Section for Biomedical Physics, Department of Radiation Oncology, University Hospital Tübingen, 72076, Tübingen, Germany.
Department of Radiation Oncology, University Hospital Tübingen, 72076, Tübingen, Germany.
Med Phys. 2019 Nov;46(11):5304-5313. doi: 10.1002/mp.13829. Epub 2019 Oct 8.
To develop, implement, and validate a full 1.5 T/7 MV magnetic resonance (MR)-Linac accelerator head and cryostat model in EGSnrc for high precision dose calculations accounting for magnetic field effects that are independent from the vendor treatment planning system.
Primary electron beam parameters for the implemented model were adapted to be in accordance with measured dose profiles of the Elekta Unity (Elekta AB, Stockholm, Sweden). Parameters to be investigated were the mean electron energy as well as the Gaussian radial intensity and energy distributions. Energy tuning was done comparing depth dose profiles simulated with monoenergetic beams of varying energies to measurements. The optimum radial intensity distribution was found by varying the radial full width at half maximum (FWHM) and comparing simulated and measured lateral profiles. The influence of the energy distribution was investigated by comparing simulated lateral and depth dose profiles with varying energy spreads to measured data. Comparison of simulations and measurements was performed by calculating average and maximum local dose deviations. The model was validated recalculating a clinical intensity-modulated radiation therapy plan for the MR-Linac and comparing the resulting dose distribution with simulations from the commercial treatment planning system Monaco using the gamma criterion.
Comparison of simulated and measured data showed that the optimum initial electron beam for MR-Linac simulations was monoenergetic with an electron energy of (7.4 ± 0.2) MeV. The optimum Gaussian radial intensity distribution has a FWHM of (2.2 ± 0.3) mm. The average relative deviations were smaller than 1% for all simulated profiles with optimum electron parameters, whereas the largest maximum deviation of 2.07% was found for the 22 × 22 cm cross-plane profile. Profiles were insensitive to energy spread variations. The IMRT plan recalculated with the final MR-Linac model with optimized initial electron beam parameters showed a gamma pass rate of 99.83 % using a gamma criterion of 3%/3 mm.
The EGSnrc MR-Linac model developed in this study showed good accordance with measurements and was successfully used to recalculate a first full clinical IMRT treatment plan. Thus, it shows the general possibility for future secondary dose calculations of full IMRT plans with EGSnrc, which needs further detailed investigations before clinical use.
开发、实施和验证一个完整的 1.5 T/7 MV 磁共振（MR）-直线加速器头和冷却是在 EGSnrc 中的模型，用于高精度剂量计算，考虑到与供应商治疗计划系统无关的磁场效应。
为实施模型而采用的初级电子束参数与 Elekta Unity（Elekta AB，斯德哥尔摩，瑞典）的测量剂量分布一致。要研究的参数是平均电子能量以及高斯径向强度和能量分布。通过比较具有不同能量的单能束的深度剂量分布来进行能量调谐。通过改变径向半高全宽（FWHM）并比较模拟和测量的横向分布，找到最佳的径向强度分布。通过比较具有不同能散的模拟横向和深度剂量分布与测量数据，研究能量分布的影响。通过计算平均和最大局部剂量偏差来比较模拟和测量。通过重新计算 MR-Linac 的临床强度调制放射治疗计划并使用伽马标准比较来自商业治疗计划系统 Monaco 的模拟结果来验证模型。
模拟与测量数据的比较表明，MR-Linac 模拟的最佳初始电子束是具有（7.4±0.2）MeV 单能电子的电子束。最佳高斯径向强度分布的 FWHM 为（2.2±0.3）mm。对于具有最佳电子参数的所有模拟分布，平均相对偏差小于 1%，而最大的最大偏差为 2.07%，为 22×22 cm 交叉平面分布。分布对能散变化不敏感。使用最终的 MR-Linac 模型和优化的初始电子束参数重新计算的 IMRT 计划显示，使用 3%/3 mm 的伽马标准，伽马通过率为 99.83%。
本研究中开发的 EGSnrc MR-Linac 模型与测量结果吻合良好，并成功用于重新计算第一个完整的临床 IMRT 治疗计划。因此，它表明在临床使用之前，使用 EGSnrc 对全 IMRT 计划进行二次剂量计算的可能性，这需要进一步详细调查。
