Performance of Multiparametric MRI and PET Imaging in Identifying and Localizing Locally Radiorecurrent Prostate Cancer in a Phase I/II Trial of Focal Salvage HDR

Abstract

<h3>Purpose/Objective(s)</h3> Salvage local therapy (SLT) with brachytherapy or SBRT for local radiorecurrence (LRR) of prostate cancer after prior radiotherapy (RT) is increasingly being used due to growing data demonstrating the safety and efficacy of SLT, as well as the availability of advanced imaging to identify LRR, such as multiparametric MRI (mpMRI) and molecular PET imaging (18F-fluciclovine or 18F-DCFPyL PSMA). Further, advanced imaging has led to investigation into focal SLT to improve the therapeutic ratio of reirradiation. Yet, data are limited on the ability of mpMRI and PET to detect LRR and how well they localize the area(s) of recurrence in the prostate to guide focal SLT. Our objective was to determine the ability of mpMRI and PET imaging to detect the presence and full extent of the areas of involvement in men with LRR. <h3>Materials/Methods</h3> We conducted a cohort study of men with LRR of prostate cancer enrolled in the FSHARP phase I/II trial of salvage focal HDR brachytherapy at our institution. Workup included PET/CT, mpMRI of the prostate, and a biopsy documenting the LRR prior to enrollment. Descriptive statistics were used to compare mpMRI and PET detection and localization with biopsy findings to determine the sensitivity/false negative rate (FNR) of each imaging modality, and the frequency with which biopsy reveals disease that would have been missed if the imaging abnormality alone was used to define the focal salvage HDR target. <h3>Results</h3> A total of 40 men were included in this analysis. Median age was 70.5 yrs (Range: 58-83). Median time from initial RT was 9.5 yrs (2-28). Initial NCCN risk groups were as follow: 13 had low risk, 21 had intermediate risk and 2 had high risk. 26 were treated with conventionally fractionated photon RT, 7 with LDR brachytherapy, 4 with protons, 2 with SBRT and 1 with neutron therapy. Regarding recurrence biopsy, 24 men had MRI/TRUS fusion biopsy and 16 had TRUS biopsy alone. A median of 14 cores were obtained and a median of 6 cores were positive. Gleason grade grouping (GG) was as follows: 1 had GG 1, 11 had GG 2, 12 had GG 3, 6 had GG 4, and 4 had GG 5. The median number of MRI target lesions was 1 (0-4), and the median number of lesions on PET was 1 (0-2). 38 men had fluciclovine and 2 had PSMA PET. The table depicts the sensitivity and FNR of each imaging modality, and frequency of pathologically identified disease outside of the imaging target. <h3>Conclusion</h3> Our study emphasizes that though mpMRI and PET are valuable tools for restaging and target volume delineation in LRR of prostate cancer, there are limitations to their use. These imaging modalities cannot be relied upon alone if pursuing focal salvage due to a high FNR, and because the full extent of disease is rarely identified, leading to the risk of marginal misses.