선형가속기의 소조사면에 대한 빔 자료 측정에서 검출기의 선량 특성 분석
Dosimetric Characteristics of Detectors in Measurement of Beam Data for Small Fields of Linear Accelerator
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선형가속기를 기반으로 하는 세기조절방사선치료와 정위적방사선수술에서는 치료계획시스템의 소조사면에 대한 신뢰할만한 선량분포를 계산하기 위해서는 우선적으로 소조사면의 정확한 빔 자료 측정이 선행되어야 한다. 특히 소조사면의 빔 자료 측정에서 조사면 가장자리에서의 급격한 선량 변화, 측면 전자비평형, 그리고 검출기의 체적 영향으로 인한 적절한 검출기 선택이 중요하다. 따라서 본 연구에서는 선형가속기의 소조사면에 대한 빔 자료 측정에 있어서 검출기의 선량 특성을 알아보고자 하였다. 검출기는 0.01cc 부피와 0.13cc 부피의 이온전리함과 정위적다이오드를 사용하였으며, 빔 자료는 광자선(6MV와 15MV)에 대하여 조사면 크기를 2×2cm2에서 5×5cm2까지 변화시켜 각 검출기를 이용하여 깊이선량백분율, 선량출력계수, 그리고 빔측면도를 측정하였다. CC01 이온전리함과 정위적다이오드 검출기를 이용한 PDD20/PDD10은 2×2cm2 조사면의 경우 6MV와 15MV에서 각각 1.02%와 0.12% 차이를 보였다. 3×3cm2 이상의 조사면에서는 각 검출기를 이용하여 얻어진 PDD20/PDD10의 차이가 6MV와 15MV에서 각각 평균 1.15%와 0.71% 이였다. CC01 이온전리함과 정위적다이오드 검출기를 이용한 선량출력계수 측정 결과, 2×2cm2 조사면의 경우 6MV와 15MV에서 0.5%와 1.5% 이내에서 일치하였다. 3×3cm2 이상의 조사면에서는 각 검출기의 차이가 0.5% 이내이였다. 3개의 깊이에서 측정된 빔측면도의 반음영은 정위적다이오드 검출기의 경우 6MV와 15MV에서 각각 평균 2.7mm와 3.5mm, CC01 이온전리함의 경우 각각 평균 3.4mm와 4.3mm, CC13 이온전리함의 경우 각각 평균 5.2mm와 6.1mm이였다. 이를 통해 깊이선량백분율과 선량출력계수 측정 시 2×2cm2 조사면에서는 CC01 이온전리함과 정위적다이오드 검출기를 3×3cm2에서 5×5cm2 조사면에서는 각 검출기의 사용이 가능할 것으로 판단된다. 또한 소조사면에 대한 정확한 빔측면도의 반음영을 측정하기 위해서는 유효체적이 작은 CC01 이온전리함과 정위적다이오드 검출기 사용하는 것이 타당하겠다.
Abstract
Aquisition of accurate beam data is very important to calculate a reliable dose distribution of the treatment planning system for small radiation fields in intensity-modulated radiation therapy(IMRT) and stereotactic radiosurgery(SRS). For the measurement of small fields, the choice of a suitable detector is important due to the shape gradient in profile penumbra, the lack of lateral electronic equilibrium, and the effect of effective detector volume. Therefore, this study was to analyze the dosimetric characteristics of various detectors in measurement of beam data for small fields of linear accelerator. 0.01cc and 0.13cc ion chambers (CC01 and CC13) and a stereotactic diode detector(SFD) were used for measurement of small fields. The beam data, including the percent depth dose, output factor, and beam profile were acquired under 6 MV and 15 MV photon beams. Measurements were performed with the field size ranging from 2×2cm2 to 5×5cm2. For 2×2cm2 field size, the differences of the ratios of PDD20 and PDD10 measured by CC01 and SFD detectors were 1.02% and 0.12% for 6MV and 15MV photon beams, respectively. For field sizes larger than 3×3cm2, the differences of values of PDD20/PDD10 obtained from each detector were 1.15% and 0.71% for 6MV and 15MV photon beams, respectively. The output factors obtained from CC01 and SFD for 2×2cm2 field size were within 0.5% and 1.5% for 6MV and 15MV, respectively. The differences in output factor of three detectors for 3×3cm2 to 5×5cm2 field sizes were within 0.5%. Profile penumbras measured by the SFD, CC01, and CC13 detectors at three depths were average 2.7mm and 3.5mm, 3.4mm and 4.3mm, and 5.2mm and 6.1mm for 6MV and 15MV photon beams, respectively. In conclusion, it could be possible to use of the CC01 and SFD detectors for the measurement of percent depth dose and output factor for 2×2cm2 field size, and to use of three detectors for 3×3cm2 to 5×5cm2 field sizes. CC01 and SFD detectors, consider ably smaller than the radiation field, should be used in order to accurately measure the profile penumbra for small field sizes.
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