Dosimetric analysis along core-needle biopsies in prostate HDR brachytherapy

Area of Science:

• Medical Physics
• Radiation Oncology
• Biomedical Engineering

Background:

• Accurate mapping of radiation dose to prostate biopsies is essential for correlating dose with treatment outcomes.
• Localization uncertainty in transrectal ultrasound (TRUS)-guided biopsies can affect dose assessment.
• High-dose-rate (HDR) brachytherapy involves complex dose distributions that require precise localization for analysis.

Purpose of the Study:

• To develop a probabilistic framework for mapping HDR brachytherapy dose and dose-gradient magnitude onto prostate core-needle biopsies.
• To explicitly propagate millimetre-scale localization uncertainty in MR-informed, TRUS-guided biopsies.
• To generate probability-weighted dose descriptors and delta metrics for assessing dose-biology correlations.

Main Methods:

• Reconstruction and voxelization (1 mm^3) of two representative second-fraction biopsy cores.
• Registration of biopsy cores to a 1 × 1 × 0.5 mm^3 clinical dose lattice.
• Modeling spatial uncertainty using 10,000 rigid translations and axial shifts, with Monte Carlo sampling for dose (D) and dose-gradient magnitude (G = ||∇D||).

Main Results:

• Two exemplar cores showed right-skewed dose distributions in high-gradient segments, with widened uncertainty bands in steep-gradient regions.
• The typical perturbation in mapped dose was substantially larger in the steep-gradient core (median |ΔDb,v,t| ≈ 7 Gy) compared to the low-gradient core (≈ 1 Gy).
• Length-scale curves indicated centimetre-scale along-core spatial structure, and voxel-pair maps revealed both near-uniform subsegments and areas with prescribed inter-voxel contrast.

Conclusions:

• The framework effectively converts biopsy localization uncertainty into probability-weighted dose descriptors and delta metrics.
• These metrics help separate nominal bias from propagated variability, aiding feasibility assessment for dose-biology studies.
• The approach supports guiding subsegment selection and provides dose/gradient covariates for various biological assays, even with partial loss of ex vivo biopsy orientation.