturer’s detailed information by use of GEANT4 MC code, and benchmarked with real clinical dosimetry in terms of percentage depth dose and beam profiles for various field sizes and depths, for a 6-MV photon beam. Two opposed tangential fields were used for the treatment of breast cases. For each beam, a phase space was generated below the wedge in absence of any magnetic field. The assumption implicit in this model was that the main magnetic field was negligible to this point due to the magnetic shielding, implicit in the Linac-MR design. The contaminant electrons were included in the present phase-space data. Each phase space was then used as a source in the GEANT4 platform. The contaminant electrons travel a large distance from the bottom of wedge tray to the skin surface in the presence of transverse magnetic field. Therefore, the contaminant electrons are likely removed from the beam before it is incident on the patient. 2.B. Technique of the applying the magnetic field
The presence of external magnetic field modifies the dose distribution of photon beams by altering the trajectories of secondary electrons due to the Lorentz force. The radius of curvature of the electrons depends on the relative density difference at tissue interface (air-soft tissue or bone-soft tissue), the electron energy, and the magnetic field strength.24, 36–38 In order to study how dose to the lung and heart could be minimized, the direction of the magnetic field was changed for two beams, i.e., the magnetic field was applied in the positive z-direction for the medial beam and in the negative zdirection for the lateral. It takes away scattering particles out of the lung centre due to the net effect of the Lorentz force (see Fig. 1). Henceforth, we will refer to this as the bidirectional magnetic field technique. To apply the bidirectional technique, the feet-heat direction of the patient must be reversed to change the effective direction of magnetic field in practice. The magnetic field starts below the physical wedge. 2.C. Phantoms and simulation setups
2.C.1. The anatomy of patients
Similar patients and treatment plan setups were used as discussed for breast cases in our previous publication.32 A total of four breast cancer patients were selected in this study: two for intact breast irradiation and other two for chest wall irradiation. CT data slices of each patient were used in the MC code as input data. Dimensions of each voxel in the reconstructed image of patients in GEANT4 were 0.8 × 0.8 × 3 mm3 . All voxels were placed as parameterized volumes on a background material of air. One slice of the reconstructed phantom is shown in Fig. 2(b).
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F IG . 1. Schematic representations of a fixed cylindrical geometry; (a) for the medial beam and (b) for the lateral beam, showing the relative positioning of the patient, Linac, magnets, beam direction, magnetic field direction, and the net Lorentz force. In the fixed cylindrical geometry, the B0 field remains fixed with respect to the patient. Between the two irradiations, the feet-head direction of the patient is changed.
In order to characterize the anatomy of patients, parameters of four different thorax shapes were defined, as illustrated in Fig. 2(a). These are as follows, A: the orthogonal distance in the center line of the chest, B: the tangential distance, the line between the middle of the patient (center line) along the most dorsal part of the clinical target volume (CTV) to the body contour, C and D: orthogonal from the tangential distance, the maximum breast and lung distance, respectively.
2.C.2. Tretment planning
For the simulations, the plannings of chest wall irradiation were similar to the actual clinical planning. The CorePlan 3.5.0.5 treatment planning system (C&J Inc, Seoul, Korea) was used for 3D-CRT planning. The external surface of the patient and lung contours were defined by automated density gradient tracking; then, it was edited and verified by physicians. The chest wall CTV was
F IG . 2. (a) Target anatomy and structure, OARs, medial and lateral beams, treatment plan beam setups (nonmastectomy case), and four thorax shape parameters (A = central distance, B = tangential distance, C = breast orthogonal distance, and D = lung orthogonal distance) of patient # 1. The ICRU reference point is indicated by the green circle. (b) CT number map of the corresponding patient reconstructed image in the GEANT4 MC code. Medical Physics, Vol. 41, No. 1, January 2014
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TABLE I. Thorax shape parameters of patients. Parametersa
Patient # 1
Patient # 2
Patient # 3
Patient # 4
LODb (cm) CDc (cm) TDd (cm) BODe (cm)
1.4 25.4 25.4 6.3
0.7 23.3 21.6 3.6
2.8 21.5 23.2 4.0
0.6 20.8 19.5 6.8
a
Thorax shape parameters of patients shown in Fig. 2(a). Lung orthogonal distance. c Central distance. d Tangential distance. e Breast orthogonal distance. b
generally delineated on the corresponding transverse CT images limited by the external skin surface anteriorly, the ribsoft tissue interface posteriorly, the inferior aspect of the clavicular head superiorly and 1-cm below the inframammary fold inferiorly. Medial and lateral borders of the CTV were delineated considering lateral border of the sternum and the mid-axillary line, respectively. The target delineation for the intact breast was not similar to the chest wall. The PTV was defined by adding 5 mm to the CTV. The contour of the PTV was outlined with a depth of 1.6 mm to the skin surface anteriorly to evaluate DVHs. The common setups for all patients are as follows: Source-to-axis distance 100 cm, identical beam weights, 6 MV primary energy of the photons with two opposed tangent fields (symmetrical fields) and isocenter setup type as shown in Fig. 2(a). The isocenter was situated inside the target volume. The nominal prescribed dose was 50 Gy in 25 fractions using 6MV photons. The treatment plan setups and the target (PTV) and OARs together with wedges and treatment beams were schematically shown in Fig. 2(a). Thorax shape parameters of patients were tabulated in Table I. For chest wall tangential radiotherapy, bolus is often used during the treatment course to ensure adequate dose to the skin. At 1.5 T, the bolus was used in order to shift the hot spot areas out of the chest wall skin. Water-equivalent material is the choice for the bolus material in the simulations. Thickness
(a)
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of the bolus must be large enough to shift the hot spot regions that were selected 0.5 cm for patients # 1 and 4, and 1 cm for patients # 2 and 3.
2.C.3. Dose distribution in the lung
To investigate the effect of the magnetic field and electron return effect (ERE) on the dose distribution at the breast-lung boundaries, the delivered dose was calculated as a function of distance from the breast-lung boundaries toward the lung (Fig. 3). The path from breast-lung boundaries perpendicular to the beam direction was divided into rows parallel to the beam direction. The breast-lung boundary has an arc shape, and the selected first row close to the boundary in the lung is accepted for the calculation, if the number of its voxels compared to longest row exceeds 90%. Therefore, voxels near the breast-lung boundaries where their numbers are few were excluded. The dose of all voxels in each row that had the same densities as lung was summed and averaged. The distance between rows was 1.1 mm and the selected slice had the maximum lung to chest wall distance. In the selected slice that was assigned a maximum lung to chest wall distance the arc of the interface introduces an uncertainty of less than 5% both in the dose distribution and lung orthogonal distance (LOD) in each row in the patients.
2.C.4. Dose calculation
In the simulations, each beam was calculated separately and the results were combined, and normalized to 100% at the ICRU report 50 reference point for multiple beam plans. Absolute dose could be obtained by multiplying all voxel doses by the dose-to-isocenter/100% of the zero field plan. When difference maps are presented in this paper, in fact, the difference of the dose in each voxel at B field from zero field was divided to the zero field (the same method was used by Kirkby et al.24 ). In order to compare the effects of different strengths of the applied magnetic field on the dose distribution,
(b)
F IG . 3. (a) A schematic representation for calculating the electron return effect in the left lung. The medium from breast-lung boundaries perpendicular to the beam direction is divided into rows parallel to the beam direction. Distance between rows is 1.1 mm and the selected slice has the maximum lung orthogonal distance. Due to the arc shape of the chest wall, the first row is selected in distance from breast-lung boundaries that have enough voxels compared to deep distances. (b) The calculated dose versus distance from breast-lung boundaries toward the lung at 0, 0.25, and 1.5 T for one slice of each patient is indicated. Medical Physics, Vol. 41, No. 1, January 2014
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F IG . 4. Energy deposition in the middle slice of the central plane of patients is indicated for (a)–(c) patient # 1, (d)–(f) patient # 2, (g)–(i) patient # 3, and (j)–(l) patient # 4. Note that the bolus was not used in the plans.
the dose-volume parameters, DVHs, and dose-area histograms (DAHs) were calculated for the PTV and OARs.
3. RESULTS AND DISCUSSION 3.A. Effect of the magnetic field on dose distribution at the breast-lung boundaries
Dose increase was expected at the breast-air and breastlung boundaries due to the ERE:34 electrons entering the lung Medical Physics, Vol. 41, No. 1, January 2014
and air described a circular path and returned to the breast, causing extra dose deposition to the breast and dose reduction to the lung. It is important to note that low density tissues or air at the exit surfaces were shown to lead to regions of dose enhancement in the presence of a conventional magnetic field due to electrons returning to more dense tissues.22, 24, 36, 37, 39 To evaluate variations of dose distribution at the breastlung boundaries, delivered dose versus distance from breastlung boundaries toward the centre of the lung, perpendicular to the radiation direction, at 0, 0.25, and 1.5 T are plotted in
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Fig. 3. As a general trend, the similar dose distribution was observed for all patients, except patient # 3, due to larger LOD (see Table I), which caused more lung volumes to be in the radiation field. In regions near the breast-lung boundaries, dose reduction is larger for 1.5 T compared to 0.25 T. Low-energy electrons are not able to penetrate far into the lung, because they are confined to their region of origin by the Lorentz force. This effect becomes stronger for higher magnetic field strengths.38 Dose reductions in lung due to use of the magnetic field were different in various patients with respect to the no magnetic field case. They were reduced by a mean (range) of 17.0% (9%–22%) and 59.7% (47%–67%) at 0.25 and 1.5 T, respectively. The effect of strong magnetic field was reduced in deep regions of the lung, and the rate of dose reduction decreased in these areas with respect to the breast-lung boundaries. This is explained by the fact that low-energy charged particles that originated far from the breast-lung boundaries in the lung had a small gyration radius and could not be taken into the breast; furthermore, there were numerous scattered photons, on which the magnetic field did not have any influence. 3.B. Effect of the magnetic field on the dose distribution in breast radiotherapy
To illustrate the differences in relative dose distribution, results of simulations were presented for different magnetic fields with respect to no magnetic field in the middle slice of the central plane for patients in Fig. 4. Figure 5 depicts the corresponding difference maps of Fig. 4 from the zero magnetic field case. The bolus was not used in Figs. 4 and 5. The difference maps were calculated to show the change in dose due to the presence of the magnetic field. At the breast-lung and breast-air interfaces, the ERE increased breast and chest wall skin doses. Because of the higher density of lung relative to the air, returning electrons from the lung lost most of their energy in the path into the breast. Therefore, the energy of returning electrons from the air toward the chest wall was more than those returning from the lung, which produced a marginal zone of high-dose distribution at the breast-air boundaries for 1.5 T. The spread of the dose distribution with and without magnetic fields was due to the different anatomy of patients and treatment plan setups. At B0 = 0.25 T, a hot spot was seen in the breast which was indicated by a dashed black arrow in Fig. 5 and cold spot in the lung was indicated by a solid white arrow. The low-density tissue and air within the lungs allowed the magnetic field to strongly influence the electron trajectories, even at this low field strength. It should be noted that, there were regions of both increased dose and decreased dose to the chest wall skin as the result of the net effect of the Lorentz force acting on the electrons which shifted the dose to the outside of a patient’s body. There appears to be a reduction in skin dose (indicated by dotted white arrow) on the order of 20%–25% at the entrance of the medial tangent field and a collection of increased dose (indicated by dotted and dashed black arrows) on the order of 10%–15%. Therefore, due to Medical Physics, Vol. 41, No. 1, January 2014
F IG . 5. Relative dose differences in the middle slice of the central plane at 0.25 and 1.5 T from the zero magnetic field case for (a) and (b) Patient # 1, (c) and (d) patient # 2, (e) and (f) patient # 3, and (g) and (h) patient # 4. Hot and cold spots in the chest wall and lung were indicated by black and white arrows, respectively. Solid white double-arrows show a shift of dose distribution distinctly to the outside of patient center although by about 1 cm. Note that the bolus was not used in the plans.
the reduction of contaminant electrons near the skin, the mean dose of chest wall skin was reduced by 7.7%, 11.4%, 3.4%, and 6.2% for patients # 1, 2, 3, and 4, respectively, compared to 0 T. At B0 = 1.5 T, the exiting electron trajectories traveled through tighter arcs and resulted in chest wall skin and breast hot spots (solid black arrow) that extended several millimeters into the breast and air. Cold spot (solid white arrow) could be seen in the lung. From Fig. 5 it becomes clear that there was high dose delivered in the air inside the main beam due to small gyration radius of the charged particles. Therefore,
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F IG . 6. Breast treatment plan of dose–volume (PTV, left lung and heart) and dose-area (chest wall skin) histograms without bolus comparing B = 0, 0.25, and 1.5 T. (a) and (b) correspond to patient # 1, (c) and (d) to patient # 2, (e) and (f) to patient # 3, and (g) and (h) to patient # 4. Those volumes in the field are considered for chest wall skin DAHs.
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F IG . 7. Breast treatment plan of dose–volume (PTV) and dose-area (chest wall skin) histograms comparing bolus and nonbolus utilized at 1.5 T in the chest wall skin and breast tissues. (a), (b), (c), and (d) are for patients # 1, 2, 3, and 4, respectively. Note that those volumes in the field are considered for chest wall skin DAHs.
the dose to the internal organs such as the heart is reduced. Furthermore, the strong magnetic field shifted the dose distribution distinctly to the outside of patient center (solid white double-arrows), even by about 1 cm, again because of the net effect of the Lorentz force. Figure 6 illustrates the effect of the magnetic field in four patients at cumulative dose-volume and dose-area histograms (DVHs and DAHs) for the PTV and each organ at risk, looking specifically at magnetic field strengths of 0, 0.25, and 1.50 T. As a general trend, at 1.5 T, the PTV DVH was shifted to the right since ERE increases the dose inside the breast near breast-air interface. This resulted in few volume elements within the PTV receiving higher doses. In contrast, at 0.25 T, there is generally a slight decrease in the steepness of the curve, indicating the effect of cold spot at the entrance of the medial tangential field and some regions in the volume are also hotter due to ERE at the breast-lung and breast-air boundaries (Fig. 5). The left lung showed noticeable dose reduction in the DVH across the investigated range of magnetic field strengths. According to Figs. 4 and 5, there were obvious changes in the dose distribution within the lung. The Lorentz force affects charged particles originating in the lung volume located directly in the beam path, and returns and/or deflects them towards the breast-lung boundaries. The magnetic field was shown to restrict the lateral spread of secondary electrons to the internal organs, resulting in dose reductions in those tissues [Fig. 3(b)]. In the heart DVHs, at 0.25 T, acceptable dose reduction can be seen since the Lorentz force consistently Medical Physics, Vol. 41, No. 1, January 2014
shifts charged particles from the lateral areas toward the beam edge for all patients. The same phenomena were observed at 1.5 T, increasing the field strength to 1.5 T, the mean free path length of charged particles was more decreased, therefore, the lateral shift of dose distribution is further than of 0.25 T. At 1.5 T, there was strong ERE at the breast-air boundaries; therefore, the dose increased in the breast and chest wall skin. In contrast, at 0.25 T, curves showed acceptable decrease of dose-volume in high-dose areas. The results showed that, even though dose reductions in the heart and ipsilateral lung were considerable at 1.5 T, the dose mainly increased inside the chest wall skin and PTV. This configuration of treatment plan is not suitable for clinical applications. Therefore, to reduce the dose to these areas, these high-dose volumes must be shifted to the outside of the chest wall skin. To achieve this goal, a build-up bolus was used. 3.C. Using strong magnetic field along with a bolus in breast radiotherapy
Tissue-equivalent material boluses, which are thick enough to provide an adequate dose build-up in the skin and superficial chest wall, are commonly used during postmastectomy radiotherapy. Skin dose contributions of boluses and the dose delivered to the skin and subcutaneous tissue are important, especially in locally advanced breast cancer.40 The purpose is to shift the high-dose area of the chest wall skin and PTV toward the bolus in order to construct a uniform dose in the breast and chest wall skin tissues. The DAHs and DVHs
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TABLE II. Variations of the relative dose-volumetric parameters in the heart, ipsilateral lung and PTV along with the use of bolus in breast radiotherapy at 1.5 T with respect to no magnetic field case. Note that the relative mean dose and V20 were calculated without bolus at 0.25 T. Patient # 1
Patient # 2
Patient # 3
Patient # 4
Organs
0.25 (T)
1.50 (T)
0.25 (T)
1.50 (T)
0.25 (T)
1.50 (T)
0.25 (T)
1.50 (T)
ILa (mean dose) V20 Heart (mean dose) PTV (mean dose)
− 17.4% − 11.6% − 8.1% +0.1%
− 27.6% − 46.0% − 38.9% − 4.2%
− 11.5% − 35.4% − 10.6% − 6.5%
− 58.6% − 53.1% − 20.2% − 5.6%
− 24.9% − 22.5% − 13.5% − 6.9%
− 43.4% − 43.6% − 24.1% − 3.8%
− 26.5% − 27.6% − 3.3% − 9.4%
− 53.8% − 77.3% − 20.8% − 4.9%
a
Ipsilateral lung.
of the chest wall skin and breast with and without bolus for all patients at B = 1.5 T are shown in Fig. 7. Using bolus, uniform dose distributions and sharp drop-off curves were created in the chest wall skin41 and breast histograms. Thus, by using the strong magnetic field, for all patients, with bolus relative to those without bolus, an unwanted condition, i.e., increase of the dose to the chest wall skin occurred. Therefore, this technique must only be used for patients whose chest surface along with the PTV are the target, such as most mastectomy patients. Radiation pneumonitis (RP) is a common toxicity caused by radiation exposure to the lung, and the incidence of RP is known to be correlated with the volume of the irradiated lung and the radiation dose. The mean lung dose and V20 are generally related to RP.42 Therefore, in order to compare the effects of different strengths of the applied magnetic field on the dose distribution, the mean dose and V20 were calculated (Table II). The dose reduction in Table II was calculated using the bolus for 1.5 T case and without bolus for 0.25 T case. At 1.5 T, the mean dose of the ipsilateral lung and heart was reduced by a mean (range) of 45.8% (27.6%–58.6%) and 26.0% (20.2%–38.9%), respectively, depending on different patients and various treatment plan setups. Considering the V20 as an another dose-volume constraints for the ipsilateral lung, its value was reduced by a mean (range) of 20.7% (11.6%–35.4%) at 0.25 T and 55.0% (43.6%–77.3%) at 1.5 T. The amount of V20 and mean dose reductions is different across the range of patients and magnetic field strengths investigated. With respect to the patient specifications listed in Table I, it seems that variations of the dose reduction in various patients presumably depend on LOD, that for patients # 2 and 4 with small LODs, one can see higher dose reductions than other patients with larger ones (Table II). Several reports have indicated that breast radiotherapy increases the risk of cardiovascular disease and long-term mortality of patients with cancer involving the left breast.4, 43 Regarding cardiac risk, a critical review published by SchultzHector, suggested that the acute single dose of 1–2 Gy to the heart significantly increased the risk of developing ischemic heart disease,44 and the excess relative risk could be linearly fitted with the slope of 17% per Gy. Due to mean dose reduction of the heart to 1.3, 0.6, 1.6, and 0.3 Gy in patients # 1, 2, 3, and 4, respectively, a decrease of excess relative risk is to be expected in all patients at 1.5 T. Medical Physics, Vol. 41, No. 1, January 2014
The objective of this work was not to calculate the risks of radiation-induced pneumonitis and cardiac complications. However, using a magnetic field in breast radiotherapy for patients with different anatomies, with respect to relative mean dose and V20 reductions in the heart and left lung tabulated in Table II, one can clearly expect the reduction of these complications. Simulation suggests a feet first position for the patient to improve dose distribution from lateral tangent field. It means that the patient should be repositioned from medial irradiation field to lateral one during radiotherapy. However, reversing of the magnetic field orientation between medial and lateral fields might be another option. But at least theoretically, it seems impractical changing the B0 field direction during clinical practice due to the required time to ramp down and ramp it back up to the high magnetic fields. It will remain for future works to finding out the optimized method of choice for improving dose distribution using bidirectional magnetic field induction proposed in the present study. 4. CONCLUSIONS Magnetic field strengths of 0.25 and 1.50 T were applied in the z-direction in the thorax area in which the breast and lung underwent photon irradiation. Simulations have shown that applying a magnetic field during breast radiotherapy reduces the dose to the lung and heart, which are OARs during breast radiotherapy, because of bending of the path of secondary charged particles toward the chest wall by the Lorentz force. The results showed that the presence of a 0.25 T magnetic field made an acceptable dose reduction to the lung, heart, and chest wall skin, which is suitable for patients whose chest wall skins are not the target volume. At 1.5 T, despite the dose reduction to the lung and heart, due to the ERE, dose distributions in the chest wall skin and PTV became worse which was undesirable in the treatment plan. The simulations established that, using a strong magnetic field (1.5 T) along with a bolus produces a uniform dose distribution in the PTV and chest wall skin with sharper edged DVH curves. The chest wall skin dose increased in all patients at 1.5 T. Therefore, for patients whose chest wall skin along with the PTV are considered the target, the use of a strong magnetic field along with the bolus is the best choice.
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In the configuration of the magnetic field-Linac setups, it seems that the practical nature of changing the B0 field direction for the lateral and medial beams in breast radiotherapy is problematic during clinical practice. It is suggested to introduce a treatment plan in which to change the positioning of patient from day to day instead of changing the B0 field direction. ACKNOWLEDGMENTS The authors are thankful from physicists of radiotherapy departments in Sh. Rajaee and Pars Hospital for their support to proper use of data and planning outputs and special thanks to Mr. Rezaii and Rezazadeh for their kindly cooperations. a) Author
to whom correspondence should be addressed. Electronic mail: [email protected] 1 S. Webb, “Conformal and intensity-modulated radiotherapy,” Handb. Radiother. Phys. 43, 943–974 (2007). 2 E. B. Elkin, C. Hudis, C. B. Begg, and D. Schrag, “The effect of changes in tumour size on breast carcinoma survival in the U.S.:1975-1999,” Cancer 104, 1149–1157 (2005). 3 W. F. Anderson, I. Jatoi, and S. S. Devesa, “Assessing the impact of screening mammography: Breast cancer incidence and mortality rates in Connecticut (1943–2002),” Breast Cancer Res. Treat. 99, 333–340 (2006). 4 S. C. Darby, P. McGale, C. W. Taylor, and R. Peto, “Long-term mortality from heart disease and lung cancer after radiotherapy for early breast cancer: Prospective cohort study of about 300,000 women in US SEER cancer registries,” Lancet Oncol. 6, 557–565 (2005). 5 D. C. Hodgson, E. S. Koh, T. H. Tran, M. Heydarian, R. Tsang, M. Pintilie, T. Xu, L. Huang, R. K. Sachs, and D. J. Brenner, “Individualized estimates of second cancer risks after contemporary radiation therapy for Hodgkin lymphoma,” Cancer 110, 2576–2586 (2007). 6 E. S. Koh, T. H. Tran, M. Heydarian, R. K. Sachs, R. W. Tsang, D. J. Brenner, M. Pintilie, T. Xu, J. Chung, N. Paul, and D. C. Hodgson, “A comparison of mantle versus involved-field radiotherapy for Hodgkin’s lymphoma: Reduction in normal tissue dose and second cancer risk,” Radiat. Oncol. 2, 13–24 (2007). 7 E. J. Hall, “Intensity-modulated radiation therapy, protons, and the risk of second cancers,” Int. J. Radiat. Oncol., Biol., Phys. 65, 1–7 (2006). 8 M. Tubiana, “Can we reduce the incidence of second primary malignancies occurring after radiotherapy? A critical review,” Radiother. Oncol. 91, 4– 15 (2009). 9 X. G. Xu, B. Bednarz, and H. Paganetti, “A review of dosimetry studies on external beam radiation treatment with respect to second cancer induction,” Phys. Med. Biol. 53, R193–R241 (2008). 10 J.-P. Pignol, B. M. Keller, and A. Ravi, “Doses to internal organs for various breast radiation techniques - Implications on the risk of secondary cancers and cardiomyopathy,” Radiother. Oncol. 6, 5–11 (2011). 11 J. W. Sohn, R. Macklis, J. H. Suh, and P. Kupelian, “A mobile shield to reduce scatter radiation to the CB during radiotherapy for breast cancer: Preclinical results,” Int. J. Radiat. Oncol., Biol., Phys. 43, 1037–1041 (1999). 12 R. Muller-Runkel and U. P. Kalokhe, “Method for reducing scatter radiation dose to the CB during tangential breast irradiation therapy,” Radiology 191, 853–855 (1994). 13 R. M. Macklis, R. L. Crownover, J. Crowe, T. WILoughby, and J. Sohn, “Reducing scatter radiation to the CB with a mobile, conformal shield during breast cancer radiotherapy,” Am. J. Clin. Oncol. 22, 419–425 (1999). 14 W. H. Bostick, “Possible techniques in direct electron beam tumour therapy,” Phys. Rev. 77, 564–565 (1950). 15 A. F. Bielajew, “The effect of strong longitudinal magnetic fields on dose deposition from electron and photon beams,” Med. Phys. 20, 1171–1179 (1993). 16 D. P. Whitmire, D. L. Bernard, and M. D. Peterson, “Magnetic modification of the electron-dose distribution in tissue and lung phantoms,” Med. Phys. 5, 409–417 (1978). 17 M. S. Weinhous, R. Nath, and R. J. Schultz, “Enhancement of electron beam dose distributions by longitudinal magnetic fields: Monte CarloSimulations and magnet system optimization,” Med. Phys. 12, 598–603 (1985). Medical Physics, Vol. 41, No. 1, January 2014
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J. E. Raaijmakers, B. W. Raaymakers, S. Van der Meer, and J. J. W. Lagendijk, “Integrating a MRI scanner with a 6 MV radiotherapy accelerator: Impact of the surface orientation on the entrance and exit dose due to the transverse magnetic field,” Phys. Med. Biol. 52, 929–939 (2007). 40 W. W. Thoms, Jr., M. D. McNeese, G. H. Fletcher, A. U. Buzdar, S. E. Singletary, and M. J. Oswald, “Multimodal treatment for inflammatory breast cancer,” Int. J. Radiat. Oncol., Biol., Phys. 17, 739–745 (1989). 41 F. Andic, Y. Ors, R. Davutoglu, S. B. Cifci, E. B. Ispir, and M. E. Erturk, “Evaluation of skin dose associated with different frequencies of bolus applications in post-mastectomy three-dimensional conformal radiotherapy,” J. Exp. Clin. Cancer Res. 28, 41–48 (2009).
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Improvement of dose distribution in breast radiotherapy using a reversible transverse magnetic field Linac-MR unit A. D. Esmaeelia) Department of Physics, Rasht Branch, Islamic Azad University, Rasht, Iran 41476-54919
S. R. Mahdavi Department of Medical Physics, Tehran University of Medical Sciences, Tehran, Iran 14174
M. Pouladian Department of Medical Radiation Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran 14778-93855
A. S. Monfared Department of Medical Physics, Babol University of Medical Sciences, Babol, Iran 47148-71167
S. Bagheri Department of Medical Radiation Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran 14778-93855
(Received 23 October 2013; revised 22 November 2013; accepted for publication 26 November 2013; published 19 December 2013) Purpose: To investigate the improvement in dose distribution in tangential breast radiotherapy using a reversible transverse magnetic field that maintains the same direction of Lorentz force between two fields. The investigation has a potential application in future Linac-MR units. Methods: Computed tomography images of four patients and magnetic fields of 0.25–1.5 Tesla (T) were used for Monte Carlo simulation. Two patients had intact breast while the other two had mastectomy. Simulations of planning and chest wall irradiation were similar to the actual clinical process. The direction of superior-inferior magnetic field for the medial treatment beam was reversed for the lateral beam. Results: For the ipsilateral lung and heart mean doses were reduced by a mean (range) of 45.8% (27.6%–58.6%) and 26.0% (20.2%–38.9%), respectively, depending on various treatment plan setups. The mean V20 for ipsilateral lung was reduced by 55.0% (43.6%–77.3%). In addition acceptable results were shown after simulation of 0.25 T magnetic field demonstrated in dose-volume reductions of the heart, ipsilateral lung, and noninvolved skin. Conclusions: Applying a reversible magnetic field during breast radiotherapy, not only reduces the dose to the lung and heart but also produces a sharp drop dose volume histogram for planning target volume, because of bending of the path of secondary charged particles toward the chest wall by the Lorentz force. The simulations have shown that use of the magnetic field at 1.5 T is not feasible for clinical applications due to the increase of ipsilateral chest wall skin dose in comparison to the conventional planning while 0.25 T is suitable for all patients due to dose reduction to the chest wall skin. © 2014 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.4845175] Key words: electron return effect, magnetic field, breast radiotherapy, GEANT4 1. INTRODUCTION Optimization of therapeutic benefit depends on maximizing the dose to the planning target volume (PTV) while minimizing the dose to normal tissues. The techniques available for modern radiotherapy enable the dose distribution to be tailored to the three-dimensional (3D) geometry of a tumor and normal tissues.1 However, while adequate tumor dose coverage can be generally achieved, the potential of these techniques is limited by normal tissue tolerance. Breast is the most common site of cancer in women and, with the widespread use of mammography, more than twothirds of breast cancers are diagnosed at an early stage.2, 3 For the patients diagnosed with breast cancer, it is essential to decrease the probability of radiation induced complications. 011709-1
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The main types of chest wall radiotherapy complications are cardiomyopathy, pneumonitis, and secondary cancers.4–6 Although their occurrence is also influenced by lifestyle and/or a predisposing genetic condition,7, 8 it is primarily related to the amount of dose deposited in specific organs.8, 9 So, the most efficient way to prevent from sequelae of treatment is to reduce the amount of dose scattered to internal organs; for example, a radiation technique such as breast IMRT, 3D-CRT partial breast radiotherapy and HDR brachytherapy can be chosen or a platform-based breast shield can be used to minimize the exposure of internal organs.7, 9–13 The effect of an external magnetic field on electron beam dose distributions was first reported more than 60 years ago.14 Monte Carlo simulations15–17 and experiments18 demonstrated that application of a strong magnetic field can provide
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a substantial improvement and control of the dose profile of clinical electron and photon radiotherapy beams. Several institutes and researchers have proposed designs to develop a radiotherapy treatment system with online MR imaging (MRI) modality because online imaging has become a useful tool for margin reduction and, thereby, treatment improvement.19–28 The various proposed designs differ in magnetic field strength and geometry. The UMC Utrecht prototyped a system in which a 6 MV linear accelerator was mounted on a 1.5 T MRI scanner.27, 29, 30 This system became feasible by adapting the Linac and the MRI system to avoid mutual disturbance by the magnetic fields. Also, the Cross Cancer Institute in Edmonton prototyped a system in which the accelerator was mounted on a 0.2 T MRI scanner.31 The effect of magnetic field on the dose distribution in lung radiotherapy has been recently studied by Kirkby et al.22, 24 A similar work for a breast case was performed by Esmaeeli et al.32 The aim of this work is to simulate the effect of magnetic field on the dose distribution in breast radiotherapy in comparison to the conventional radiotherapy process. The emphasis of this work is on the radiobiological preference of the magnetic field application and its direction to be applied on the treatment region. The directions of the magnetic field were in the cranial-caudal and vice versa for medial and lateral tangential beams, respectively. The GEANT4 Monte Carlo (MC) code and DICOM computed tomography (CT) image data files were used for simulation of the treatment planning process.
2. METHODS 2.A. Monte Carlo simulations in the presence of a magnetic field
For all simulations, the MC toolkit GEANT4 (version 9.3) has been used to characterize the dose distribution in a reconstructed patient-derived phantom in the presence of a magnetic field. As discussed in previous work,32 GEANT4 is a freely available code and open source program, created by the GEANT4 Collaboration.33 All regular physical processes for medical applications were actuated from the Low EPhysics Processes package. The most common variance reduction method is energy cutoff. The MC simulation stops transportation of the particle if its energy falls below the cutoff energy and it was set to 0.4 mm for all particles. This means that secondary particles are generated only if their energy is high enough to travel at least 0.4 mm through the medium in which they are generated. Differential equation solver (classical Runge–Kutta method33 ) was used for calculation of the trajectory of charged particles in a magnetic field. This code was shown to provide accurate results for gamma ray dose distributions in the presence of a magnetic field.34 In simulations, statistical uncertainty in the dose distribution was less than 1% (1 SD) for all voxels with the dose of more than 1% of maximum dose. The mathematical phantoms were reconstructed from the DICOM images of the patients by means of GEANT4 DIMedical Physics, Vol. 41, No. 1, January 2014
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COM toolkit. The human body tissues such as lung, liver, chest wall, and bones are modeled according to the International Commission on Radiation Units and measurements (ICRU) report 46. In our previous work,35 the head of a clinical linear accelerator (Siemens Primus) was simulated based on the manufacturer’s detailed information by use of GEANT4 MC code, and benchmarked with real clinical dosimetry in terms of percentage depth dose and beam profiles for various field sizes and depths, for a 6-MV photon beam. Two opposed tangential fields were used for the treatment of breast cases. For each beam, a phase space was generated below the wedge in absence of any magnetic field. The assumption implicit in this model was that the main magnetic field was negligible to this point due to the magnetic shielding, implicit in the Linac-MR design. The contaminant electrons were included in the present phase-space data. Each phase space was then used as a source in the GEANT4 platform. The contaminant electrons travel a large distance from the bottom of wedge tray to the skin surface in the presence of transverse magnetic field. Therefore, the contaminant electrons are likely removed from the beam before it is incident on the patient. 2.B. Technique of the applying the magnetic field
The presence of external magnetic field modifies the dose distribution of photon beams by altering the trajectories of secondary electrons due to the Lorentz force. The radius of curvature of the electrons depends on the relative density difference at tissue interface (air-soft tissue or bone-soft tissue), the electron energy, and the magnetic field strength.24, 36–38 In order to study how dose to the lung and heart could be minimized, the direction of the magnetic field was changed for two beams, i.e., the magnetic field was applied in the positive z-direction for the medial beam and in the negative zdirection for the lateral. It takes away scattering particles out of the lung centre due to the net effect of the Lorentz force (see Fig. 1). Henceforth, we will refer to this as the bidirectional magnetic field technique. To apply the bidirectional technique, the feet-heat direction of the patient must be reversed to change the effective direction of magnetic field in practice. The magnetic field starts below the physical wedge. 2.C. Phantoms and simulation setups
2.C.1. The anatomy of patients
Similar patients and treatment plan setups were used as discussed for breast cases in our previous publication.32 A total of four breast cancer patients were selected in this study: two for intact breast irradiation and other two for chest wall irradiation. CT data slices of each patient were used in the MC code as input data. Dimensions of each voxel in the reconstructed image of patients in GEANT4 were 0.8 × 0.8 × 3 mm3 . All voxels were placed as parameterized volumes on a background material of air. One slice of the reconstructed phantom is shown in Fig. 2(b).
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F IG . 1. Schematic representations of a fixed cylindrical geometry; (a) for the medial beam and (b) for the lateral beam, showing the relative positioning of the patient, Linac, magnets, beam direction, magnetic field direction, and the net Lorentz force. In the fixed cylindrical geometry, the B0 field remains fixed with respect to the patient. Between the two irradiations, the feet-head direction of the patient is changed.
In order to characterize the anatomy of patients, parameters of four different thorax shapes were defined, as illustrated in Fig. 2(a). These are as follows, A: the orthogonal distance in the center line of the chest, B: the tangential distance, the line between the middle of the patient (center line) along the most dorsal part of the clinical target volume (CTV) to the body contour, C and D: orthogonal from the tangential distance, the maximum breast and lung distance, respectively.
2.C.2. Tretment planning
For the simulations, the plannings of chest wall irradiation were similar to the actual clinical planning. The CorePlan 3.5.0.5 treatment planning system (C&J Inc, Seoul, Korea) was used for 3D-CRT planning. The external surface of the patient and lung contours were defined by automated density gradient tracking; then, it was edited and verified by physicians. The chest wall CTV was
F IG . 2. (a) Target anatomy and structure, OARs, medial and lateral beams, treatment plan beam setups (nonmastectomy case), and four thorax shape parameters (A = central distance, B = tangential distance, C = breast orthogonal distance, and D = lung orthogonal distance) of patient # 1. The ICRU reference point is indicated by the green circle. (b) CT number map of the corresponding patient reconstructed image in the GEANT4 MC code. Medical Physics, Vol. 41, No. 1, January 2014
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TABLE I. Thorax shape parameters of patients. Parametersa
Patient # 1
Patient # 2
Patient # 3
Patient # 4
LODb (cm) CDc (cm) TDd (cm) BODe (cm)
1.4 25.4 25.4 6.3
0.7 23.3 21.6 3.6
2.8 21.5 23.2 4.0
0.6 20.8 19.5 6.8
a
Thorax shape parameters of patients shown in Fig. 2(a). Lung orthogonal distance. c Central distance. d Tangential distance. e Breast orthogonal distance. b
generally delineated on the corresponding transverse CT images limited by the external skin surface anteriorly, the ribsoft tissue interface posteriorly, the inferior aspect of the clavicular head superiorly and 1-cm below the inframammary fold inferiorly. Medial and lateral borders of the CTV were delineated considering lateral border of the sternum and the mid-axillary line, respectively. The target delineation for the intact breast was not similar to the chest wall. The PTV was defined by adding 5 mm to the CTV. The contour of the PTV was outlined with a depth of 1.6 mm to the skin surface anteriorly to evaluate DVHs. The common setups for all patients are as follows: Source-to-axis distance 100 cm, identical beam weights, 6 MV primary energy of the photons with two opposed tangent fields (symmetrical fields) and isocenter setup type as shown in Fig. 2(a). The isocenter was situated inside the target volume. The nominal prescribed dose was 50 Gy in 25 fractions using 6MV photons. The treatment plan setups and the target (PTV) and OARs together with wedges and treatment beams were schematically shown in Fig. 2(a). Thorax shape parameters of patients were tabulated in Table I. For chest wall tangential radiotherapy, bolus is often used during the treatment course to ensure adequate dose to the skin. At 1.5 T, the bolus was used in order to shift the hot spot areas out of the chest wall skin. Water-equivalent material is the choice for the bolus material in the simulations. Thickness
(a)
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of the bolus must be large enough to shift the hot spot regions that were selected 0.5 cm for patients # 1 and 4, and 1 cm for patients # 2 and 3.
2.C.3. Dose distribution in the lung
To investigate the effect of the magnetic field and electron return effect (ERE) on the dose distribution at the breast-lung boundaries, the delivered dose was calculated as a function of distance from the breast-lung boundaries toward the lung (Fig. 3). The path from breast-lung boundaries perpendicular to the beam direction was divided into rows parallel to the beam direction. The breast-lung boundary has an arc shape, and the selected first row close to the boundary in the lung is accepted for the calculation, if the number of its voxels compared to longest row exceeds 90%. Therefore, voxels near the breast-lung boundaries where their numbers are few were excluded. The dose of all voxels in each row that had the same densities as lung was summed and averaged. The distance between rows was 1.1 mm and the selected slice had the maximum lung to chest wall distance. In the selected slice that was assigned a maximum lung to chest wall distance the arc of the interface introduces an uncertainty of less than 5% both in the dose distribution and lung orthogonal distance (LOD) in each row in the patients.
2.C.4. Dose calculation
In the simulations, each beam was calculated separately and the results were combined, and normalized to 100% at the ICRU report 50 reference point for multiple beam plans. Absolute dose could be obtained by multiplying all voxel doses by the dose-to-isocenter/100% of the zero field plan. When difference maps are presented in this paper, in fact, the difference of the dose in each voxel at B field from zero field was divided to the zero field (the same method was used by Kirkby et al.24 ). In order to compare the effects of different strengths of the applied magnetic field on the dose distribution,
(b)
F IG . 3. (a) A schematic representation for calculating the electron return effect in the left lung. The medium from breast-lung boundaries perpendicular to the beam direction is divided into rows parallel to the beam direction. Distance between rows is 1.1 mm and the selected slice has the maximum lung orthogonal distance. Due to the arc shape of the chest wall, the first row is selected in distance from breast-lung boundaries that have enough voxels compared to deep distances. (b) The calculated dose versus distance from breast-lung boundaries toward the lung at 0, 0.25, and 1.5 T for one slice of each patient is indicated. Medical Physics, Vol. 41, No. 1, January 2014
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F IG . 4. Energy deposition in the middle slice of the central plane of patients is indicated for (a)–(c) patient # 1, (d)–(f) patient # 2, (g)–(i) patient # 3, and (j)–(l) patient # 4. Note that the bolus was not used in the plans.
the dose-volume parameters, DVHs, and dose-area histograms (DAHs) were calculated for the PTV and OARs.
3. RESULTS AND DISCUSSION 3.A. Effect of the magnetic field on dose distribution at the breast-lung boundaries
Dose increase was expected at the breast-air and breastlung boundaries due to the ERE:34 electrons entering the lung Medical Physics, Vol. 41, No. 1, January 2014
and air described a circular path and returned to the breast, causing extra dose deposition to the breast and dose reduction to the lung. It is important to note that low density tissues or air at the exit surfaces were shown to lead to regions of dose enhancement in the presence of a conventional magnetic field due to electrons returning to more dense tissues.22, 24, 36, 37, 39 To evaluate variations of dose distribution at the breastlung boundaries, delivered dose versus distance from breastlung boundaries toward the centre of the lung, perpendicular to the radiation direction, at 0, 0.25, and 1.5 T are plotted in
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Fig. 3. As a general trend, the similar dose distribution was observed for all patients, except patient # 3, due to larger LOD (see Table I), which caused more lung volumes to be in the radiation field. In regions near the breast-lung boundaries, dose reduction is larger for 1.5 T compared to 0.25 T. Low-energy electrons are not able to penetrate far into the lung, because they are confined to their region of origin by the Lorentz force. This effect becomes stronger for higher magnetic field strengths.38 Dose reductions in lung due to use of the magnetic field were different in various patients with respect to the no magnetic field case. They were reduced by a mean (range) of 17.0% (9%–22%) and 59.7% (47%–67%) at 0.25 and 1.5 T, respectively. The effect of strong magnetic field was reduced in deep regions of the lung, and the rate of dose reduction decreased in these areas with respect to the breast-lung boundaries. This is explained by the fact that low-energy charged particles that originated far from the breast-lung boundaries in the lung had a small gyration radius and could not be taken into the breast; furthermore, there were numerous scattered photons, on which the magnetic field did not have any influence. 3.B. Effect of the magnetic field on the dose distribution in breast radiotherapy
To illustrate the differences in relative dose distribution, results of simulations were presented for different magnetic fields with respect to no magnetic field in the middle slice of the central plane for patients in Fig. 4. Figure 5 depicts the corresponding difference maps of Fig. 4 from the zero magnetic field case. The bolus was not used in Figs. 4 and 5. The difference maps were calculated to show the change in dose due to the presence of the magnetic field. At the breast-lung and breast-air interfaces, the ERE increased breast and chest wall skin doses. Because of the higher density of lung relative to the air, returning electrons from the lung lost most of their energy in the path into the breast. Therefore, the energy of returning electrons from the air toward the chest wall was more than those returning from the lung, which produced a marginal zone of high-dose distribution at the breast-air boundaries for 1.5 T. The spread of the dose distribution with and without magnetic fields was due to the different anatomy of patients and treatment plan setups. At B0 = 0.25 T, a hot spot was seen in the breast which was indicated by a dashed black arrow in Fig. 5 and cold spot in the lung was indicated by a solid white arrow. The low-density tissue and air within the lungs allowed the magnetic field to strongly influence the electron trajectories, even at this low field strength. It should be noted that, there were regions of both increased dose and decreased dose to the chest wall skin as the result of the net effect of the Lorentz force acting on the electrons which shifted the dose to the outside of a patient’s body. There appears to be a reduction in skin dose (indicated by dotted white arrow) on the order of 20%–25% at the entrance of the medial tangent field and a collection of increased dose (indicated by dotted and dashed black arrows) on the order of 10%–15%. Therefore, due to Medical Physics, Vol. 41, No. 1, January 2014
F IG . 5. Relative dose differences in the middle slice of the central plane at 0.25 and 1.5 T from the zero magnetic field case for (a) and (b) Patient # 1, (c) and (d) patient # 2, (e) and (f) patient # 3, and (g) and (h) patient # 4. Hot and cold spots in the chest wall and lung were indicated by black and white arrows, respectively. Solid white double-arrows show a shift of dose distribution distinctly to the outside of patient center although by about 1 cm. Note that the bolus was not used in the plans.
the reduction of contaminant electrons near the skin, the mean dose of chest wall skin was reduced by 7.7%, 11.4%, 3.4%, and 6.2% for patients # 1, 2, 3, and 4, respectively, compared to 0 T. At B0 = 1.5 T, the exiting electron trajectories traveled through tighter arcs and resulted in chest wall skin and breast hot spots (solid black arrow) that extended several millimeters into the breast and air. Cold spot (solid white arrow) could be seen in the lung. From Fig. 5 it becomes clear that there was high dose delivered in the air inside the main beam due to small gyration radius of the charged particles. Therefore,
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F IG . 6. Breast treatment plan of dose–volume (PTV, left lung and heart) and dose-area (chest wall skin) histograms without bolus comparing B = 0, 0.25, and 1.5 T. (a) and (b) correspond to patient # 1, (c) and (d) to patient # 2, (e) and (f) to patient # 3, and (g) and (h) to patient # 4. Those volumes in the field are considered for chest wall skin DAHs.
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F IG . 7. Breast treatment plan of dose–volume (PTV) and dose-area (chest wall skin) histograms comparing bolus and nonbolus utilized at 1.5 T in the chest wall skin and breast tissues. (a), (b), (c), and (d) are for patients # 1, 2, 3, and 4, respectively. Note that those volumes in the field are considered for chest wall skin DAHs.
the dose to the internal organs such as the heart is reduced. Furthermore, the strong magnetic field shifted the dose distribution distinctly to the outside of patient center (solid white double-arrows), even by about 1 cm, again because of the net effect of the Lorentz force. Figure 6 illustrates the effect of the magnetic field in four patients at cumulative dose-volume and dose-area histograms (DVHs and DAHs) for the PTV and each organ at risk, looking specifically at magnetic field strengths of 0, 0.25, and 1.50 T. As a general trend, at 1.5 T, the PTV DVH was shifted to the right since ERE increases the dose inside the breast near breast-air interface. This resulted in few volume elements within the PTV receiving higher doses. In contrast, at 0.25 T, there is generally a slight decrease in the steepness of the curve, indicating the effect of cold spot at the entrance of the medial tangential field and some regions in the volume are also hotter due to ERE at the breast-lung and breast-air boundaries (Fig. 5). The left lung showed noticeable dose reduction in the DVH across the investigated range of magnetic field strengths. According to Figs. 4 and 5, there were obvious changes in the dose distribution within the lung. The Lorentz force affects charged particles originating in the lung volume located directly in the beam path, and returns and/or deflects them towards the breast-lung boundaries. The magnetic field was shown to restrict the lateral spread of secondary electrons to the internal organs, resulting in dose reductions in those tissues [Fig. 3(b)]. In the heart DVHs, at 0.25 T, acceptable dose reduction can be seen since the Lorentz force consistently Medical Physics, Vol. 41, No. 1, January 2014
shifts charged particles from the lateral areas toward the beam edge for all patients. The same phenomena were observed at 1.5 T, increasing the field strength to 1.5 T, the mean free path length of charged particles was more decreased, therefore, the lateral shift of dose distribution is further than of 0.25 T. At 1.5 T, there was strong ERE at the breast-air boundaries; therefore, the dose increased in the breast and chest wall skin. In contrast, at 0.25 T, curves showed acceptable decrease of dose-volume in high-dose areas. The results showed that, even though dose reductions in the heart and ipsilateral lung were considerable at 1.5 T, the dose mainly increased inside the chest wall skin and PTV. This configuration of treatment plan is not suitable for clinical applications. Therefore, to reduce the dose to these areas, these high-dose volumes must be shifted to the outside of the chest wall skin. To achieve this goal, a build-up bolus was used. 3.C. Using strong magnetic field along with a bolus in breast radiotherapy
Tissue-equivalent material boluses, which are thick enough to provide an adequate dose build-up in the skin and superficial chest wall, are commonly used during postmastectomy radiotherapy. Skin dose contributions of boluses and the dose delivered to the skin and subcutaneous tissue are important, especially in locally advanced breast cancer.40 The purpose is to shift the high-dose area of the chest wall skin and PTV toward the bolus in order to construct a uniform dose in the breast and chest wall skin tissues. The DAHs and DVHs
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TABLE II. Variations of the relative dose-volumetric parameters in the heart, ipsilateral lung and PTV along with the use of bolus in breast radiotherapy at 1.5 T with respect to no magnetic field case. Note that the relative mean dose and V20 were calculated without bolus at 0.25 T. Patient # 1
Patient # 2
Patient # 3
Patient # 4
Organs
0.25 (T)
1.50 (T)
0.25 (T)
1.50 (T)
0.25 (T)
1.50 (T)
0.25 (T)
1.50 (T)
ILa (mean dose) V20 Heart (mean dose) PTV (mean dose)
− 17.4% − 11.6% − 8.1% +0.1%
− 27.6% − 46.0% − 38.9% − 4.2%
− 11.5% − 35.4% − 10.6% − 6.5%
− 58.6% − 53.1% − 20.2% − 5.6%
− 24.9% − 22.5% − 13.5% − 6.9%
− 43.4% − 43.6% − 24.1% − 3.8%
− 26.5% − 27.6% − 3.3% − 9.4%
− 53.8% − 77.3% − 20.8% − 4.9%
a
Ipsilateral lung.
of the chest wall skin and breast with and without bolus for all patients at B = 1.5 T are shown in Fig. 7. Using bolus, uniform dose distributions and sharp drop-off curves were created in the chest wall skin41 and breast histograms. Thus, by using the strong magnetic field, for all patients, with bolus relative to those without bolus, an unwanted condition, i.e., increase of the dose to the chest wall skin occurred. Therefore, this technique must only be used for patients whose chest surface along with the PTV are the target, such as most mastectomy patients. Radiation pneumonitis (RP) is a common toxicity caused by radiation exposure to the lung, and the incidence of RP is known to be correlated with the volume of the irradiated lung and the radiation dose. The mean lung dose and V20 are generally related to RP.42 Therefore, in order to compare the effects of different strengths of the applied magnetic field on the dose distribution, the mean dose and V20 were calculated (Table II). The dose reduction in Table II was calculated using the bolus for 1.5 T case and without bolus for 0.25 T case. At 1.5 T, the mean dose of the ipsilateral lung and heart was reduced by a mean (range) of 45.8% (27.6%–58.6%) and 26.0% (20.2%–38.9%), respectively, depending on different patients and various treatment plan setups. Considering the V20 as an another dose-volume constraints for the ipsilateral lung, its value was reduced by a mean (range) of 20.7% (11.6%–35.4%) at 0.25 T and 55.0% (43.6%–77.3%) at 1.5 T. The amount of V20 and mean dose reductions is different across the range of patients and magnetic field strengths investigated. With respect to the patient specifications listed in Table I, it seems that variations of the dose reduction in various patients presumably depend on LOD, that for patients # 2 and 4 with small LODs, one can see higher dose reductions than other patients with larger ones (Table II). Several reports have indicated that breast radiotherapy increases the risk of cardiovascular disease and long-term mortality of patients with cancer involving the left breast.4, 43 Regarding cardiac risk, a critical review published by SchultzHector, suggested that the acute single dose of 1–2 Gy to the heart significantly increased the risk of developing ischemic heart disease,44 and the excess relative risk could be linearly fitted with the slope of 17% per Gy. Due to mean dose reduction of the heart to 1.3, 0.6, 1.6, and 0.3 Gy in patients # 1, 2, 3, and 4, respectively, a decrease of excess relative risk is to be expected in all patients at 1.5 T. Medical Physics, Vol. 41, No. 1, January 2014
The objective of this work was not to calculate the risks of radiation-induced pneumonitis and cardiac complications. However, using a magnetic field in breast radiotherapy for patients with different anatomies, with respect to relative mean dose and V20 reductions in the heart and left lung tabulated in Table II, one can clearly expect the reduction of these complications. Simulation suggests a feet first position for the patient to improve dose distribution from lateral tangent field. It means that the patient should be repositioned from medial irradiation field to lateral one during radiotherapy. However, reversing of the magnetic field orientation between medial and lateral fields might be another option. But at least theoretically, it seems impractical changing the B0 field direction during clinical practice due to the required time to ramp down and ramp it back up to the high magnetic fields. It will remain for future works to finding out the optimized method of choice for improving dose distribution using bidirectional magnetic field induction proposed in the present study. 4. CONCLUSIONS Magnetic field strengths of 0.25 and 1.50 T were applied in the z-direction in the thorax area in which the breast and lung underwent photon irradiation. Simulations have shown that applying a magnetic field during breast radiotherapy reduces the dose to the lung and heart, which are OARs during breast radiotherapy, because of bending of the path of secondary charged particles toward the chest wall by the Lorentz force. The results showed that the presence of a 0.25 T magnetic field made an acceptable dose reduction to the lung, heart, and chest wall skin, which is suitable for patients whose chest wall skins are not the target volume. At 1.5 T, despite the dose reduction to the lung and heart, due to the ERE, dose distributions in the chest wall skin and PTV became worse which was undesirable in the treatment plan. The simulations established that, using a strong magnetic field (1.5 T) along with a bolus produces a uniform dose distribution in the PTV and chest wall skin with sharper edged DVH curves. The chest wall skin dose increased in all patients at 1.5 T. Therefore, for patients whose chest wall skin along with the PTV are considered the target, the use of a strong magnetic field along with the bolus is the best choice.
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In the configuration of the magnetic field-Linac setups, it seems that the practical nature of changing the B0 field direction for the lateral and medial beams in breast radiotherapy is problematic during clinical practice. It is suggested to introduce a treatment plan in which to change the positioning of patient from day to day instead of changing the B0 field direction. ACKNOWLEDGMENTS The authors are thankful from physicists of radiotherapy departments in Sh. Rajaee and Pars Hospital for their support to proper use of data and planning outputs and special thanks to Mr. Rezaii and Rezazadeh for their kindly cooperations. a) Author
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