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비균질 팬텀에서 소조사면에 대한 필름측정, 회선/중첩 모델과 몬테 카를로 모사의 비교 연구

Comparison of Film Measurements, Convolution/Superposition Model and Monte Carlo Simulations for Small Fields in Heterogeneous Phantoms

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대한의용생체공학회 (The Korea Society Of Medical & Biological Engineering)|2004.04|의공학회지 (Journal of biomedical Engineering Research)|제25권 제2호|pp. 1-1

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초록

세기조절방사선치료(IMRT)에서는 일반적인 방사선 치료에서 사용되는 조사면에 비해 비교적 작은 크기의 빔조각(beamlet)을 사용하여 방사선의 세기를 조절하는 새로운 치료법으로 이에 대한 비균질 효과는 많은 연구가 필요하다. 우리는 기하학적으로 일정한 비균질 팬텀들에서 몬테카를로 시뮬레이션에 의한 선량값을 라디오크로믹 필름에 의한 선량값과 회선/중첩 방법에 의한 선량 계산값과 서로 비교하였다. 몬테 카를로 모사를 위하여 EGS4 코드 기반의 BEAM 코드를 사용하였으며 이를 이용하여 Varian 2300C/D 선형가속기의 두부를 모사하였다. 측정과 모사에 사용된 조사면은 1×1㎠, 2×2㎠, 그리고 5×5㎠이었다. 또한 팬텀의 물질은 솔리드 워터, 폐 등가 물질, 뼈 등가 물질을 사용하여 세 경우의 비균질 팬텀들을 설정하여 방사선을 조사하였다. 회선/중첩 방법과 몬테 카를로 방법에 의한 선량 계산치는 광자 측면선량의 경우 ±1 ㎜, 깊이선량의 경우 ±2% 이내로 선량측정치와 잘 일치함을 볼 수 있었다. 결론적으로 회선/중첩 방법과 몬테 카를로 방법이 소조사면에서도 필름 측정 데이터와 잘 일치함을 확인할 수 있었다.

Abstract

Intensity-modulated radiation therapy (IMRT) often uses small beam segments. The heterogeneity effect is well known for relatively large field sizes used in the conventional radiation treatments. However, this effect is not known in small fields such as the beamlets used in IMRT. There are many factors that can cause errors in the small field i.e. electronic disequilibrium and multiple electron scattering. This study prepared geometrically regular heterogeneous phantoms, and compared the measurements with the calculations using the Convolution/Superposition algorithm and Monte Carlo method for small beams. This study used the BEAM00/EGS4 code to simulate the head of a Varian 2300C/D. The commissioning of a 6MV photon beam were performed from two points of view, the beam profiles and depth doses. The calculated voxel size was 1×1×2 ㎣ with field sizes of 1×1㎠, 2×2㎠, and 5×5㎠. The XiOTM TPS (Treatment Planning System) was used for the calculation using the Convolution/Superposition algorithm. The 6MV photon beam was irradiated to homogeneous (water equivalent) and heterogeneous phantoms (water equivalent + air cavity, water equivalent + bone equivalent). The beam profiles were well matched within ±1 ㎜ and the depth doses were within ±2%. In conclusion, the dose calculations of the Convolution/Superposition and Monte Carlo simulations showed good agreement with the film measurements in the small field.

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