Medical Physics Unit, McGill University and Cedars Cancer Center, 1001 Boulevard Décarie, Montréal, QC H4A 3J1, Canada.
Medical Physics Unit, McGill University and Cedars Cancer Center, 1001 Boulevard Décarie, Montréal, QC H4A 3J1, Canada.
Phys Med. 2020 Aug;76:7-15. doi: 10.1016/j.ejmp.2020.06.006. Epub 2020 Jun 20.
Owing to its short computation time and simplicity, the Ray-Tracing algorithm (RAT) has long been used to calculate dose distributions for the CyberKnife system. However, it is known that RAT fails to fully account for tissue heterogeneity and is therefore inaccurate in the lung. The aim of this study is to make a dosimetric assessment of 219 non-small cell lung cancer CyberKnife plans by recalculating their dose distributions using an independent Monte Carlo (MC) method. For plans initially calculated by RAT without heterogeneity corrections, target coverage was found to be significantly compromised when considering MC doses. Only 35.4% of plans were found to comply to their prescription doses. If the normal tissue dose limits were respected in the treatment planning dose, the MC recalculated dose did not exceed these limits in over 97% of the plans. Comparison of RAT and recalculated-MC doses confirmed the overestimation of RAT doses observed in previous studies. An inverse correlation between the RAT/MC dose ratio and the target size was also found to be statistically significant (p<10), consistent with other studies. In addition, the inaccuracy and variability in target coverage incurred from dose calculations using RAT without heterogeneity corrections was demonstrated. On average, no clinically relevant differences were observed between MC-calculated dose-to-water and dose-to-medium for all tissues investigated (⩽1%). Patients receiving a dose D larger than 119 Gy in EQD2 (or ≈52 Gy in 3 fractions) as recalculated by MC were observed to have significantly superior loco-regional progression-free survival rates (p=0.02) with a hazard ratio of 3.45 (95%CI: 1.14-10.5).
由于其计算时间短且简单，光线追踪算法（RAT）长期以来一直用于计算 CyberKnife 系统的剂量分布。然而，众所周知，RAT 未能充分考虑组织异质性，因此在肺部不够准确。本研究的目的是通过使用独立的蒙特卡罗（MC）方法重新计算其剂量分布，对 219 例非小细胞肺癌 CyberKnife 计划进行剂量评估。对于最初未进行异质性校正的 RAT 计算的计划，考虑 MC 剂量时，发现靶区覆盖率显著降低。只有 35.4%的计划符合其处方剂量。如果在治疗计划剂量中遵守正常组织剂量限制，则超过 97%的计划中 MC 重新计算的剂量不会超过这些限制。RAT 和重新计算的 MC 剂量之间的比较证实了之前研究中观察到的 RAT 剂量高估。还发现 RAT/MC 剂量比与靶区大小之间存在显著的负相关（p<10），与其他研究一致。此外，还证明了不进行异质性校正的 RAT 剂量计算导致靶区覆盖的不准确和变化。平均而言，对于所有研究的组织，MC 计算的水剂量与介质剂量之间没有观察到临床相关的差异（⩽1%）。接受 MC 重新计算的剂量 D 大于 119 Gy 的等效剂量（EQD2）（或 3 个部分的 ≈52 Gy）的患者，其局部区域无进展生存率显著提高（p=0.02），危险比为 3.45（95%CI：1.14-10.5）。
