An Analysis of the Distribution of PSMA PET/CT-Positive Lymph Nodes and Their Coverage by Different Elective Nodal Radiation Volumes in Postoperative Prostate Cancer Patients

Salvage elective nodal radiotherapy (ENRT) is a treatment option for patients with biochemically persistent or recurrent prostate cancer who have lymph node metastases (LNs) after prostatectomy. Possible ENRT templates were proposed by the Radiation Therapy Oncology Group (RTOG; 2009), the PIVOTAL trialists (2015), and the NRG Oncology Group (2021). The goal of this study was to analyze the distribution of prostate-specific membrane antigen (PSMA) PET/CT- positive LNs and to compare the templates regarding their LN coverage. Methods: We analyzed the PSMA PET/CT scans of 105 patients with PET-positive LNs treated with salvage ENRT from 2014 to 2019. All LNs were mapped in an exemplary dataset, classified by region, and assessed with regard to their potential coverage by the 3 ENRT templates. The primary endpoint was the number of missed LNs. The secondary endpoint was the number of patients with full coverage. To compare the templates, a t test and McNemar test were used. Results: Three hundred thirty-five LNs were contoured (3.19 per patient; 95% CI, 2.43-3.95). Most frequently, LNs were seen in the internal iliac (n = 94, 28.1%), external iliac (n = 60, 17.9%), periaortic (n = 58, 17.3%), common iliac (n = 55, 16.4%), perirectal (n = 26, 7.8%), and presacral (n = 19, 5.7%) regions. The NRG template missed fewer LNs per patient (1.01, 31.7%) than the RTOG (1.28, 40.1%, P < 0.001) and PIVOTAL templates (1.19, 37.3%, P = 0.003). No difference was observed in the number of patients with full coverage of all LNs: 52 (49.5%) with the NRG template versus 50 (47.6%) with the RTOG (P = 0.625) and 49 (46.7%) with the PIVOTAL template (P = 0.250). Conclusion: The NRG template showed better coverage than the RTOG and PIVOTAL templates. Nevertheless, in this cohort, it would have missed almost one third of all contoured LNs and would have resulted in incomplete coverage in half the patients. This result underlines the importance of advanced imaging, such as PSMA PET/CT scans, before salvage ENRT and shows the need for further individualization of ENRT fields.