Clinical relevance of positron emission tomography/computed tomography-positive inguinal nodes in rectal cancer after neoadjuvant chemoradiation

Aim Inguinal nodes may be a possible route for lymphatic spread in patients with distal rectal cancer. The outcome was examined for patients with distal rectal cancer undergoing neoadjuvant chemoradiation (CRT) and having 2‐fluorine‐18‐fluoro‐2‐deoxy‐d‐glucose (FDG)‐avid inguinal nodes using positron emission tomography/computed tomography (PET/CT) imaging. Method Ninety‐nine consecutive patients with cT2‐4N0‐2M0 distal rectal adenocarcinoma were enrolled in a clinical trial (NCT00254683) and underwent baseline PET/CT followed by 54 Gy and 5‐fluorouracil‐based CRT. After CRT, patients underwent 6‐ and 12‐week PET/CT. Patients with positive inguinal node uptake were compared with patients with negative uptake. The inguinal region was not included in the field of radiation therapy. Results Seventeen (17%) patients had baseline positive inguinal node FDG uptake. They were more likely to have the tumour closer to the anal verge (2.0 vs 4.2 cm; P = 0.001). Of these, eight (47%) demonstrated a positive inguinal uptake at PET/CT after 12 weeks from CRT. Patients with inguinal node FDG uptake after CRT (positive PET at baseline and 12 weeks) had a significantly worse 3‐year overall and disease‐free survival (P = 0.02 and P = 0.03). After a median follow‐up period of 22 months, none of these patients had developed inguinal recurrence. Conclusion Uptake of inguinal nodes at PET/CT may be present in up to 17% of patients with distal rectal cancer, particularly with ultra‐low tumours. Nearly half of these nodes no longer show uptake after CRT despite the groin area not being included in the radiation field. Persistence of inguinal node uptake 12 weeks after CRT completion may be a marker for worse oncological outcome.