An analysis of PSMA-PET/CT-positive lymph node distribution and their coverage by different elective nodal radiation volumes in postoperative prostate cancer patients.

Salvage elective nodal radiotherapy (sENRT) is a treatment option for biochemical persistent or recurrent patients with lymph node metastases (LN) after prostatectomy. Possible ENRT templates were proposed by the RTOG (2009), the PIVOTAL trialists (2015) and the NRG Oncology Group (2021). The goal of this study was to analyze the distribution of PSMA-PET/CT-positive LN and to compare the templates with regard to their LN coverage. Methods: We analyzed PSMA-PET/CT scans of 105 patients with PET-positive LN treated with sENRT from 2014 to 2019. All LN were mapped in an exemplary dataset, classified by region, and assessed with regard to their potential coverage by the three ENRT templates. Primary endpoint was the number of missed LN. Secondary endpoint was the number of patients with full coverage. To compare the templates, t-test and McNemar test were used. Results: Three hundred and thirty-five LN were contoured (3.19 per patient, 95%-CI 2.43 - 3.95). Most frequently, LN 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%) region. The NRG template missed less LN per patient (1.01, 31.7%) than the RTOG (1.28, 40.1%, p<.001) and PIVOTAL templates (1.19, 37.3%, P = .003). No difference was observed in the number of patients with full coverage of all LN: 52 (49.5%) with the NRG template vs. 50 (47.6%) with the RTOG ( P = .625) and 49 (46.7%) with the PIVOTAL template ( P = .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 LN and would have resulted in incomplete coverage in half of the patients. This result underlines the importance of advanced imaging, such as PSMA-PET/CT, before sENRT and shows the need for further individualization of ENRT fields.