Dosimetry modeling for focal high-dose-rate prostate brachytherapy

Abstract

The dosimetry of focal high-dose-rate prostate brachytherapy was assessed. Dose volume histogram parameters, robustness to source position errors, and Monte Carlo (MC) simulations were compared for whole-gland (WG), hemi-gland (HEMI), and ultra-focal (UF) treatment plans. Tumor volumes were delineated based on MRI and template biopsy results for 9 patients. WG, HEMI, and UF plans were produced assuming 19Gy single fraction monotherapy treatments. For UF plans, a 6-mm margin was applied to the visible tumor to create a focal-planning target volume (F-PTV). Systematic source position shifts of 1-4mm were applied to assess plan robustness. The dosimetric impact of steel catheters was assessed using MC simulation. Mean D90 and V100 were 20.4Gy and 97.9% for prostate in WG plans, 22.2Gy and 98.1% for hemi-prostate in HEMI plans, and 23.0Gy and 98.2% for F-PTV in UF plans. Mean urethra D10 was 20.3, 19.7, and 9.2Gy in WG, HEMI, and UF plans, respectively. Mean rectal D2cc was 12.5, 9.8, and 4.6Gy in WG, HEMI, and UF plans, respectively. Focal treatment plans were sensitive to source position errors-2mm systematic shifts reduced mean prostate D90 by 0.7%, hemi-prostate D90 by 2.6%, and F-PTV D90 by 8.3% in WG, HEMI, and UF plans, respectively. MC simulation results were similar for all plan types with most dose volume histogram parameters reduced by <2%. HEMI and UF treatments can achieve higher D90 values compared with WG treatments with reduced organ at risk dose. Focal treatments are more sensitive to systematic source position errors than WG treatments.