Dosimetric and Clinical Insights In Transitioning from TG-43 to Advanced TG-186 Dose Calculations for Permanent Prostate Brachytherapy
Abstract
Purpose
To quantify the dosimetric impact of transitioning from water-based TG-43 dose evaluations to advanced model-based TG-186 calculations in permanent prostate brachytherapy, and to assess the effects on target coverage, organ-at-risk doses, and the appropriateness of current clinical dose prescriptions. Associations between dosimetric changes, implant parameters, and prostate-specific antigen (PSA) outcomes were also explored.
Methods
Retrospective data for 1,297 patients treated at four Canadian centers were analyzed. Post-implant dose-volume metrics were recalculated using both TG-43 and TG-186 Monte Carlo-based algorithms (egs_brachy’s eb_gui software). Paired Wilcoxon signed-rank tests were used to compare TG-43– and TG-186–calculated dosimetric parameters. Correlations between dose metrics and calcification volume, seed number, and target volume were evaluated using Pearson and Spearman correlation coefficients. PSA nadir (lowest post-treatment PSA value, reflecting treatment success) was analyzed in relation to target dose.
Results
TG-186 dosimetry resulted in lower median doses for CTV D90 (-2.3%), CTV V100 (-1.06%), and Urethra D5cc (-7.99%) (all p<0.001), with smaller differences observed for rectum D2cc. Strong negative correlations were found between TG-186/TG-43 ratios of target doses and the presence of calcifications, indicating that calcifications reduce effective target coverage. Median PSA nadir decreased from 0.26 to 0.05 ng/mL as the actual dose calculated by TG-186 approached the TG-43 estimate, suggesting improved biochemical response. Low TG-186 doses relative to TG-43 predictions reflect greater reductions due to heterogeneities, which are associated with less tumor control and higher PSA nadir.
Conclusion
TG-43 dose calculations tend to overestimate the actual delivered dose, particularly in the presence of calcifications. The overestimation is also associated with PSA outcomes, highlighting the clinical relevance of accurate dose modeling. These findings underscore TG-186’s potential to guide dose optimization and improve clinical decision-making in prostate brachytherapy. While the current prescription dose appears broadly appropriate, TG-186 modeling provides valuable insight for patient-specific dose assessment.