Department of Radiation Oncology, University Hospital, LMU Munich, Marchioninistr. 15, 81377 Munich, Germany.
Department of Radiation Oncology, University Hospital, LMU Munich, Marchioninistr. 15, 81377 Munich, Germany; Department of Radiation Oncology, University Hospital Cologne, Kerpener Str. 62, 50937 Cologne, Germany.
Z Med Phys. 2022 Aug;32(3):312-325. doi: 10.1016/j.zemedi.2022.01.006. Epub 2022 Mar 16.
To date, no universally accepted technique for the evaluation of the overall dosimetric performance of hybrid integrated magnetic resonance imaging (MR) - linear accelerators (linacs) is available. We report on the suitability and reliability of a novel phantom with modular inserts for combined polymer gel (PG) and ionisation chamber (IC) measurements at a 0.35 T MR-linac.
Three 3D-printed, modular head phantoms, based on real patient anatomy, were used for repeated (2 times) PG irradiations of cranial treatment plans on a 0.35 T MR-linac. The PG readout was performed on two 1.5 T diagnostic MR-scanners to reduce scanning time. The PG dose volumes were normalised to the IC dose (normalised dose N1) and to the median planning target volume dose (normalised dose N2). Linearity of the PG dose response was validated and dose profiles, centres of mass (COM) of the 95% isodoses and dose volume histograms (DVH) were compared between planned and measured dose distributions and a 3D gamma analysis was performed.
Dose linearity of the PG was good (R> 0.99 for all linear fit functions). High agreement was found between planned and measured dose volumes in the dose profiles and DVHs. The largest dose deviation was found in the intermediate dose region (mean dose deviation 0.2Gy; 5.6%). A mean COM offset of 1.2mm indicated high spatial accuracy. Mean 3D gamma passing rates (2%, 2mm) of 83.3% for N1 and 91.6% for N2 dose distributions were determined. When comparing repeated PG measurements to each other, a mean gamma passing rate of 95.7% was found.
The new modular phantom was found practical for use at a 0.35 T MR-linac. In contrast to the high dose region, larger mean deviations were found in the mid dose range. The PG measurements showed high reproducibility. The MR-linac performed well in a non-adaptive setting in terms of spatial and dosimetric accuracy.
目前，尚无用于评估混合集成磁共振成像（MR）-直线加速器（linac）整体剂量性能的通用技术。我们报告了一种新型模体的适用性和可靠性，该模体具有模块化插件，可用于在 0.35 T MR-linac 上进行聚合物凝胶（PG）和电离室（IC）联合测量。
使用基于真实患者解剖结构的三个 3D 打印模块化头部模体，对 0.35 T MR-linac 上的颅部治疗计划进行了两次 PG 照射。PG 读出在两个 1.5 T 诊断磁共振扫描仪上进行，以减少扫描时间。PG 剂量体积被归一化为 IC 剂量（归一化剂量 N1）和中位数计划靶区剂量（归一化剂量 N2）。验证了 PG 剂量响应的线性度，并比较了计划剂量分布和测量剂量分布之间的剂量分布、剂量分布中心（COM）和剂量体积直方图（DVH），并进行了 3D 伽马分析。
PG 的剂量线性度很好（所有线性拟合函数的 R>0.99）。在剂量分布和 DVH 中的剂量剖面和 DVH 中发现了计划剂量体积和测量剂量体积之间的高度一致性。在中间剂量区域发现了最大剂量偏差（平均剂量偏差 0.2Gy；5.6%）。1.2mm 的平均 COM 偏移表明了较高的空间精度。N1 剂量分布的平均 3D 伽马通过率（2%，2mm）为 83.3%，N2 剂量分布的平均 3D 伽马通过率为 91.6%。当将重复的 PG 测量彼此进行比较时，发现平均伽马通过率为 95.7%。
新型模块化模体在 0.35 T MR-linac 中使用方便。与高剂量区域相比，中剂量区域的平均偏差较大。PG 测量显示出很高的可重复性。MR-linac 在空间和剂量学精度方面表现良好，无需自适应设置。
