68Ga-PSMA-11 PET-Based Prostate Cancer Lymph Node Atlas Reveals Patterns of Potential Geographic Miss in Consensus Pelvic Nodal Contours

Abstract

Gallium-68 prostate-specific membrane antigen positron emission tomography (68Ga-PSMA-11 PET) enables highly sensitive and specific detection of metastatic prostate cancer. We aimed to characterize patterns of prostate cancer lymph node metastasis through development of a PSMA PET-based lymph node atlas. We then compared our results to the RTOG consensus prostate cancer pelvic nodal atlas to reveal areas of potential geographic miss during radiation therapy. From December 2015 to October 2018, 378 patients were enrolled in a prospective study evaluating the use of 68Ga-PSMA-11 PET in the staging of prostate cancer. We identified 84 previously untreated prostate cancer patients in the study cohort who underwent staging 68Ga-PSMA PET demonstrating pelvic lymph node metastasis. DICOM images were exported to the commercially available deformable registration algorithm, and PET avid lymph nodes were manually contoured. Each node was categorized by laterality, location and volume. Finally, rigid and deformable image fusion was performed, and the location of involved pelvic lymph nodes was compared to the RTOG consensus prostate cancer pelvic nodal contours. We identified 242 68Ga-PSMA-11 PET avid lymph nodes with an average volume of 2.3mL (range 0.9-55.96mL). Among all patients, external iliac and internal iliac nodal basins were most often involved with avid nodes found in 60.7% and 34.5% of patients respectively. Pelvic (32.1%), common iliac (25.0%), perirectal (13.1%), and presacral (10.7%) nodal basins were also commonly involved followed by low frequency involvement of obturator, inguinal, anterior bladder, and retropubic regions (1.2% each). In total, 57 of 242 (24%) PSMA PET avid lymph nodes were located outside of RTOG consensus prostate cancer pelvic nodal contours; missed nodes were found in 37 of the 84 (44%) patients. The most common sites of geographic miss were in the proximal common iliac, distal anterior external iliac, low presacral and perirectal regions. We used 68Ga-PSMA-11 PET imaging to generate a prostate cancer lymph node atlas in previously untreated prostate cancer patients, revealing that more than 1 in 5 involved lymph nodes may be prone to geographic miss by current consensus pelvic nodal contours. These findings suggest that 68Ga-PSMA-11 PET may be used to supplement conventional target delineation. Further research may refine clinical scenarios in which extended lymph node delineation is warranted.