Probabilistic quality assurance of biopsy DVH thresholds in HDR prostate brachytherapy under localization uncertainty.

BACKGROUND

MR-US guided biopsies support biologic correlation in HDR prostate brachytherapy, but steep dose gradients and millimeter-scale localization uncertainty limit the validity of nominal biopsy doses.

PURPOSE: To assess how localization uncertainty affects the robustness of biopsy DVH dose thresholds in HDR prostate brachytherapy and to quantify the nominal dose margins corresponding to 95% pass probabilities.

METHODS

Twenty-seven biopsies from 15 HDR monotherapy patients were reconstructed as sets of 1 m m voxels and sampled on the clinical HDR dose lattice. Localization uncertainty was modeled with 10 000 Monte Carlo trials per biopsy using isotropic 3D Gaussian translations ( σ = 1.25 m m ) plus an axial tissue-deficit offset. For each voxel we computed nominal and Monte Carlo summaries of dose and dose gradient magnitude; for each biopsy we evaluated DVH metrics D 2 % , D 50 % , D 98 % , and V 150 % . For four robustness thresholds, Monte Carlo pass probabilities p b were related to nominal distance-from-threshold using logistic models, and geometric, spatial, and radiomics predictors were tested as secondary covariates.

RESULTS

The pooled voxel-dose and dose-gradient distributions were broad, positively skewed, and consistent with log-normal models. Nominal voxel-level doses tended to be higher than localization-perturbed samples, with mean absolute per-trial | Δ D b , v , t | ≈ 7.6 Gy . Margin-only logistic models captured most of the variability in pass probabilities; the nominal margins required for a 95% pass probability were ≈ 6 Gy ( D 98 % ), 21 Gy ( D 50 % ), 49 Gy ( D 2 % ), and 49 percentage points ( V 150 % ). Adding geometric, spatial, or radiomics predictors altered these margins only modestly. Along biopsy cores, mean absolute dose differences between voxel pairs approached ≈ 10 Gy at separations of 1 - 1.5 c m , indicating decorrelation on that scale.

CONCLUSIONS

Localization uncertainty causes nominal biopsy DVH metrics to systematically overestimate Monte Carlo-propagated doses. The reported 95% margins should be interpreted as biopsy-scale robustness buffers under the stated uncertainty model, not as clinically prescriptive focal-boost increments. Dose decorrelated over distances of order 1 - 1.5 c m along biopsy cores, supporting spatially explicit, Monte Carlo-based reporting when interpreting biopsy dosimetry in HDR prostate brachytherapy.