MRI assisted focal boost integrated with HDR monotherapy study in low and intermediate risk prostate cancer (MARS): Results from a phase II clinical trial.

Abstract

PURPOSE\nThere is growing concern that single-fraction HDR monotherapy to a dose of 19\u202fGy is suboptimal for the treatment of localized prostate cancer. We report the results of a phase II prospective trial of single-fraction 19\u202fGy HDR monotherapy with MRI-guided simultaneous focal boost.\n\n\nMETHODS\nEligible patients had low or intermediate risk prostate cancer and an identified lesion on MRI. TRUS based single-fraction HDR monotherapy with MRI fusion was delivered. The dose prescribed was 19\u202fGy to the prostate and ≥23\u202fGy to the dominant intraprostatic lesion (DIL). ADT was not used. The purpose is to report early efficacy results.\n\n\nRESULTS\n60 patients were enrolled, with a median follow-up of 39\u202fmonths. With MRI T-stage incorporated into the risk-group criteria, 8% had low-risk, 35% had favorable intermediate-risk and 57% had unfavorable intermediate-risk disease. The median dose to 90% of the DIL (D90) was 27.2\u202fGy, and the median prostate V100% was 96.9%. No acute or late grade ≥3 bowel or urinary toxicity was observed. The cumulative BF probability was 15.2% at 36\u202fmonths and 31.6% at 48\u202fmonths. All patients that were fully investigated had local failure only, and 88% of the local failures were at the site of original DIL. The median PSA nadir was 0.79\u202fng/ml, with a median time to nadir of 32\u202fmonths.\n\n\nCONCLUSIONS\nFocal boost to the MRI-specified gross tumor was well tolerated, but did not adequately improve local control. Single-fraction HDR monotherapy to 19\u202fGy for prostate cancer provides suboptimal local control, and should not be offered outside of clinical trials.