调强放射治疗多叶光栅叶片到位精确度质量核查方法研究

目的 研究用胶片（film）测量调强放射治疗（IMRT）多叶光栅（MLC）叶片到位精确度方法。方法 中国参加国际多放射治疗中心研究。固体水模体30 cm×30 cm，经CT扫描，影像传给放射治疗计划系统（TPS）制定治疗计划，多叶光栅形成5条栅栏野，3 cm×6 mm，条状与条状之间距离3 cm，在最大剂量点（d_max）处，源轴距离100 cm，6 MV X射线，每条栅栏野照射监督单位250 MU。放射性免冲洗胶片EBT2放在固体模体上，实施调强放射治疗5条栅栏野计划。选择加速器多、物理师水平较高的江苏、湖北、河南和四川4省的27家医院30台加速器参与验证研究。验证程序和方法与国际多放射治疗中心研究程序相同。照射后的胶片分别邮给国际原子能机构（IAEA）剂量学实验室和外部检查组（EAG）测量分析并计算。结果 按IAEA要求，胶片测量与TPS计划每条栅栏野多叶光栅条状位置结果应≤±0.5 mm，本研究结果分别为0.3、0.2、0、-0.1、-0.2 mm，符合要求。4省的30台加速器中，5台加速器多叶光栅条状位置结果在±0.6~1.0 mm范围内，不符合要求；25台加速器条状位置结果均在±0.5 mm范围内，符合要求。EAG验证研究结果表明，6台加速器多叶光栅条状位置结果在0.6~1.0 mm范围内，不符合要求；24台加速器条状位置结果均在±0.5 mm范围内，符合要求。按IAEA要求，胶片测量每对与每条所有多叶光栅叶片位置偏差应≤±0.5 mm。本研究结果为0.04 mm，符合IAEA要求。EAG验证研究结果表明，所有条状位置偏差均<0.3 mm，符合要求；除1台加速器条状位置偏差为-0.7 mm、超出±0.5 mm外，其余29台加速器条状位置偏差均在±0.5 mm范围内，符合要求。按IAEA要求，胶片测量每对与每条所有多叶光栅叶片最小与平均宽度差值，最大与平均宽度差值应≤±0.75 mm。本研究结果，最小与平均宽度差值为-0.2 mm，最大与平均宽度差值为0.4 mm，符合要求。30台中有6台加速器多叶光栅叶片最大宽度与平均宽度，最小宽度与平均宽度差值超出±0.75 mm，不符合要求；24台加速器多叶光栅叶片最大宽度与平均宽度，最小宽度与平均宽度差值均<±0.75 mm，符合要求。5台加速器多叶光栅最大宽度与平均宽度，最小宽度与平均宽度差值超出±0.75 mm，不符合要求；25台均<±0.75 mm，符合要求。按IAEA要求，胶片测量每对与每条所有对多叶光栅叶片实际宽度位置标准偏差应≤0.3 mm。本研究结果为0.12 mm，符合要求。4台加速器多叶光栅叶片实际宽度位置标准偏差超出0.3 mm，不符合要求；26台在0.3 mm范围内，符合要求。4省的30台加速器IAEA验证研究结果与EAG相同。结论 胶片剂量学验证多叶光栅叶片到位精确度方法，方便适用，作为质量核查，便于邮寄，可反复多次测量，适合在医疗机构大规模开展质量核查。

Development of methodology for the positioning accuracy of MLC leaves in IMRT

Objective To develop measurement methodology using film for the positioning accuracy of MLC leaves in IMRT. Methods The solid water phantom of 30 cm x 30 cm was scanned and the scanned images were transferred to TPS for treatment plan formulation. The five MLC strip picket fence pattern was formed by MLC leaves, each 3.0 cm long×6.0 mm wide. The separation between strip and strip is 3.0 cm. SAD is 100 cm at d_max for 6 MV X-ray, with 250 MU per MLC strip. EBT2 radiochomic film was put on the phantom for delivery of IMRT, for each MLC strip. The present study focused on 30 accelerators of Varian, Elekta and Siemens designs at 27 hospitals with highly skilled physicists all over Jiangsu, Sichuan, Hubei and Henan provinces. The study was conducted in the same way as used in international multi-radiotherapy center (IMRC). The irradiated films were sent respectively to IAEA dosimetry laboratory and external audit group (EGA) of China for measurement, analysis and calculation. Results According to IAEA requirements, the differenc of film-measured and TPS-planned of MLC leaf position for each strip should be within ±0.5 mm. China had participated in the research of IMRC, with the result of 0.3, 0.2, 0.0, -0.1, and -0.2 mm, respectively. For 30 accelerators in four provinces involved in the study, the IAEA''s verification results of MLC leaf position were within 0.6-1.0 mm for 5 accelerators and within ±0.5 mm for other 25 ones. Whereas the verification results of EAG were within 0.6-1.0 mm for 6 accelerators and within ±0.5 mm for other 24 ones. According to IAEA requirements, the film-measured MLC leaf position deviation for each pair of leaves and average all pairs of leaves should be within ±0.5 mm. China had participated in IMRC''s research, with the measured result being 0.04 mm. The verification result of EGA for 30 accelerators showed the measured MLC leaf position deviations were all <0.3 mm per strip, consistent with IAEA requirements. The IAEA''s result showed the measured deviations of MLC leaf position for 29 accelerators were within ±0.5 mm, with only other one being -0.7 mm not consistent with the IAEA requirements. As required by IAEA, the difference of film-measured difference of MLC opening width should be within ±0.75 mm between each pair and average all pairs of leaves. China''s result in research of IMRC showed the difference of minimum width to mean width was -0.2 mm whereas the difference of maximum width to mean width was 0.4 mm. For 30 accelerators involved in IAEA'' verification study, the measured result shown that the difference between maximum and average of film-measured of MLC leaf width, and between minmum and average, were within ±0.75 mm for 24 accelerators, in line with the IAEA requirements. For other 6 ones, the values were beyond ±0.75 mm, not in line with the IAEA requirements. For the verification result of EAG, the difference between maximum and average widths and between minmum and average widths for 25 accelerators were within ±0.75 mm as required by IAEA, whereas for other 5 ones the value were beyond ±0.75 mm, not consistent with IAEA requirements. The standard deviation of film-measured MLC opening width between each pair and average all pairs should be within 0.3 mm as required by IAEA. China''s IMRC result was 0.12 mm. The verification result of IAEA shown the standard deviation of MLC opening position were <0.3 mm for 26 accelerators and > 0.3 mm for other 4 accelerators. EAG verification result were the same as IAEA result. Conclusions The method using radiochromic film for measuring accuracy of MLC leaf position is convenient and practicable as a quality audit. It is suitable for quality verification in medical institutions owing to easy to post and repeated measurements.