The Importance of Multiparametric Magnetic Resonance Imaging, Positron Emission Tomography/Computed Tomography, and Biopsy for Identifying and Delineating the Extent of Intraprostatic Radiorecurrent Prostate Cancer: A Secondary Analysis of the F-SHARP Clinical Trial

Abstract

Purpose: Up to 50% of clinical recurrences after curative-intent prostate cancer radiation are intraprostatic radiorecurrences (IPRRs). Salvage local therapy (SLT) is increasingly offered, particularly as focal SLT, to reduce toxicity due to prior radiation. Limited data exist on the relative value of magnetic resonance imaging (MRI), positron emission tomography/computed tomography (PET/CT), and biopsy on SLT target delineation. We compared MRI, PET/CT, and biopsy in patients with IPRRs and the impact each modality has on identifying IPRRs and defining the extent of prostatic involvement.

Methods and materials: We performed a secondary analysis of 62 patients enrolled in a phase 1/2 clinical trial of salvage high-dose-rate brachytherapy. The IPRR was delineated using each imaging modality and by defining the involved regions of the prostate on biopsy. The exact binomial distribution was used to estimate the sensitivity of MRI and PET/CT to detect the IPRR. Exact conditional logistic regression was used to compare the tumor identified by MRI and PET/CT with the areas of biopsy involvement (gold standard) and estimate the proportion of patients with prostatic involvement outside of the image-defined targets.

Results: The sensitivity for detecting the IPRR was 91.8% for MRI and 85.5% for PET/CT. Most patients had biopsy-proven cancer outside of the MRI-defined (70.5%) and PET/CT-defined (73.8%) target. Delineating the brachytherapy target using imaging only would have missed the full extent of recurrence in 63.9%.

Conclusions: Although MRI and PET/CT are valuable, a thorough biopsy is a mandatory tool to avoid missing areas of imaging-occult prostatic involvement when delivering focal SLT.