Large scale retrospective Monte Carlo dosimetric study for permanent implant prostate brachytherapy

Publication date: Available online 23 November 2016
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): N. Miksys, E. Vigneault, A.G. Martin, L. Beaulieu, R.M. Thomson
PurposeTo retrospectively compare water-based and full tissue model Monte Carlo dose calculations in a large cohort of permanent implant prostate brachytherapy (PIPB) patients.Methods and MaterialsFor 613 patients, EGSnrc BrachyDose dose calculations are performed in two virtual patient models: TG43sim (simulated AAPM TG-43 conditions) and MCref (CT derived heterogeneous tissue model with interseed effects). A sensitivity analysis is 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.ResultsIn the target volume, average D90 (V100) is lower with MCref than TG43sim by 5.9 ± 1.6% (2.6 ± 1.7%). Patients with prostatic calcifications can have substantial under-dosed volumes due to calcification shielding, lowering D90 up to 25%. In the urethra, average D5 (D30) is lower with MCref than TG43sim by 4.4 ± 1.8% (4.7 ± 1.9%). In the rectum (bladder) average D0.1cc is lower (higher) with MCref than TG43sim by 5.2 ± 1.8% (1.3 ± 1.8%). Doses to target and OARs can increase or decrease by several percent depending on the assumed tissue elemental composition. In patients with calcifications, differences between approaches to model calcifications can vary target and OAR dose metrics by upwards of 10%.ConclusionsTG43sim typically overestimates target and OAR doses by several percent, on average, compared to MCref. The considerable variation in relative TG43sim and MCref doses between patients, and the larger dose differences for calcified patients, suggests that clinical adoption of Monte Carlo dose calculations for PIPB should be pursued. The substantial sensitivity of Monte Carlo dose calculations to patient modelling approach supports the adoption of a consensus modelling scheme, such as MCref described herein, to assure consistency of practice.



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