Lower inguinal lymph node metastases in anal cancer: prevalence, predictors, and exact locations

Abstract Background In anal caner, the lower inguinal subregion is the most common site of regional lymph node metastases (LNM) occurring outside the borders of the major contouring guidelines. With the aim of informing future contouring guidelines, the present study investigated the exact locations and predictors of lower inguinal LNM. Methods Baseline FDG-PET-CTs from a consecutive population-based series of anal cancer patients ( n  = 190) were used to identify LNM in the lower inguinal subregion, defined as ≥ 10 mm below the saphenofemoral junction (SFJ). Lymph nodes with an FDG uptake above the mediastinal blood pool were considered metastatic irrespective of size. The distances from the center of each LNM to the SFJ, the great saphenous vein (GSV), and the femoral vessels were measured. The characteristics of patients with lower inguinal LNM were compared with other patients using non-parametric tests. Results Lower inguinal LNM were identified in 8.4% of the patients. For every lower inguinal LNM the distance was shorter to the GSV vein (median 7 mm; range 4–19) compared to the femoral vessels (median 21 mm; range 9–37). Only one lower inguinal LNM was located more than 10 mm medial to the GSV. A distal margin of 2.5 cm from the SFJ covered all lower inguinal LNM in 97 of 103 (95%) inguinal regions with LNM and in 375 of 380 (98.7%) inguinal regions in all patients. Patients with lower inguinal LNM had larger tumors and more often stage T4, which might open for an individualization of the inferior border of the elective clinical target volume (eCTV). Conclusions In the lower inguinal subregion LNM follow the GSV rather than the femoral vessels. To capture the different anatomical compartments of the inguinal region future contouring guidelines should present separate recommendations for different inguinal subregions. In addition, they should be aiming at providing individual risk-adapted recommendations rather than a ‘one size fits all’.