调强放疗多叶光栅野剂量质量核查方法研究

Development of methodology for dose quality audit of multi-leaf collimator in intensity modulated radiotherapy

目的 研究用热释光剂量计（TLD）和胶片（film）测量调强放疗（IMRT）多叶光栅（MLC）野吸收剂量和二维剂量分布质量核查方法，为我国IMRT剂量质量核查的应用提供指导。方法 根据放疗开展情况，选择江苏、四川、湖北、河南4省的27家医院30台不同型号加速器（瓦里安17台，医科达10台，西门子3台）。同批次胶片和TLD，分别放入2 cm固体模体中固定，装入国际原子能机构（IAEA）提供的15 cm×15 cm×15 cm聚苯乙烯固体模体中。源皮距90 cm，深度10 cm，照射野10 cm×10 cm，照射1组不同剂量点的剂量，分别建立胶片和TLD标准剂量曲线。照射后的TLD和胶片返回中国外部核查组（EAG）测量并计算吸收剂量。测量的胶片或TLD吸收剂量与放疗计划系统（TPS）计算吸收剂量进行比较。胶片测量IMRT多叶光栅二维剂量分布，参加IEAE国际多放射治疗中心研究。30 cm×30 cm均质固体模体，经CT扫描，影像传给TPS制定治疗计划，25 cm×25 cm的胶片放在固体模体表面，源皮距95 cm，深度5 cm，实施IMRT计划，分次完成6 Gy照射。照射后的胶片邮寄到IAEA剂量学实验室测量和估算。IMRT二维剂量分布验证研究验证程序与国际多放射治疗中心研究程序相同。计算3 mm，3%二维剂量分布通过率。胶片测量与TPS计算二维剂量分布通过率进行比较。结果 按IAEA要求，TLD或胶片测量吸收剂量与TPS计算吸收剂量相对偏差应在±5%内符合要求。江苏、四川、湖北、河南4省的TLD及胶片测量吸收剂量与TPS计算吸收剂量的相对偏差结果分别为±0.7%~±8.5%和±0.3%~±7.8%范围内。IAEA要求，胶片测量与TPS计划二维剂量分布通过率3 mm/3%比较，90%符合要求。中国参与国际多放射治疗中心研究结果为94%。江苏、湖北、河南、四川4省参加二维剂量分布验证研究结果在70.0%~99.9%的范围内。结论 TLD和胶片核查IMRT多叶光栅野吸收剂量和二维剂量分布方法，科学适用，可操作性强。建立的TLD和胶片核查方法，适合在国内医院大范围开展IMRT剂量质量核查。

Objective To develop the methodology for using TLDs and films to measure absorbed dose and 2D dose distribution produced by the multi-leaf collimator （MLC） in intensity modulated radiotherapy （IMRT）, in order to provide the guidance on dose quality audit in IMRT. Methods A total of 30 different-typed accelerators were selected from 27 hospitals in Jiangsu, Sichuan, Hubei and Henan provinces, including 17 Varian accelerators, 10 Elektas and 3 Simens. The same batch of films and TLDs were put in a 2 cm-thick solid plate for fixation and then loaded in a 15 cm×15 cm×15 cm polysyrene solid phantom supplied by International Atomic Energy Agency（IAEA） in terms of 90 cm SSD, 19 cm depth, 10 cm×10 cm field at different doses. The standard dose curves were established for film and TLD, respectively. The irradiated film was measured and then sent to the External Audit Group （EAG） in China. The TLD-and film-absorbed doses were compared with TPS-calculated doses. The 2D dose distribution on the IRMT MLC field was measured using films. The homogeneous phantom of 30 cm×30 cm was scanned by CT and the image was transferred to the TPS. The IMRT was implemented with 6 Gy fractionated irradiation by placing a 25 cm×25 cm film on the phantom surface at 95 cm SSD and at 5 cm depth. The irradiated film was sent to the IAEA dosimetry laboratory for measurement and calculation. 2D dose distribution verification was conducted in the same way consistent with the procedure of international multi-radiotherapy center. The 3 mm/3% passing rate was calculated for 2D dose distribution and compared with the film-measured and TPS calculated result. Results IAEA requires the relative deviation of TLD and film measured absorbed dose are with in ±5%.The relative deviation of TLD-and film-measured to TPS-calculated absorbed dose was within the range of ±0.7%-±8.5% and within ±0.3%-±7.8% in Jiangsu, Sichuan, Hubei and Henan provinces, respectively. IAEA requires the 3 mm/3% passing rate of film-measured 2D distribution to be 90%. The result of the present study were up to 94.0%. The verification result of 2D dose distribution were within 70.0%-99.9% in Sichuan, Jiangsu, Hubei and Henan provinces. Conclusions The adsorbed dose and 2D distribution can be audited using TLDs and films for MLC in IRMT. The method is scientific and applicable, economical and convenient for development of dose quality audit for a wide range of IRMT.