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Abstract
The present study verified and evaluated the dosimetric effects of protons scattered from a snout and an aperture in clinical practice, when a range compensator was included. The dose distribution calculated by a treatment planning system (TPS) was compared with the measured dose distribution and the dose distribution calculated by Monte Carlo simulation at several depths. The difference between the measured and calculated results was analyzed using Monte Carlo simulation with filtration of scattering in the snout and aperture. The dependence of the effects of scattered protons on snout size, beam range, and minimum thickness of the range compensator was also investigated using the Monte Carlo simulation. The simulated and measured results showed that the additional dose compared with the results calculated by the TPS at shallow depths was mainly due to protons scattered by the snout and aperture. This additional dose was filtered by the structure of the range compensator so that it was observed under the thin region of the range compensator. The maximum difference was measured at a depth of 16 mm (8.25%), with the difference decreasing with depth. Analysis of protons contributing to the additional dose showed that the contribution of protons scattered from the snout was greater than that of protons scattered from the aperture when a narrow snout was used. In the Monte Carlo simulation, this effect of scattered protons was reduced when wider snouts and longer-range proton beams were used. This effect was also reduced when thicker range compensator bases were used, even with a narrow snout. This study verified the effect of scattered protons even when a range compensator was included and emphasized the importance of snout-scattered protons when a narrow snout is used for small fields. It indicated that this additional dose can be reduced by wider snouts, longer range proton beams, and thicker range compensator bases. These results provide a better understanding of the additional dose from scattered protons in clinical practice.

그림 1. 측정된 선량 프로필(EBT3 필름, MatriXX)과 치료 계획 시스템(TPS) 계산 결과 및 몬테카를로(MC) 시뮬레이션 결과와의 비교: (a) y-axis / 16mm 측정 깊이 (b) x-axis / 30mm 측정 깊이 (c) x-axis / 40 mm 측정 깊이 (d) y-axis / 56 mm 측정 깊이 (e) y-axis / 69 mm 측정 깊이

그림 2. Snout의 사이즈(100mm, 180mm, 250mm)에 따른 중심축에서의 산란된 양성자 빔의 선량 변화 비교: (a) TPS 계산 결과, (b) 몬테카를로 시뮬레이션.

그림3. Range compensator의 base thickness 변화에 따른 노즐을 통과하면서 산란되는 양성자 수의 비교

Affiliations

Chankyu Kim  1 , Yeon-Joo Kim  1 , Nuri Lee  2 , Sang Hee Ahn  3 , Kwang Hyeon Kim  4 , Haksoo Kim  1 , Dongho Shin  1 , Young Kyung Lim  1 , Jong Hwi Jeong  1 , Dae Yong Kim  1 , Wook-Geun Shin  5 , Chul Hee Min  6 , Se Byeong Lee  1
1 Proton Therapy Center, National Cancer Center Korea, Gyeonggi-do, Republic of Korea.
2 Department of Radiation Oncology, National Medical Center, Seoul, Republic of Korea.
3 Department of Radiation Oncology, Yonsei Cancer Center, Yonsei University College of Medicine, Seoul, Republic of Korea.
4 Department of Neurosurgery, Inje University Ilsan Paik Hospital, Gyeonggi-do, Republic of Korea.
5 Department of Radiation Oncology, Seoul National University Hospital, Seoul, Republic of Korea.
6 Department of Radiation Convergence Engineering, Yonsei University, Gangwon-do, Republic of Korea.