Using Molecular Imaging to Create Guidelines for the Optimal Clinical Target Volume for Para-Aortic Lymph Nodes in Prostate Cancer

Abstract

<h3>Purpose/Objective(s)</h3> Molecular imaging enables improved understanding of the anatomic regions involved with metastatic prostate cancer (PC) due to the improved sensitivity compared to conventional imaging. Consequently, there is increasing awareness that PC metastasizes relatively frequently to the para-aortic (PA) lymph node region, compelling some radiation oncologists to electively cover this area if it is determined to be at high risk or if nodes are already involved. However, the optimal contouring boundaries are unknown. The objective of this study is to use molecular imaging to develop guidelines for contouring the PA clinical target volume (CTV) in patients with PC. <h3>Materials/Methods</h3> We conducted a retrospective cohort study of patients with PC undergoing 18F-fluciclovine (FLU) or 18F-DCFPyL PSMA PET/CT at our institution. Images of patients with PET-positive PA lymph node involvement were imported into the treatment planning system and the avid nodes were contoured. The radial distance from the epicenter of each node was measured in all dimensions relative to the inferior vena cava and aorta, and superiorly relative to the left renal vein. The PA CTV was delineated per the 2021 NRG/RTOG post-operative endometrial/cervical cancer atlas, as principles of these malignancies are frequently used to delineate this region. Descriptive statistics were used to assess the performance of this gynecologic atlas and generate recommendations for contouring in PC patients. <h3>Results</h3> 246 men had molecular PET imaging at our institution from 10/2016 to 1/2022 (48% FLU, 52% PSMA). 31 men (13%) had evidence of PA nodal metastasis. In these patients, 16% had de novo, 52% recurrent, and 32% castration-resistant metastatic PC. The median number of positive PA lymph nodes per patient was 1 (IQR 1-3.5). The distance from central vasculature and left renal vein are shown in the Table. Only 66% of PA nodes were covered using the left renal vein as the superior border and 75% were covered using 1.5 cm above the left renal vein. Overall, the gynecologic atlas missed 25% of nodes (57% due to left sided and 30% due to superior misses), suggesting unique target delineation should be considered for PC patients. We recommend expanding the PC PA CTV in the left lateral and superior dimensions, with detailed recommendations to be included in our presentation. <h3>Conclusion</h3> We used molecular PET imaging to determine the anatomic patterns of PA metastasis to develop contouring guidelines for creating a PA CTV in patients with PC. Although the optimal patient selection and clinical benefits of PA RT remain uncertain, our results will aid in delineating the optimal target and are currently being validated in an independent dataset. Table. Distance from central vasculature and left renal vein (cm)