Catheter Geometries For Prostate Tumor-Targeted Hdr Brachytherapy

Magnetic Resonance Imaging (MRI)-guided High Dose Rate (HDR) brachytherapy is a novel technique being investigated for the salvage treatment of local-only failure after external beam radiation therapy for prostate cancer. Delivering a radiation boost to MRI-defined sites of local recurrence may potentially improve the therapeutic ratio. The purpose of this study was to investigate whether tumor targeted catheter implantation geometries would translate to improved dosimetry when treating the prostate gland with a boost to the MRI-defined gross tumor volume (GTV). The MRI dataset for 10 men previously treated with tumor targeted MRI-guided salvage HDR brachytherapy were used to create 180 alternative brachytherapy plans using either a standard modified peripheral loading pattern (S) or two alternative tumor targeted catheter implantation geometries (TT1 and TT2). Two additional catheters were implanted in the tumor for TT1 and the standard pattern was adjusted to have one catheter in the center of the tumor for TT2. To determine if any observed difference between the geometries was dependent on the number of catheters implanted or the boost dose prescribed, plans using an equivalent total of 12, 14, or 16 catheters were optimized with the Inverse Planning Simulated Annealing (IPSA) algorithm (using class solution optimization parameters) to deliver a minimum peripheral dose to the prostate gland and two alternative boost dose levels (138% and 175%) to the MRI-defined GTV. Plans using the tumor targeted catheter geometries showed improved GTV coverage. The TT1 pattern showed the largest improvement and this was consistent across plans using 12, 14, or 16 catheters. For plans using 14 catheters and optimized to deliver a 138% boost, mean coverage of the GTV by the prescribed boost isodose was 99.0%, 98.3%, and 97.1% for the TT1, TT2, and S geometries respectively (p = 0.008 on multiple comparison testing between TT1 and S). The absolute improvement in tumor coverage by the boost isodose was greater when plans were optimized to deliver the higher 175% boost level. There was no statistically significant difference in dose to the organs at risk (OAR). As the number of implanted catheters was increased from 12 to 16, there was a trend for reduced dose to the OAR and improved dose conformity to the GTV for all geometries. On subgroup analysis, improved target coverage was greatest in men with small tumors (GTV: prostate volume ≤ 0.1). Tumor targeted catheter implantation geometries improve the dosimetry of a boost dose to the tumor in HDR brachytherapy for prostate cancer and this strategy is more important in men with small volume tumors. Accurate MRI-guidance strategies during catheter implantation may assist in this regard.