Preliminary Results of MR-Guided Focal Salvage HDR Brachytherapy for Locally Recurrent Prostate Cancer after Primary Radiotherapy

To report preliminary outcomes of focal salvage MR-guided HDR brachytherapy for locally recurrent prostate cancer after external beam radiotherapy (EBRT). Patients entered on this prospective study had biopsy proven local recurrence of prostate carcinoma visible on MRI, with a PSA doubling time > 6 months, at least 18 months after definitive external beam radiotherapy. The GTV was defined prior to the brachytherapy procedure using multiparametric MRI (T2-weighted, diffusion-weighted, dynamic contrast-enhanced) and biopsy acquired during transperineal MR-guided mapping biopsy. The prostate gland and GTV contours were deformably co-registered to T2w images obtained at the time of brachytherapy after catheter placement using a biomechanical-based deformable image registration (MORFEUS). CTV margin expansion (5 mm in all directions) was restricted to adjacent OARs and 2 mm beyond the prostate boundary where applicable. PTV margins of 2 mm cranio-caudally and 1mm elsewhere were then applied. Focal HDR brachytherapy was delivered to a total dose of 26 Gy in two separate implants 7-14 days apart. Patients received no androgen deprivation therapy. 15 patients were enrolled; median follow-up was 12 months (range, 3-24). The median age at time of enrollment was 72 years (range, 62-77). The median pre-salvage treatment PSA was 3.96 ng/ml (range, 1.68 - 8.39). All 15 patients had a single GTV. Median GTV and PTV volumes were 1.45 cc (range, 0.66-6.76) and 6.45 cc (range 3.5-17.39), respectively. A median of 8 catheters (range, 4-12) were required for the focal implants. All 15 patients had an initial biochemical response after salvage brachytherapy with 14 having more than 50% reduction from baseline. To date 10 patients have achieved PSA < 1 ng/ml, while 9 of 12 patients with > 12 months follow up have PSA level < 1ng/ml. Of 2 patients who have experienced biochemical failure, 1 had sufficient follow-up to have imaging and biopsy evidence of a marginal local recurrence but neither has developed evidence of metastatic disease. Acute toxicity (CTCAE v4.0) was mild. Only one patient had grade 2 toxicity (urinary obstruction requiring temporary catherisation after his second implant). All acute toxicity has resolved in patients who have greater than 3 months of follow-up. These early results suggest that in selected patients, MR-guided focal salvage HDR brachytherapy is associated with minimal acute toxicity and initial biochemical response appears promising. Further follow up will determine if this approach results in acceptable local oncologic control with minimal associated late toxicity in the re-treatment setting.