Patterns of Inguinal Lymph Node Metastases in Anal Cancer: Recommendations for Elective Clinical Target Volume (CTV) Delineation

Abstract

In the era of conformal radiotherapy for anal cancer, the optimal CTV for inguinal lymph node irradiation remains uncertain. The purpose of this study was to document the location of radiographically involved inguinal lymph nodes in patients with anal canal cancer and to propose guidelines for elective inguinal CTV delineation. Patients with squamous cell carcinoma of the anal canal with inguinal lymph node metastases presenting to our institution between 2003 and 2017 were identified from a departmental database. Inguinal lymph nodes were considered involved based on at least one of the following: >1.5 cm on computed tomography (CT) or magnetic resonance imaging (MRI), FDG avidity greater than background liver on positron emission tomography (PET), or positive histology from lymph node biopsy. Distances from the center of the involved lymph node to the closest adjacent femoral vessel and to the inferior extent of the primary tumor were measured on a pre-treatment diagnostic imaging study. Position of lymph nodes relative to the right femoral vessels was recorded on a clock face with 12:00 anterior, 3:00 medial, 6:00 posterior, and 9:00 lateral. The anatomic location of each lymph node was transferred onto a representative CT scan of a patient in treatment position and compared to the inguinal lymph node CTV in the Australasian Gastrointestinal Trials Group (AGITG) consensus guidelines for intensity-modulated radiotherapy for anal cancer. We identified 41 patients with 81 positive inguinal lymph nodes. Median lymph node size was 16.1 mm (range 7-59 mm) in short axis and 21.4 mm (range 10-65 mm) in long axis. Median distances from the nearest femoral artery and vein were 19.8 mm (range 8-40 mm) and 17.1 mm (range 8-46 mm), respectively. Median distance from the inferior aspect of the primary tumor was 47.9 mm (range 4-108 mm). Relative to the femoral vessels, lymph nodes were located at 12:00 (n=7); 1:00 (n=28); 2:00 (n=35), 3:00 (n=5); 4:00 (n=1); 10:00 (n=1); 11:00 (n=4). No lymph nodes were identified in the 5:00-9:00 positions (lateral or posterior to the vessels). Volumes contoured according to AGITG consensus guidelines covered 70% of lymph nodes anteriorly and 85% of lymph nodes medially. Margins from the nearest femoral vessel required to cover 90% and 95% of lymph nodes were 29 mm and 30 mm anteriorly and 25 mm and 26 mm medially, respectively. Inferior borders of the inguinal CTV required to cover 90% and 95% of lymph nodes were 18 mm and 12 mm, respectively, from the inferior aspect of the primary tumor. Published guidelines for CTV delineation of the inguinal lymph node region in anal cancer may result in inadequate coverage of high-risk areas. Updated contouring guidelines based on the present study would ensure coverage of areas at high risk for harboring metastatic disease and may reduce toxicity by sparing low-risk areas.