湖北省7台医用直线加速器调强放射治疗多叶光栅叶片到位精度验证方法研究

目的 用放射性免冲洗胶片测量调强放射治疗（IMRT）多叶光栅（MLC）叶片到位精确度验证方法研究，为IMRT质量控制提供参考依据。方法 选择湖北省7家三甲医院的7台不同型号医用直线加速器，放射治疗计划系统（TPS）计划5条宽度为0.6 cm条状的栅栏，每条状野之间的距离为3.0 cm。用EBT2免冲洗胶片测量5条状野位置、多叶光栅叶片位置偏差和实际宽度。结果 按国际原子能机构（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。编号5和编号7的加速器MLC条状野位置偏差分别为0.7和-1.0 mm，不符合IAEA要求。按IAEA要求，胶片测量每对与每条所有多叶光栅叶片平均位置偏差应≤±0.5 mm。7台加速器MLC每对叶片位置偏差均在±0.5 mm内，符合IAEA要求；IAEA要求单对MLC叶片开野宽度与该机器5条条状野平均开野宽度的差值不超过±0.75 mm，7台加速器每对MLC开野宽度与平均值的差值范围为-0.6~0.5 mm，符合要求。所有叶片开野宽度标准偏差应≤0.3 mm，7台加速器标准偏差在0.1~0.2 mm范围内，符合要求。结论 用EBT2免冲洗胶片验证MLC位置精确度方法操作简单、检测精度高，是质量控制（QA）的重要手段之一，适合大范围核查使用。

Verification of positioning accuracy of MLC leaves of 7 medical linear accelerators in IMRT in Hubei province

Objective To verify the leaf positioning accuracy of multi-leaf collimator (MLC) in intensity modulated radiation therapy (IMRT) by using radiochromic films to provide references for IMRT quality control. Methods Medical linear accelerators of different designs, owned by 7 first-class hospitals at grade-3 in Hubei, were selected for this verification study. Five strip picket fence pattern was created by treatment planning system (TPS). Each strip was 0.6 cm wide with a 3 cm strip separation. MLC leaf positions,positioning bias and opening widths were then measured with EBT2 radiochromic films. Results According to IAEA standards, the film-measured and TPS planned MLC leaf position difference was ±0.5 mm. The difference of MCL strip position of No.5 and No. 7 accelerator was 0.7 and -1.0 mm, respectively, not in line with the IAEA standards. The difference of film-measured MLC leaf position between each pairs and all pairs of leaves of 7 accelerators were all within ±0.5 mm, in line with the IAEA requirements. As required by IAEA, the difference of opening width of MLC leave of each pair relative to the averaged widths of all pairs should be within ±0.75 mm. The filem-measured values from 7 accelerators ranged from -0.6 to 0.5 mm, all in line with the IAEA standards. Standard deviation of opening width of all leaves were required to be within 0.3 mm. The measured values from 7 accelerators ranged from 0.1 to 0.2 mm, in line with the IAEA standard. Conclusions The use of EBT2 radiochromic films to verify MLC leaf positioning accuracy is an important means of quality control owing to its simplicity and high measurement accuracy. It is recommended for verification use at a large scale.