S3826 Appendiceal Neuroendocrine Tumor Masquerading as Endometriosis: A Rare Case

Introduction: Appendiceal endometriosis is rare, representing only about 1% of all endometriosis cases. However, it is identified in approximately 8%–13% of patients with deep infiltrating endometriosis, particularly in those with more severe disease. Neuroendocrine tumors are the most common type of appendiceal neoplasm and can sometimes be mistaken for endometriotic lesions, especially when multiple pelvic endometriosis implants are present. Case Description/Methods: A 26-year-old woman with prior medical history Insulinoma presents to the office with a 1- month history of right sided abdominal pain. She had went to the ER multiple times with cross-sectional imaging consistent with a 6.0 cm complex left adnexal cyst, with MR appearance compatible with an endometrioma. After multiple other tests including EGD, HIDA scan being inconclusive, she underwent an exploratory laparotomy which showed severe endometriosis involving right and left paraureters, anterior rectum, rectovaginal wall, right and left hypogastric, left uterine artery, and, fibroadipose tissue on the anterior abdominal wall. The appendix was also removed which contained tumor involving the tip of the appendix, 0.5 cm in greatest dimension consistent with well differentiated neuroendocrine tumor pT3, G1. Discussion: This case report underscores the importance of thoroughly evaluating the entire peritoneal cavity—including the appendix—during laparoscopy in patients with deep infiltrating endometriosis (DIE). If suspicious lesions are observed, an appendectomy is warranted. Prior research has suggested that endometriosis may act as a precursor to malignancy, as genetic mutations identified in endometriosis-associated cancers have also been detected in nearby endometriotic tissue. Neuroendocrine tumors (NETs) are identified in approximately 0.16%–2.3% of all appendectomy specimens. These tumors generally have a favorable prognosis, with a 5-year survival rate exceeding 95%. Appendectomy is the recommended treatment for appendiceal NETs smaller than 1 cm. Right hemicolectomy is advised for tumors located at the base of the appendix, those larger than 2 cm, or when there is lymphovascular invasion, mesoappendiceal involvement, lymph node metastasis, high mitotic activity, peritoneal dissemination, angioinvasion, or tumors classified as intermediate to high grade. In cases where only the proximal surgical margin is involved, local conservative re-excision may be considered as an alternative to more extensive surgery.