Carleton Laboratory for Radiotherapy Physics, Department of Physics, Carleton University, Ottawa, Ontario, Canada.
Département de Physique et Centre de recherche sur le cancer, Québec, Québec, Canada; Département de Radio-oncologie et Centre de recherche du CHU de Québec, Québec, Québec, Canada.
Int J Radiat Oncol Biol Phys. 2017 Mar 1;97(3):606-615. doi: 10.1016/j.ijrobp.2016.11.025. Epub 2016 Nov 23.
To retrospectively compare water-based and full tissue model Monte Carlo dose calculations in a large cohort of patients undergoing I permanent implant prostate brachytherapy.
For 613 patients, EGSnrc BrachyDose dose calculations were performed in 2 virtual patient models: TG43sim (simulated American Association of Physicists in Medicine Task Group Report 43 conditions) and MCref (computed tomography-derived heterogeneous tissue model with interseed effects). A sensitivity analysis was performed in a patient subset (25 with and 25 without prostatic calcifications) to explore dose calculation dependence on organ-at-risk (OAR) and calcification tissue elemental compositions and modelling approach.
In the target volume, the minimum radiation dose delivered to 90% of prostate (D) (volume of prostate receiving at least 100% of prescription dose [V]) was lower with MCref than with TG43sim by 5.9% ± 1.6% (2.6% ± 1.7%), on average. Patients with prostatic calcifications can have substantial underdosed volumes due to calcification shielding, lowering the D by ≤25%. In the urethra, the average D (D) was lower with MCref than with TG43sim by 4.4% ± 1.8% (4.7% ± 1.9%). In the rectum (bladder), the minimum dose to the hottest 0.1 cm (D_0.1cm) of the contoured organ was lower (higher) with MCref than with TG43sim by 5.2% ± 1.8% (1.3% ± 1.8%). Doses to the target and OARs can increase or decrease by several percentages, depending on the assumed tissue elemental composition. In patients with calcifications, differences between approaches to model calcifications can change the target and OAR dose metrics by upward of 10%.
TG43sim typically overestimates the target and OAR doses by several percentages, on average, compared with MCref. The considerable variation in the relative TG43sim and MCref doses between patients, and the larger dose differences for patients with calcification, suggests that clinical adoption of Monte Carlo dose calculations for permanent implant prostate brachytherapy should be pursued. The substantial sensitivity of the Monte Carlo dose calculations to the patient modelling approach supports the adoption of a consensus modelling scheme, such as MCref described in the present study, to ensure consistency of practice.
回顾性比较水基和全组织模型 Monte Carlo 剂量计算在接受 I 期永久性植入前列腺近距离放射治疗的大量患者中的应用。
对 613 例患者，使用 EGSnrc BrachyDose 剂量计算，在 2 个虚拟患者模型中进行：TG43sim（模拟美国医学物理学家协会任务组报告 43 条件）和 MCref（基于 CT 的异质组织模型，包含种植间效应）。在患者亚组（25 例有前列腺钙化和 25 例无前列腺钙化）中进行敏感性分析，以探讨剂量计算对危及器官（OAR）和钙化组织元素组成以及建模方法的依赖性。
在靶区，用 MCref 计算的 90%前列腺接受的最小辐射剂量（D）（接受至少 100%处方剂量的前列腺体积[V]）比 TG43sim 低 5.9%±1.6%（平均 2.6%±1.7%）。由于钙化屏蔽，有前列腺钙化的患者可能会出现大量剂量不足的体积，使 D 降低≤25%。在尿道中，用 MCref 计算的平均 D（D）比 TG43sim 低 4.4%±1.8%（4.7%±1.9%）。在直肠（膀胱）中，勾画器官最热的 0.1cm（D_0.1cm）的最小剂量，用 MCref 计算比 TG43sim 低 5.2%±1.8%（1.3%±1.8%）。靶区和 OAR 的剂量取决于假设的组织元素组成，可能会增加或减少几个百分点。在有钙化的患者中，模拟钙化的方法之间的差异可能会使靶区和 OAR 剂量指标变化超过 10%。
与 MCref 相比，TG43sim 通常会高估靶区和 OAR 的剂量，平均高估几个百分点。患者之间 TG43sim 和 MCref 剂量的差异较大，且有钙化的患者剂量差异更大，这表明应该为永久性植入前列腺近距离放射治疗采用 Monte Carlo 剂量计算。Monte Carlo 剂量计算对患者建模方法的显著敏感性支持采用共识建模方案，如本研究中描述的 MCref，以确保实践的一致性。
