Low-Grade Appendiceal Mucinous Neoplasm in a Patient With Ulcerative Colitis Detected on Routine Surveillance Colonoscopy: A Case Report and Review of Literature.

Appendiceal mucocele (AM) is characterized by the dilation of the appendix due to the accumulation of mucinous material within its lumen, often discovered incidentally on imaging. Symptoms of AM are typically nonspecific or absent, but patients may present with clinical features mimicking appendicitis or an adnexal mass. Ultrasound and CT imaging of the abdomen and pelvis are key in establishing the diagnosis. Early diagnosis is crucial, as a neoplastic mucocele can rupture, leading to pseudomyxoma peritonei (PMP), a clinical syndrome characterized by diffuse intra-abdominal gelatinous ascites with mucinous peritoneal involvement. While patients with ulcerative colitis (UC) are at increased risk for colorectal cancer, appendiceal neoplasms are rarely reported in this group. This case presentation involves a 70-year-old female patient with a history of UC who presented for a routine surveillance colonoscopy in the fall of 2024. She was diagnosed with mild UC in 2006 and had been in clinical remission since December 2018, and a colonoscopy performed in 2021 showed no mucosal inflammation. However, during the most recent surveillance colonoscopy in 2024, the appendiceal orifice was noted to be bulging, with mucus extruding, suspicious of a mucinous appendiceal neoplasm. Cold forceps biopsies were obtained, and histology showed only inflammatory changes in the appendiceal orifice. A contrast-enhanced CT of the abdomen and pelvis revealed a 3.2 x 2.0 cm tubular cystic structure originating from the base of the cecum, consistent with an AM, without signs of rupture or nodularity. The patient underwent a robotic-assisted partial cecectomy and appendectomy, and pathology confirmed a well-differentiated, low-grade appendiceal mucinous neoplasm (LAMN).