四川省7台加速器调强放射治疗多叶光栅叶片到位精确度验证方法研究

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

目的用放射性免冲洗胶片验证调强放射治疗(IMRT)多叶光栅(MLC)叶片到位精确度方法研究。方法选择瓦里安、医科达、西门子3个厂家的医用电子直线加速器共7台,用25cm×25cm的放射性免冲洗胶片放在30cm×30cm、厚3.0cm的均质固体模体上,厚度2.0cm的固体模体板覆盖在胶片上面,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条条状栅栏野,能量6MVX射线束,每条栅栏野长3cm,宽0.6cm,每条条状野间隔3cm,在最大剂量点处,胶片到源距离100cm,每条栅栏野给出监督剂量250MU。照射后邮寄到国际原子能机构(IAEA)剂量学实验室测量和计算。结果6台加速器胶片测量与TPS计划每条栅栏野MLC条状位置偏差符合IAEA要求的±0.5mm,1台加速器偏差不符合要求。7台加速器胶片测量每对与每条多叶光栅叶片位置偏差均在IAEA要求0.5mm以内,符合要求。6台加速器胶片测量每对与每条所有MLC叶片实际宽度差值在0.75mm范围内,1台加速器为-0.8mm,不符合要求。6台加速器胶片测量每条多叶光栅叶片实际宽度标准偏差在0.3mm范围内,符合要求。1台加速器为0.4mm,不符合要求。结论用放射性免冲洗胶片验证调强放射治疗多叶光栅片到位精确度的方法简单,快速精确,建议广泛应用到临床。

Objective To verify the accuracy of multi-leave collimators (MLC) leaves position in intensity modulated radiation therapy (IMRT) using radiochromic films. Methods 7 medical electronic linear accelerators of Varian, Elekta or Siemens design were chosen. 25 cm×25 cm films were put on 30 cm×30 cm×3.0 cm homogeneous solid phantom and covered with a 2.0 cm thick of homogeneous solid phantom. The CT scanned images were transmitted to TPS for plan preparation. A 5 strip picket fence pattern was formed by MLC leaf, each 3 cm long and 0.6 cm wide, with 3.0 cm wide strip separation. At the maximum dose point, the SSD was 100 cm, with 250 MU per strip. After exposure, the films were transmitted to the dosimetry laboratory at IAEA for measurement and calculation. Results For 6 of 7 accelerators chosen, the differences of film-measured and TPS-planned MLC leaf position for every fence were within ± 0.5 mm as required by IAEA, and the other one not consistent with the requirements. The difference of film-measured MLC leaf position between each pair and all pair for 7 accelerators were within ± 0.5 mm, in line with IAEA′s requirements. The differences of film-measured MLC leaf actual width were within 0.75 mm, as required by IAEA, for 6 accelerators and -0.8 mm for the other one, not consistent with the IAEA requirements. The standard deviations of film-measured MLC leaf actual width for all pairs for 6 accelerators were <0.3 mm, in line with IAEA requirements, but 0.4 mm for the other one, not consistent with IAEA requirements. Conclusions It is simple, fast and accurate to use radiochromic films for verification of the accuracy of MLC leaf position in IMRT. Therefore it is advisable to widely use radiochromic films in IMRT clinical practice.