Xu Zhengzheng, Warrell Gregory, Lee Soyoung, Colussi Valdir, Zheng Yiran, Ellis Rodney, Machtay Mitchell, Podder Tarun
Department of Radiation Oncology, University Hospitals Seidman Cancer Center, Cleveland, OH, USA.
School of Medicine, Case Western Reserve University, Cleveland, OH, USA.
J Appl Clin Med Phys. 2019 Jan;20(1):68-75. doi: 10.1002/acm2.12492. Epub 2018 Nov 7.
Dosimetric accuracy is critical when switching a patient treated with stereotactic body radiation therapy (SBRT) or stereotactic fractionated radiotherapy (SRT) among beam-matched linacs. In this study, the dose delivery accuracy of volumetric modulated arc therapy (VMAT) plans for SBRT/SRT patients were evaluated on three beam-matched linacs.
Beam data measurements such as percentage depth dose (PDD ), beam profiles, output factors, and multi-leaf collimator (MLC) leaf transmission factor for 6 MV photon beam were performed on three beam-matched linacs. The Edge™ diode detector was used for measurements of beams of field size less than 5 × 5 cm . Ten lung and 15 brain plans were generated using VMAT with the same beam model. Modulation complexity score of the VMAT plan (MCSv) was used as a plan complexity indicator. Doses were measured using ArcCHECK™ and GafChromic™ EBT3 films. The measurements were compared with calculated doses through absolute dose gamma comparison using 3%/2 mm and 2%/2 mm criteria. Correlation between difference in passing rates among beam-matched linacs and MCSv was evaluated using the Pearson coefficient. Point doses were measured with the A1SL micro ion chamber.
Difference in beam outputs, beam profiles, and MLC leaf transmission factors of beam-matched linacs were all within ±1%, except the difference in output factor for 1 × 1 cm field between linac 1 and 3 (1.3%). For all 25 cases, passing rates of measured doses on three linacs were all higher than 90% when using 2%/2 mm gamma criteria. The average difference in point dose measurements among three beam-matched linacs was 0.1 ± 0.2% (P > 0.05, one-way ANOVA).
Minimal differences in beam parameters, point doses, and passing rates among three linacs proved the viability of swapping SBRT/SRT using VMAT among beam-matched linacs. The effect of plan complexity on passing rate difference among beam-matched linacs is not statistically significant.
当在射束匹配的直线加速器之间切换接受立体定向体部放射治疗（SBRT）或立体定向分割放射治疗（SRT）的患者时，剂量测定精度至关重要。在本研究中，在三台射束匹配的直线加速器上评估了SBRT/SRT患者的容积调强弧形治疗（VMAT）计划的剂量传递精度。
在三台射束匹配的直线加速器上进行了射束数据测量，如6MV光子束的百分深度剂量（PDD）、射束轮廓、输出因子和多叶准直器（MLC）叶片透射因子。使用Edge™二极管探测器测量场大小小于5×5cm的射束。使用相同的射束模型通过VMAT生成了10个肺部计划和15个脑部计划。VMAT计划的调制复杂度评分（MCSv）用作计划复杂度指标。使用ArcCHECK™和GafChromic™EBT3胶片测量剂量。通过使用3%/2mm和2%/2mm标准的绝对剂量伽马比较，将测量结果与计算剂量进行比较。使用Pearson系数评估射束匹配的直线加速器之间通过率差异与MCSv之间的相关性。使用A1SL微型电离室测量点剂量。
射束匹配的直线加速器的射束输出、射束轮廓和MLC叶片透射因子的差异均在±1%以内，但直线加速器1和3之间1×1cm射野的输出因子差异为1.3%。对于所有25例病例，当使用2%/2mm伽马标准时，三台直线加速器上测量剂量的通过率均高于90%。三台射束匹配的直线加速器之间点剂量测量的平均差异为0.1±0.2%（P>0.05，单因素方差分析）。
三台直线加速器之间在射束参数、点剂量和通过率方面的差异极小，证明了在射束匹配的直线加速器之间使用VMAT切换SBRT/SRT的可行性。计划复杂度对射束匹配的直线加速器之间通过率差异的影响无统计学意义。
