2157 - Validation of Presacral Lymph Node Delineation in Non-Operative Treatment Strategies for Locally Advanced Rectal Cancer

Purpose/Objective(s): In total mesorectal excision (TME) for rectal cancer, the upper border is the inferior mesenteric artery root. However, in preoperative radiotherapy with mandatory TME, routinely dissected presacral lymph node regions are not fully included in the clinical target volume. Contouring guidelines exist for patients undergoing preoperative radiotherapy. In nonoperative management (NOM), surgery is avoided in patients with a complete clinical response. We hypothesized that the guideline-defined presacral lymph node regions do not cover metastatic presacral lymph nodes sufficiently in NOM, and evaluated lymph node coverage using contouring guidelines and explored the optimal presacral lymph node definition for NOM.

Materials/Methods: This study included 28 patients with enlarged presacral lymph nodes between the rectosigmoid junction and promontory who underwent total neoadjuvant therapy between December 2020 and July 2023. We contoured the enlarged lymph nodes, superior rectal artery (SRA), and anterior sacral border on planning computed tomography. A diagnostic radiologist identified lymph nodes based on their size and morphology as seen on magnetic resonance imaging. We calculated the centroid inclusion rates for enlarged lymph nodes in the presacral regions defined by the guidelines and contours created using various margins from anatomical landmarks. We delineated the lymph nodes and anatomical landmarks using the Eclipse Treatment Planning System (version 16.0, Varian Medical Systems).

Results: The median age was 60 years (range, 26–84); 19 patients were male. The tumor location, T stage, and N stage were Ra/Rab/Rb, T3/T4, and N0/N1/N2 in 1/8/19, 16/12, and 1/11 (N1a/N1b: 2/9)/16 (N2a/N2b: 9/7) patients, respectively. We identified a total of 160 enlarged presacral lymph nodes (median: 5/patient; range: 1–17). The median enlarged lymph node size was 5 mm (range: 3–19). Contouring guideline-defined presacral regions included 24.4% (Radiation Therapy Oncology Group), 42.5% (Australasian Gastrointestinal Trials Group), and 79.4% (International consensus guidelines) of the lymph node centroids. The 2.0/3.0/4.0-cm margins from the anterior sacral border included 56.9%/79.4%/90.0%, respectively. +10–15-mm margins around the SRA with or without a 1-cm margin from the anterior sacral border included 92.5–97.5% and 91.9–97.5%, respectively. Both a = 12-mm margin around the SRA with and without 1-cm margins from the anterior sacral border achieved coverage beyond 95%.

Conclusion: Enlarged presacral lymph nodes between the rectosigmoid junction and promontory were insufficiently covered by guideline-defined presacral regions. A contour defined by a 12-mm margin around the SRA, with and without a 1-cm margin from the anterior sacral border, provided over 95 % coverage. Presacral lymph node delineation reassessment and anatomical optimization may be necessary for NOM-based treatment strategies.