S3798 Intussusception, Infiltration, and a Mucinous Surprise: A Case of LAMN

Introduction: Acute abdominal pain often signals common gastrointestinal disorders, but uncommon neoplasms may lurk behind familiar symptoms. Low-grade appendiceal mucinous neoplasms (LAMNs) are indolent lesions with malignant potential, accounting for less than 2% of appendiceal tumors. This case illustrates an unexpected diagnosis of LAMN in a patient with nonspecific abdominal complaints and ileocecal intussusception. Case Description/Methods: A 53-year-old man presented with 4 days of diffuse abdominal pain, bloating, and watery diarrhea. He denied fever, travel, or sick contacts. Vitals were stable; labs showed WBC 11.87 without other abnormalities. Computed tomography abdomen/pelvis revealed a 4.2 cm cecal mass with ileocecal intussusception. Colonoscopy identified a necrotic 6–7 cm polypoid lesion occupying over half of the cecal lumen. Biopsies were obtained. The patient underwent robotic-assisted right colectomy; pathology revealed LAMN with negative margins and no lymph node involvement (0/21). Histology showed mucinous epithelial proliferation confined to the appendix. The terminal ileum was uninvolved. Postoperative recovery included transient high-grade fever and ileus requiring NG decompression. He was discharged on postoperative day 9. Subsequent outpatient oncology evaluation confirmed Stage IIA LAMN. No adjuvant therapy was recommended per NCCN guidelines. Discussion: Low-grade appendiceal mucinous neoplasms (LAMNs) are rare, comprising 0.2%–0.3% of appendectomy specimens, and are classified as mucinous neoplasms by the WHO. They typically present with vague gastrointestinal symptoms such as abdominal pain or altered bowel habits, and less commonly with intussusception, as in our case. Though indolent, LAMNs carry malignant potential due to the risk of peritoneal spread if ruptured. While imaging may suggest mucinous pathology, definitive diagnosis depends on histology. Surgery remains the cornerstone of treatment. Appendectomy may suffice for localized disease, but right hemicolectomy is warranted when malignancy is suspected, the appendiceal base is involved, or nodal sampling is needed, as with our patient. Preoperative biopsy of mucinous lesions is discouraged due to the risk of pseudomyxoma peritonei (PMP), a debilitating condition from mucinous ascites. There are no standardized follow-up protocols. Recurrence risk depends on rupture, margins, and peritoneal spread. Favorable long-term outcomes after complete resection underscore the importance of early recognition and appropriate surgical planning.