Challenges in Diagnostic Imaging of Appendiceal Signet Ring Cell Carcinoma: A Case Report.

Appendiceal signet ring cell carcinoma (ASRC) is an exceptionally rare and aggressive malignancy, representing < 0.1% of gastrointestinal neoplasms. Its nonspecific imaging manifestations frequently mimic inflammatory conditions such as appendicitis, leading to delayed diagnosis. We report a diagnostically challenging case of a 75-year-old woman with recurrent right lower abdominal pain over 6 months. Initial ultrasonography revealed appendiceal wall thickening (19.5 mm diameter) without layered structure loss, suggestive of chronic appendicitis. Follow-up computed tomography (CT) demonstrated cystic distal appendiceal dilatation (5.6 × 2.5 cm) with mural nodular enhancement. Despite three ultrasound examinations and CT scans, no definitive malignant features were identified preoperatively. Intraoperative findings revealed an 8.0 cm enlarged appendix with serosal infiltration, and histopathology confirmed the diagnosis of high-grade appendiceal mucinous neoplasm (HAMN) with associated signet ring cell carcinoma. This case highlights the limitations of conventional imaging in detecting ASRC, particularly when mucinous components obscure tumor invasion. The key to distinguishing ASRC from appendicitis lies in recognizing its progressive nature on serial imaging. Discriminatory features include: rapid and asymmetric wall thickening (often > 10 mm), the evolution from a preserved to a lost "target ring sign" on ultrasonography, and the presence of mural nodular enhancement on CT.