Patterns of inguinal lymph node metastases in anal canal cancer and recommendations for elective clinical target volume (CTV) delineation

Purpose: Optimal clinical target volume (CTV) for inguinal lymph node irradiation in anal cancer remains uncertain. This study documents the location of radiographically involved inguinal lymph nodes and proposes guidelines for CTV delineation. Materials and Methods: Patients with anal canal squamous cell carcinoma with inguinal lymph node metastases were identified. Criteria for lymph node involvement included: >15 mm short axis or suspicious morphology on CT or MRI, increased avidity on 18-FDG-PET, or positive biopsy. Distances from center of involved nodes to femoral vessels and inferior pubic symphysis were measured. Results: Forty patients with 79 inguinal lymph nodes were included. Relative to right femoral vessels, nodes were located: 12:00 (n = 6); 1:00 (n = 28); 2:00 (n = 35), 3:00 (n = 5); 4:00 (n = 1); 10:00 (n = 1); 11:00 (n = 3). No nodes were identified lateral or posterior to vessels. Published AGITG guidelines covered 68% of nodes anteriorly and 85% medially. Margins from nearest femoral vessel to cover 95% of nodes were 30 mm anteriorly and 26 mm medially. Inferior margin to cover 95% of nodes was 14 mm below inferior pubic symphysis. Proposed borders include cranial, where external iliac vessels leave bony pelvis; caudal, 14 mm below inferior pubic symphysis; posterior, posterior border of femoral vessels; lateral, lateral border of femoral vessels; anterior, 30 mm margin on femoral vessels and medial, 26 mm margin on femoral vessels, including radiographically suspicious nodes. Conclusions: Published guidelines for inguinal CTV in anal cancer may result in inadequate coverage of high risk areas. Updated guidelines based on this study ensure coverage of at-risk areas.