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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]

To assess the performance of multiparametric (mp) MRI in identification of intraprostatic tumor deposits in patients previously treated with local radiation therapy (RT) using a systematic and targeted MR-gided transperineal prostate biopsy technique. Thirty patients with a rising PSA and negative metastatic work up after prior external beam RT, brachytherapy, or combined external beam RT and brachytherapy for localized prostate cancer underwent a combined systematic and targeted MRI-guided prostate biopsy using a transperineal approach. For each patient, a pre-biopsy mpMRI scan was obtained and imported into a prostate biopsy planning system. Transverse T2-weighted images were then reoriented from the supine to dorsal lithotomy position. Dividing the prostate into an apical and base section, a systematic array of transperineal biopsies spaced approximately 10mm apart was planned with additional biopsies targeting any mpMRI-identified PI-RADS 3, 4, or 5 lesion. Biopsy procedures were carried out under general anesthesia in the dorsal lithotomy position using a transrectal ultrasound with stepper-stabilizer and template grid. Matching of the planning mpMRI images to live ultrasound images was achieved using the template grid as a reference. All patients successfully underwent biopsy without post-procedure urinary obstruction or infection. The median prostate volume was 36cc (range 12-90, IQR 24-51). The median PSA prior to biopsy was 4.7 ng/mL (range 1.9-24.6, IQR 3.0-7.3). The median number of biopsy specimens obtained per patient was 21 (range 13-34, IQR 17-24). A total of 32 PI-RADS lesions were identified in 30 patients. Overall, the positive predictive value (PPV) of any PI-RADS 3-5 lesion for prostate cancer (based on pathologic confirmation) was 84% (27/32). The individual PPV of PI-RADS 3, 4 and 5 lesions were 40% (2/5), 89% (17/19), and 100% (8/8), respectively. Forty-three percent (13/30) of patients harbored mpMRI-unidentified prostate cancer. Thirty percent (9/30) of patients harbored mpMRI-unidentified GS≥7 disease. Using prostate mpMRI to identify intraprostatic tumor deposits after local radiation therapy results in a high PPV, particularly for PI-RADS 4 and 5 lesions. However, given the high rate of mpMRI-unidentified prostate cancer, the addition of systematic transperineal prostate biopsy information appears necessary to most accurately identify all intraprostatic tumor deposits. These data would suggest that a combination of systematic and mpMRI targeted biopsies are necessary to optimally perform focal salvage therapy in the setting of local recurrence after radiation therapy. [ABSTRACT FROM AUTHOR]