Design of a generally applicable abdominal shield for reducing fetal dose during radiotherapy of common malignancies in pregnant patients

Background: In most cancer cases, the treatment choice for a pregnant patient is radiotherapy. In these patients, the abdomen is usually not exposed; therefore fetus exposure is due to peripheral dose (PD). The purpose of this study was to estimate the fetal dose (the maximum PD in each pregnancy stage) for modalities available and to fabricate and evaluate a generally applicable fetal shield. Materials and Methods: PD values were measured for brain, breast and mediastinum irradiation in a whole body anthropomorphic phantom using a NE 2571 ionization chamber. An external shield was then designed to reduce the fetal dose to the standard dose limit, 5 mSv. Results: The range of PD values as a function of distance from the field’s edge were as follows 1) 9.4-259 cGy for Mantel field; 2) 6.5-95 cGy for chest wall irradiation with 10 MeV electrons, 3) 8.5- 52.5 cGy for tangential field with Co-60 and 4) 4.8-7.8 cGy for brain radiotherapy with 9 MV photon. PD values for the same setups using the fetal shield were as follows: 1) 1.4-22 cGy, 2) 0.5-4 cGy, 3) 1.5-5 cGy and 4) under 1 cGy. Conclusions: The measured PD data sets can be used to estimate fetal dose for specific treatment setups and pregnancy stages. The use of external shield designed in this research reduced the fetal dose effectively to under the threshold (a 70-90% reduction), except for the final stages of pregnancy in Hodgkin’s patients

Out-of-field beam characteristics of a 6 MV photon beam: Results of a Monte Carlo study

Detailed characteristics of particles in the periphery of a 6 MV photon beam resulting from the exposure of a water phantom were analyzed. The characteristics at the periphery were determined with respect to particles’ origin and charge, using Monte Carlo simulations. Results showed that in the peripheral regions, the energy fluence and the mean energy distribution of particles are independent of depth, and the majority of charged particles originate in the irradiated volume. The results are used to examine out-of-field dosimetry factors

Design and Implementation of a Monte Carlo Framework for Assessment of Spoiler Applications in Abutting Electron Fields

Background: Field matching problems in abutting electron fields can be man-aged by using spoilers. Objective: The aim of this study was to design a Monte Carlo framework for the assessment of spoiler application in abutting electron fields. Material and Methods: In this experimental study, a Siemens Primus treatment head was simulated for a 5 MeV electron beam using BEAMnrc, DOSXYZnrc and EGSnrc user codes. Validation of beam model was done by measurement using a MP3-M water tank and a Semi-flex Chamber-31010 (PTW, Freiburg, Germany). An in-house routine was developed to calculate the combined isodose curves result-ing from simulated adjacent fields. The developed framework was analyzed using PMMA and chromium spoilers. Results: The penumbra width increased from 27.5 mm for open fields to 42 mm for PMMA and 40 mm for chromium. The maximum junction dose reduced from 115% for open fields to 107% for PMMA and 108% for chromium. R90 reduced about 6 mm for PMMA and 3 mm for chromium. Uniformity index reduced from 93% to 77% for both spoilers. Surface dose increased from 79% to 89% for PMMA and 88% for chromium. Conclusion: Using spoilers, penumbra width at the surface was increased, size and depth of hot spots as well as the therapeutic range were reduced and dose homo-geneity at the junction of abutting electron fields was improved. For both spoilers, the uniformity index reduced, and surface percent dose increased. The results of this research can be used to optimize dose distribution in electron beam treatment using abutting fields

Evaluation of Therapeutic Properties of a Low Energy Electron Beam Plus Spoiler for Local Treatment of Mycosis Fungoides: A Monte Carlo Study

Background: When using low-energy electron beams for the treatment of skin lesions, such as Mycosis Fungoides (MF), a beam spoiler is used to decrease electron therapeutic depth (R90) while increasing the surface dose. Objective: The aim of this study was to evaluate the characteristics of a 5 MeV electron beam when using a spoiler for the local treatment of MF skin lesions by Monte Carlo (MC) simulation. Methods: A Siemens Primus treatment head and an acrylic spoiler, positioned at the end of applicator, were simulated using BEAMnrc, an EGSnrc user code. The modelled beam was validated by measurement using MP3-M water tank, Roos parallel plate chamber and Semi flex Chamber-31013 (all from PTW, Freiburg, Germany). For different spoiler thicknesses, dose distributions in water were calculated for 2 field sizes and were compared to those for the corresponding open fields. Results: For a 1.3 cm spoiler, therapeutic range changed from 1.5 cm (open field) to 0.5 cm and 0.4 cm for 10 × 10 cm2 and 20 × 20 cm2 field sizes, respectively. Maximum increase in penumbra width was 2.8 and 3.8 cm for 10 × 10 cm2 and 20 × 20 cm2 field sizes, respectively. Maximum increase in bremsstrahlung contamination was %2 in both field sizes. Conclusion: R90 decreased exponentially with increase in spoiler thickness. The effect of field size on penumbra was much larger for spoiled beam compared to the open beam. The results of this research can be applied to optimize the radiation treatment of MF patients in our hospital

Measurement of Collimator Scatter Factor for Photon Fields Using Gafchromic EBT2 Film and Ion Chamber

Background and Objective: In radiotherapy, measurement of the output factor in the air is one of the input parameters for Implementation of treatment planning systems and beam modeling. The aim of this study is to design a mini-phantom and select the appropriate detector for measuring the output factor. Subjects and Methods: In-air output factor was measured for Siemens linear accelerator 6 MV energy and field sizes of 4, 6, 8, 10, 15, 2 cm irradiated by 200MU. Measurements were performed using a Gafchromic film EBT2 and 0.6 cc Farmer ion chamber and then the results were compared with Report of AAPM Therapy Physics Committee Task Group 74. For lateral electron equilibrium and electron contamination removal, two top of the water and Cerrobend were designed for film dosimetry and plexiglass buildup cap for Farmer chamber was applied. Results: The output factor increases with field size, and this increase in Cerrobend top is less than water top and Farmer dosimeter. In comparison between the results reported by TG74 and this study shows that film dosimetry with water top and Farmer chamber have 0.35 and 0.68 mean percentage differences with TG74 respectively and meanwhile the film dosimetry with Cerrobend top presents 1.48 mean percentage differences. Conclusion: EBT2 film with mini phantom made of water as a good alternative to ion chamber dosimeters known as a standard dosimeter especially the film can be used for a wider range of field size

Evaluation of Therapeutic Properties of a Low Energy Electron Beam Plus Spoiler for Local Treatment of Mycosis Fungoides: A Monte Carlo Study

Background: When using low-energy electron beams for the treatment of skin lesions, such as Mycosis Fungoides (MF), a beam spoiler is used to decrease electron therapeutic depth (R90) while increasing the surface dose. Objective: The aim of this study was to evaluate the characteristics of a 5 MeV electron beam when using a spoiler for the local treatment of MF skin lesions by Monte Carlo (MC) simulation. Material and Methods: In this experimental study, a Siemens Primus treatment head and an acrylic spoiler, positioned at the end of applicator, were simu-lated using BEAMnrc, an EGSnrc user code. The modelled beam was validated by measurement using MP3-M water tank, Roos parallel plate chamber and Semi flex Chamber-31013 (all from PTW, Freiburg, Germany). For different spoiler thickness-es, dose distributions in water were calculated for 2 field sizes and were compared to those for the corresponding open fields. Results: For a 1.3 cm spoiler, therapeutic range changed from 1.5 cm (open field) to 0.5 cm and 0.4 cm for 10 × 10 cm2 and 20 × 20 cm2 field sizes, respectively. Maximum increase in penumbra width was 2.8 and 3.8 cm for 10 × 10 cm2 and 20 × 20 cm2 field sizes, respectively. Maximum increase in bremsstrahlung contamination was %2 in both field sizes. Conclusion: R90 decreased exponentially with increase in spoiler thickness. The effect of field size on penumbra was much larger for spoiled beam compared to the open beam. The results of this research can be applied to optimize the radiation treatment of MF patients in our hospital

Monte Carlo Study of Fetal Dosimetry Parameters for 6 MV Photon Beam

Because of the adverse effects of ionizing radiation on fetuses, prior to radiotherapy of pregnant patients, fetal dose should be estimated. Fetal dose has been studied by several authors in different depths in phantoms with various abdomen thicknesses (ATs). In this study, the effect of maternal AT and depth in fetal dosimetry was investigated, using peripheral dose (PD) distribution evaluations. A BEAMnrc model of Oncor linac using out of beam components was used for dose calculations in out of field border. A 6 MV photon beam was used to irradiate a chest phantom. Measurements were done using EBT2 radiochromic film in a RW3 phantom as abdomen. The followings were measured for different ATs: Depth PD profiles at two distances from the field′s edge, and in-plane PD profiles at two depths. The results of this study show that PD is depth dependent near the field′s edge. The increase in AT does not change PD depth of maximum and its distribution as a function of distance from the field′s edge. It is concluded that estimating the maximum fetal dose, using a flat phantom, i.e., without taking into account the AT, is possible. Furthermore, an in-plane profile measured at any depth can represent the dose variation as a function of distance. However, in order to estimate the maximum PD the depth of Dmax in out of field should be used for in-plane profile measurement

Out-of-field beam characteristics of a 6MV photon beam: Results of a Monte Carlo study

Detailed characteristics of particles in the periphery of a 6 MV photon beam resulting from the exposure of a water phantom were analyzed. The characteristics at the periphery were determined with respect to particles’ origin and charge, using Monte Carlo simulations. Results showed that in the peripheral regions, the energy fluence and the mean energy distribution of particles are independent of depth, and the majority of charged particles originate in the irradiated volume. The results are used to examine out-of-field dosimetry factors

Evaluation of the photon dose calculation accuracy in radiation therapy of malignant pleural mesothelioma

Background: Photon dose distribution of malignant pleural mesothelioma (MPM) in matched photon-electron technique is influenced by media inhomogeneity, lateral electronic disequilibrium at interfaces and narrow field. These may influence the dose calculation accuracy, calculated by treatment planning systems (TPS). This study aimed to evaluate the dose calculation accuracy of TiGRT TPS in radiation therapy of MPM. Materials and Methods: 18 MV photon beams of ONCOR Siemens linear accelerator was simulated using EGSnrc Monte Carlo (MC) code. Data of four patients were used to compare TPS and MC results in different regions included: Open and in-field, under shied and out of field regions. Results: Compared to MC results, the TPS overestimated the pleura dose coverage (90% of prescribed dose) about 3-12 mm, and also it overestimated the dose in under the shielded regions of lung (4-74%). While the TPS underestimated the dose profile width about 1-16 mm in low dose region (<50% prescribed dose) as well as the out of field region dose (4-100%). Conclusions: Results showed that TPS underestimated the dose in out of field and overestimated the dose in under the shielded regions. Unlike MC measurements, TPS calculation showed adequate pleural dose coverage. Based on the results, MC calculation can be used in matched photon-electron beam radiation therapy of MPM to modify the TPS photon dose calculations in the presence of heterogeneity, interfaces, and shield in MPM radiotherapy

Sensitivity analysis of a 6 MeV Photon Beam Monte Carlo model

Background: This study aimed to optimize efficiency in Monte Carlo (MC) simulation using sensitivity analysis of a beam model. Methods: The BEAMnrc-based model of 6 MV beam of a Siemens Primus linac was developed. For sensitivity analysis, the effect of the electron source, treatment head, and virtual phantom specifications on calculated percent depth dose (PDD) and lateral dose profiles was evaluated. Results: The optimum mean energy (E) and the full width at half maximum (FWHM) of the intensity distribution of the electron beam were calculated as 6.7 MeV and 3 mm, respectively. Increasing E from 6.1 to 6.7 MeV, increased the PDD in the fall-off region by 4.70 and decreased the lateral profile by 8.76. Changing the FWHM had a significant effect on the buildup region of PDD and the horns and out-of-field regions of the lateral profile. Increasing the collimators opening by 0.5 mm, PDD increased by 2.13 and the central and penumbra regions of profiles decreased by 1.98 and 11.40 respectively. Collimator properties such as thickness and density were effective in changing the penumbra (11.32 for 0.25 cm increment) and the out-of-field (22.82 for 3 g/cm 3) regions of the lateral profiles. Conclusion: Analysis of a 6 MV model showed that PDD profiles were more sensitive to changes in energy than to FWHM of the electron source. The lateral profiles were sensitive to E, FWHM, and collimator opening. The density of the collimator affected only the out-of-field region of lateral profiles. The findings of this study may be used to make benchmarking of an MC beam model more efficient