Dosimetric Impact of Perirectal Hydrogel Spacer for Focal Intraprostatic Lesion Ablative Microboost: Which Patients Benefit?

Purpose/Objective(s)Recent trials have demonstrated a benefit in biochemical free survival with focal ablative microboost (FAM) to the dominant intraprostatic lesion (DIL) in localized prostate cancer (LPC). Full DIL coverage was not always achieved due to prioritization of organs at risk. A dosimetric study was conducted to characterize the advantage of iodinated hydrogel (HG) spacer placement based on lesion location in both the IMRT and SBRT setting.Materials/MethodsTen prostate patients with pre- and post- HG placement imaging were evaluated. A GU-radiologist delineated all DILs on post-HG placement MRIs. IMRT (77-95 Gy/35 fx) and SBRT (36.25-40-50 Gy/5 fx) plans were generated on both pre- and post-HG imaging, with and without urethral constraints. Normal tissue constraints were utilized from prior randomized trials. Full coverage of the DIL was defined as D95 and V95 ≥95% of volume. Additional plans with theoretical DILs in varying positions throughout the prostate were created to develop a generalized model of HG benefit for FAM radiotherapy. The model was then applied to 125 patients with biopsy-confirmed DILs from the Prostate-MRI-US-Biopsy (PMUB) dataset to better approximate the real-world benefit of HG placement.ResultsThe mean prostate-rectal separation with HG placement at mid prostate was 12.8 mm (SD 2.85). The mean lesion size was 9 mm (range 4.3-14.8). There were no post-operative complications from HG placement. In the majority of patients both pre- and post-HG plans were able to achieve pre-specified dose-constraints for both SBRT and IMRT plans. In two patients (20%), full coverage of the DIL was not able to be achieved until after HG placement due to proximity of the rectum for both SBRT and IMRT. Post-HG plans had lower maximum rectal point dose, D0.1cc, and achieved significant reduction of D1cc to the rectum for SBRT (p=0.045) and IMRT (p=0.015). In the model, DIL coverage was compromised when the DIL was located within 1.5 mm of the urethra, 2.0 mm of the rectum or 1.5 mm of the bladder. In the PMUB dataset, an estimated 32 (25.6%) patients could potentially benefit from HG placement to improve DIL coverage if planned for FAM with either SBRT or IMRT.ConclusionWe developed a model to assess the benefit of HG spacer for FAM in LPC. Our data suggest HG spacer placement may provide benefit in improving DIL coverage for select patients, typically for LPC with DILs that are within 2.0 mm of the posterior prostate-rectum interface. The majority of prostate patients do not require HG placement to achieve both full DIL coverage and standard dose constraints.