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To evaluate the dosimetric outcomes of focal salvage low dose rate (LDR) brachytherapy for biopsy proven local recurrence of prostate cancer after definitive external beam radiation therapy (EBRT) utilizing an MR-guided transperineal mapping biopsy to delineate the target volume. Patients with a Phoenix definition PSA recurrence (nadir + 2.0 ng/dL) after definitive EBRT for localized prostate cancer and a negative metastatic work up underwent MR-guided transperineal mapping biopsy of the prostate with both targeted and systematic tissue sampling. Using a transperineal template grid, systematic biopsies were spaced 10 mm or less from each other or the edge of the prostate and multiple biopsies were taken of MR-identified lesions. Patients with a pathologically confirmed recurrence underwent focal salvage LDR brachytherapy using iodine-125 seeds to a prescription dose of 145 Gy. The brachytherapy planning target volume (PTV) was defined by the positions of the pathologically negative core biopsies adjacent to the positive core biopsies as recorded on the template grid during the biopsy procedure. An intraoperative treatment plan was developed to cover the PTV with the 145 Gy isodose line. All patients underwent one-month post-operative dosimetry CT scan. The PTV was reproduced using the dimensions from the intraoperative plan. The PTV D90, V100; urethral V150, D30 and D1; and rectal V100 and D30 were measured. Ten patients with biopsy proven local recurrence underwent focal salvage LDR brachytherapy. The median prior EBRT dose was 7805 cGy (interquartile range IQR 7560-7920) and the median time from completion of EBRT to brachytherapy was 81 months (IQR 68-134). The median PSA prior to brachytherapy was 3.2 ng/dL (IQR 2.3 - 4.5). The median prostate volume was 23.7 cc (IQR 17.6-27.2) and the median PTV volume was 7.8 cc (IQR 3.8-23.9). The median ratio of PTV to prostate volume was 32.9% (IQR 20.4-49.4). A median of 22 seeds (IQR 19-28) were used to achieve PTV coverage with median activity per seed of 0.456 millicuries (mCi) per seed (IQR 0.451-0.456). The median PTV V100 and D90 were 94.9% (IQR 89.9-96.5) and 112.3% (IQR 99.7-121.8). The urethral V150 for all patients was 0 cc. The median urethral D30 and D1 were 72.2% (IQR 55.5-97.8) and 102.6% (IQR 73.6-121.7). The median rectal V100 and D30 were 0.01cc (IQR 0.00-0.03) and 17.8% (IQR 10.8-24.4). High dose focal salvage LDR brachytherapy can be delivered with excellent dosimetric coverage of the target volume and low dose to the urethra and rectum utilizing a MR-guided transperineal mapping biopsy technique to delineate the target volume. [ABSTRACT FROM AUTHOR]