基于 0.35TMR-LINAC 用 LiF 热释光探测器和聚合物水凝胶进行绝对 3D 剂量测量的可行性研究。

Department of Medical Physics in Radiation Therapy, German Cancer Research Center, Im Neuenheimer Feld 280, Heidelberg, Germany. National Center for Radiation Research in Oncology (NCRO), Heidelberg Institute for Radiation Oncology (HIRO), Im Neuenheimer Feld 280, Heidelberg, Germany.

Phys Med Biol. 2020 Oct 26;65(21):215002. doi: 10.1088/1361-6560/aba6d7.

BACKGROUND AND PURPOSE

As shown in our previous study, highly accurate absolute dosimetry in 3D is feasible by combining polymer gels (PG) with thermoluminescence dosimetry (TLD). In this setup, the thermoluminescence (TL)-based point dose information is used to renormalize the PG. This new PG-TLD reference system is now extended to measurements in magnetic fields.

MATERIALS AND METHODS

Experiments were carried out on a conventional 6 MV linear accelerator (LINAC) and a 6 MV 0.35 T magnetic resonance (MR)-LINAC. Signal stability of TLD600 and TLD700 was examined without and with magnetic field. Afterwards, the combination of PAGAT PG and TL detectors was employed within a cylindrical phantom in presence of the magnetic field. Two scenarios were tested: (I) an air-filled phantom and (II) a water-filled phantom. For each scenario, two plans were irradiated: (a) opposed beams with a field size of 10 × 10 cm and (b) a 3D conformal plan assuring homogeneous target coverage using three equally distributed coplanar beams.

RESULTS

Mean relative uncertainty of TL calibration reproducibility for TLD600/TLD700 was 0.49%/0.85% at the MR-LINAC and 0.48%/0.83% for the conventional LINAC. Individual TL calibration coefficients of TLD600 and TLD700 behaved differently in the presence of the magnetic field. An average difference of (3.29 ± 0.89)% occurred for all TLD600, whereas the result for TLD700 is not quite as clear with (1.09 ± 0.89)% after excluding some outliers. Using the TL dose information for PG renormalization, high 3D gamma passing rates were achieved using the 3%/2 mm criteria: 91.0% (Ia), 92.6% (Ib), 94.3% (IIa), 97.4% (IIb).

CONCLUSION

This study shows that TL signal reproducibility is not affected by a low magnetic field. Nevertheless, absolute calibration coefficients of the individual detectors indicate a dependency on the magnetic field. Hence, a calibration at the appropriate LINAC type is recommended. Furthermore, the previously established renormalization method for PG was applied to measurements at a MR-LINAC and was verified as suitable for evaluations of homogeneous dose distribution in the target volume.

背景与目的

在我们之前的研究中，通过将聚合物凝胶（PG）与热释光剂量测定（TLD）相结合，实现了 3D 中高精度的绝对剂量测定。在这种设置中，基于热发光（TL）的点剂量信息用于对 PG 进行归一化。这个新的 PG-TLD 参考系统现在已经扩展到磁场中的测量。

材料与方法

实验是在常规 6 MV 线性加速器（LINAC）和 6 MV 0.35 T 磁共振（MR）-LINAC 上进行的。检查了 TLD600 和 TLD700 在没有和有磁场的情况下的信号稳定性。然后，在存在磁场的情况下，将 PAGAT PG 和 TL 探测器组合在圆柱形体模中。测试了两种情况：（I）充满空气的体模和（II）充满水的体模。对于每种情况，都对两个计划进行了照射：（a）具有 10×10 cm 场大小的对光束和（b）使用三个均匀分布的共面光束确保靶区均匀覆盖的 3D 适形计划。

结果

在 MR-LINAC 上，TLD600/TLD700 的 TL 校准重现性的平均相对不确定度为 0.49%/0.85%，在常规 LINAC 上为 0.48%/0.83%。TLD600 和 TLD700 的单独 TL 校准系数在磁场存在下表现不同。对于所有 TLD600，平均差异为（3.29±0.89）%，而 TLD700 的结果则不太清楚，在排除一些异常值后为（1.09±0.89）%。使用 PG 归一化的 TL 剂量信息，使用 3%/2 mm 标准实现了高的 3D 伽马通过率：91.0%（Ia）、92.6%（Ib）、94.3%（IIa）、97.4%（IIb）。