Spatial pattern of intra-prostatic recurrence after definitive external-beam radiation therapy for prostate cancer: Implications for focal boost to intra-prostatic dominant lesion

Background/Purpose We retrospectively investigated spatial pattern associations between primary and recurrent tumor sites following definitive external-beam radiation therapy (EBRT) for prostate cancer, using positron emission tomography/computed tomography (PET/CT) with a prostate-specific membrane antigen (PSMA)-targeted probe, 18F-FSU-880. Methods Among the patients who received PET/CT with 18F-FSU-880 in a phase 2 trial, which evaluated the tumor detection efficacy of PSMA-PET/CT for recurrent prostate cancer, patients with local intra-prostatic recurrence detected by PSMA-PET/CT following definitive EBRT were retrospectively analyzed. The prostate and seminal vesicles were divided into 14 sections in total. Two diagnostic radiologists separately re-evaluated the intra-prostatic location of the primary tumor on magnetic-resonance imaging and that of the recurrent tumor on PSMA-PET/CT, respectively, and the rate of overlap between primary and recurrent tumors was calculated. The overlap rate was defined as “the number of sections which overlapped between the primary tumor and recurrent tumor” divided by “the total number of sections of recurrent tumor”. A recurrent tumor was considered to be at the same location with an overlap rate equal to or greater than 75%, and a partial overlap was defined as an overlap rate between 25 and 74%. Results Twelve patients had local recurrence detected by PSMA-PET/CT. The median time to the diagnosis of local recurrence was 9.1 (range: 2.2–12.3) years after definitive EBRT.The recurrent tumor was detected at the same location in 25.0%, and a partial overlap was noted in 41.7%. Conclusions Local intra-prostatic recurrence after definitive EBRT often occurs at the same site or at a partially overlapping site adjacent to the primary IPDL. Our results support the merit of focal dose-escalation for intra-prostatic dominant lesions in definitive EBRT.