The Accuracy of Multiparametric MRI in Identification of Intraprostatic Tumor Deposits after Local Prostate Radiation Therapy

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<h3>Purpose/Objective(s)</h3> To determine the accuracy of multiparametric (mp) MRI in identification of intraprostatic tumor deposits in prostate cancer patients previously treated with local radiation therapy (RT) using a systematic and targeted MR-guided transperineal prostate biopsy technique as a reference to guide and improve clinical workflow processes. <h3>Materials/Methods</h3> Twenty-three patients with localized prostate cancer who had been previously treated with external beam RT, brachytherapy, or combined external beam RT and brachytherapy, underwent a combined systematic and targeted MRI-guided prostate biopsy using a transperineal approach under general anesthesia. 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 lesions. Biopsy procedures were carried out 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. <h3>Results</h3> All patients successfully underwent their biopsy procedure as planned. The median PSA prior to biopsy was 5.5 ng/mL (range 2.1-24.6). The median number of biopsy specimens obtained per patient was 21 (range 13-34). Overall, the positive predictive value (PPV) of any PI-RADS 3-5 lesion (based on pathologic confirmation) was 81% (21/26). The individual PPV of PI-RADS 3, 4 and 5 lesions were 25% (1/4), 89% (16/18), and 100% (4/4), respectively. Forty-eight percent (11/23) of patients harbored mpMRI-unidentified prostate cancer. Thirty-five percent (8/23) of patients harbored mpMRI-unidentified GS≥7 disease. <h3>Conclusion</h3> 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 improve the accuracy of mpMRI identification of intraprostatic tumor deposits.