Location of local recurrence after MRI-guided partial prostate brachytherapy targeting only the peripheral zone: Implications for focal therapy.

149 Background: Prostate cancer local recurrences usually occur at the same site as the dominant primary tumor in patients treated with radiation therapy to the whole gland. We characterized location of local recurrences in patients who were treated with MRI Guided Partial Brachytherapy in which only the peripheral zone was targeted. Methods: We retrospectively reviewed ten patients with initial cT1c, Gleason score 3+4 or less prostate cancer who developed biopsy proven local recurrences and had available imaging after MRI Guided Partial Brachytherapy targeting the peripheral zone from 1998 to 2006. All 10 patients had 1.5T endorectal coil MRI at diagnosis, performed primarily for staging and not for tumor localization, while at recurrence 8 had 3T endorectal coil MRI and 2 had 1.5T endorectal coil MRI. Scans consisted of at least T1 and T2 sequences. Two radiologists (C.T. and T.P.) blinded to clinical data reviewed diagnosis MRI scans together and quantified likelihood of tumor on a 1 to 5 scale in each section of an eight part prostate in both pre-treatment and recurrence scans. Local recurrence was judged to be in the same location as the baseline tumor if at least 50% of the tumor location overlapped. Results: Only 3 of 10 patients had local recurrences at the same location as the baseline tumor with a mean overlap of 64%. 7 of 10 patients had local recurrences at a different location with a mean overlap of 5%. 5 of 10 patients had recurrences in the central zone of the prostate which did not definitively show tumor on review of the initial 1.5T staging scan. Conclusions: After MRI-guided brachytherapy targeting only the peripheral zone in men initially staged with 1.5T MRI, 50% of the local recurrences occurred at the non-targeted central zone, raising the possibility that focal therapy directed only at the dominant tumor will result in increased out-of-field recurrences. Whether the superior ability of modern 3T multiparametric MRI to detect and precisely localize occult prostate cancer foci will reduce this risk is the subject of current study.