Large-scale Retrospective Monte Carlo Dosimetric Study for Permanent Implant Prostate Brachytherapy

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Permanent implant prostate brachytherapy relies on accurate dose calculations for optimal treatment outcomes.
• Traditional dosimetry models like TG43sim may not fully account for patient-specific anatomy and tissue heterogeneity.
• Monte Carlo (MC) methods offer a more physically realistic approach to dose calculation.

Purpose of the Study:

• To compare dose calculations from a water-based model (TG43sim) and a full tissue model (MCref) using MC methods in a large cohort of prostate brachytherapy patients.
• To evaluate the impact of prostatic calcifications and tissue elemental composition on dose calculation accuracy.

Main Methods:

• Retrospective analysis of 613 patients undergoing ¹²⁵I permanent implant prostate brachytherapy.
• Dose calculations performed using EGSnrc BrachyDose in two models: TG43sim and MCref (CT-derived heterogeneous model).
• Sensitivity analysis conducted on a subset of patients to assess the impact of calcifications and tissue composition.

Main Results:

• MCref generally resulted in lower target (D₉₀) and organ-at-risk (OAR) doses compared to TG43sim.
• Prostatic calcifications significantly reduced D₉₀ (up to 25%) due to shielding effects.
• Dose differences varied based on tissue composition, with calcifications causing >10% variations in dose metrics.

Conclusions:

• TG43sim often overestimates target and OAR doses compared to the more realistic MCref model.
• Significant inter-patient variability and larger discrepancies in calcified patients underscore the need for MC dosimetry.
• Adoption of a consensus MC modeling scheme, like MCref, is recommended for consistent and accurate prostate brachytherapy dosimetry.