Primary Signet Ring Cell Adenocarcinoma of the Appendix Masquerading as Acute Appendicitis: A Diagnostic Pitfall.

Background & Objective: Primary signet ring cell adenocarcinoma (PSRCA) of the appendix is a rare and aggressive malignancy accounting for approximately 4% of all appendiceal neoplasms. These tumors often present at an advanced stage with nonspecific clinical and radiological findings and frequently mimic acute appendicitis. Hence, histopathological evaluation remains essential for definitive diagnosis. Case Presentation: A 64-year-old female presented with clinical features suggestive of acute appendicitis. Radiological evaluation revealed appendicolithiasis. The patient underwent appendectomy, and histopathological examination demonstrated a signet ring cell adenocarcinoma diffusely infiltrating through the appendiceal wall into the mesoappendix (pT3). Lymphovascular invasion was present. Both the proximal and mesenteric margins were positive for tumor infiltration. No lymph nodes were submitted in the specimen. Immunohistochemistry showed tumor cells positive for CDX2, CK20, and MUC-1 and negative for CK7, supporting a primary lower gastrointestinal tract origin and favoring an appendiceal origin. Conclusion: PSRCA of the appendix is an aggressive malignancy often presenting at an advanced stage and mimicking acute appendicitis. Accurate histopathological evaluation is essential, as the diagnosis has important therapeutic implications, including the need for further oncological management beyond appendectomy.