Inguinal CTV contouring in anal cancer: comparative validation of inguinal lymph nodes contouring guidelines and...

Contouring Inguinal lymph nodes (ILNs) in anal squamous cell cancer (ASCC) remains challenging. Accurate delineation is essential to optimize treatment efficacy with sparing of organs at risk (OaRs). Garda et al. proposed a contouring guideline based on the anatomical distribution of inguinal lymphadenopathy. This study evaluates their effectiveness in ensuring adequate ILNs coverage.

We retrospectively analyzed ASCC patients with radiologically positive inguinal lymph nodes treated with IMRT in our institution between 2015 and 2024. ILNs were mapped relative to the femoral vessels and delineated based on the Garda's guidelines. Descriptive analyses were conducted.

Among 46 patients with positive ILNs, 27 (59%) had cT3-T4 stage and 27 (59%) had bilateral inguinal nodal involvement. Overall, 203 ILNs were analyzed: 88% (178 nodes) were anteromedial to the femoral vessels. Garda's CTV encompassed 182 nodes (90%), while 22 nodes (11%) across 14 patients (30%) were not covered. Most of the uncovered nodes were in the superolateral region (n = 15, 7% of all nodes), whereas 6 nodes (3%) from six patients were identified in the inferior region. Only one node was found in the superomedial quadrant, while no nodes were located posteriorly.

These findings validate the proposed guidelines, confirming their effectiveness in reducing unnecessary irradiation of the posterolateral quadrant. However, coverage gaps highlight the need for further refinements to optimize target delineation. These findings offer disease-specific evidence to inform the harmonization of inguinal CTV delineation and support the development of standardized, consensus-based contouring protocols.