F18-DCFPyL PSMA-PET/CT vs. mpMRI for Localizing the Gross Target Volume for Radiotherapy in Prostate Cancer

Abstract

Local recurrences in prostate cancer (Pca) after radiotherapy generally occur in the site of dominant tumor burden, and recent evidence based on multi parametric (mpMRI) supports tumor-targeted escalation of dose to improve biochemical disease-free survival outcomes. With the recent emergence of prostate cancer targeted radiotracers, we hypothesize that PSMA-PET and mpMRI may not equally depict the pathological gross target volume (pGTV) at risk of recurrence after radiotherapy. Subjects who underwent both mpMRI and 18F DCFPyl PSMA-PET/CT prior to radiotherapy were identified from a prospective REB-approved registry. Patients who were naive to radiotherapy and patients with local recurrences were evaluated. Each patient underwent standard of care systematic biopsies prior to imaging and treatment. Sextants from prostate biopsy were analyzed on an independent basis. Per previous published work, pGTV at risk of recurrence was defined as the pathological dominant lesions with peak PCL (percentage core length) involvement and any sextants with ≽ 40% PCL involvement. Each imaging method was analyzed independently to determine spatial correspondence of the visible tumor to pGTV at risk of recurrence. Lesions scored 4-5 (PIRADSv2.1) were identified on mpMRI. For PSMA-PET, regions with uptake higher than the adjacent background with scaling SUVmin-max 0-10 were identified. Forty-seven patients with histopathological proven intermediate to high-risk Pca represented the study cohort. Of the 320 sextants, 135 were involved with PCa. Thirty-six percent (17∕47) of patients had a history of prior radiotherapy. PSMA-PET identified the pGTV in 89% (42∕47) of patients, while mpMRI yielded a higher detection rate at 98% (46∕47) (89% vs 98% p = 0.038). The only pGTV not detected on mpMRI was also missed by PSMA-PET, and had received EBRT in 2011. Of the four other pGTVs at risk of recurrence missed by PSMA-PET, three had no history of prior radiotherapy and one was treated with brachytherapy in 2005. Overall, we found no differences in imaging detection rates between previously treated and untreated patients. More detailed volumetric analyses are ongoing and will be presented. mpMRI may provide superior detection of the pGTV at risk of recurrence in comparison to PSMA-PET. Until further research is completed, clinicians should exercise caution in omitting mpMRI and utilizing PSMA-PET as the sole imaging method for GTV targeting.