Presentation Monte Carlo study on out-of-field exposure in carbon-ion radiotherapy with passive beam: Organ doses in prostate cancer treatment

米内, 俊祐  ,  松藤, 成弘  ,  赤羽, 恵一

Purpose: The aim of this work is to estimate typical dose equivalents to out-of-field organs during CIRT with passive beam for prostate cancer treatment. In addition, sensitivity analyses of organ doses for various beam parameters and phantom height were performed.Methods: Since the out-of-field dose in CIRT depends on the beam parameters such as beam energy and irradiation field size etc., the typical beam parameters were determined from the statistical data on target properties of selected 165 patients received CIRT with the same treatment protocol at HIMAC. By using the typical beam parameters, we estimated typical out-of-field organ dose equivalents during CIRT for prostate cancer treatment by Monte Carlo simulations with PHITS and the ICRP reference phantom. The method to calculate dose equivalents in CIRT with passive beam with PHITS developed in the previous paper[1] was used in this study. Sensitivities of organ doses to beam parameters and phantom height were analyzed by changing the typical beam parameters and the voxel size of ICRP reference phantom.Results: The results showed that the organ dose equivalent decreased with the distance from the target organ with a range from 116 mSv in testes to 7 mSv in brain. Also, the comparison with photon radiotherapies showed that the organ dose equivalents per treatment dose in CIRT with passive beam were lower than those in IMRT with 6 MV and those in brachytherapy with Ir-192 source for organs within 40 cm from the target organ. Sensitivity analyses of organ doses for possible beam parameters and phantom height found out that the differences from typical values were within ~30% for organs except for sigmoid colon.Conclusions: We showed the typical organ dose equivalents outside the irradiation field during CIRT with passive beam using Monte Carlo simulations. The low sensitivity of the dose equivalent in organs father than 10 cm from the target organ indicated that individual dose assessments required for the retrospective epidemiological study may be limited in organs around the target organ in case of CIRT with passive beam for prostate cancer.[1] Yonai S, Matsufuji N. and Namba M. Calculation of out-of-field dose distribution in carbon-ion radiotherapy by Monte Carlo simulation. Medical Physics 2012; 39(8):5028-539
International Conference on Monte Carlo Techniques for Medical Applications

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