Primary signet ring cell carcinoma of the rectum

Signet ring cell carcinoma (SRCC) is a rare subtype of colorectal cancer, defined histologically when ≥50% of tumour cells contain abundant intracytoplasmic mucin with a peripherally displaced nucleus, producing signet ring morphology. SRCC is disproportionately diagnosed in younger adults and is associated with diffuse infiltrative growth, peritoneal dissemination and poorer outcomes than conventional adenocarcinoma. We report a man in his early 30s who presented with acute bowel obstruction due to a rectal stricture. An inflammatory aetiology was initially considered because biopsies were superficial and non-diagnostic, there was no strong clinical history to suggest malignancy, and carcinoembryonic antigen was within the normal range. The diagnosis became apparent only at laparotomy, with subsequent histopathology confirming primary rectal SRCC. This case illustrates how SRCC, a rare subtype of colorectal cancer, can masquerade as an inflammatory disease and evade superficial biopsies and standard biomarkers despite its poor-prognosis biology.