Low-Grade Appendiceal Mucinous Neoplasm Mimicking Pelvic Inflammatory Disease: A Case Report.

Low-grade appendiceal mucinous neoplasms (LAMN) are rare clinical entities. These lesions are characterized by an expansive growth pattern and a significant propensity for peritoneal dissemination, potentially leading to pseudomyxoma peritonei. Preoperative diagnosis of appendiceal mucinous neoplasm remains a formidable challenge, particularly in female patients, as clinical presentations often mimic common gynecological pathologies. This report describes the case of a 63-year-old female with a history of schizophrenia and previous pelvic surgeries who presented with generalized colicky abdominal pain, vomiting, and diarrhea. Initial diagnostic workup, including computed tomography, suggested a massive pyo/hydrometra and severe pelvic inflammatory disease, for which she received intravenous antibiotic therapy. Despite treatment, persistent pain led to a follow-up imaging study revealing a significant pelvic collection. During exploratory laparotomy, free mucin was identified in the right iliac fossa, and intraoperative biopsy reported acellular mucinous implants. A right hemicolectomy with a terminal ileostomy was performed after identifying a perforated appendix with a tumor base exceeding two centimeters. Histopathological analysis confirmed a low-grade appendiceal mucinous neoplasm, staged as pTis N0 pM1a (Stage IVA), with 16 negative lymph nodes and clear surgical margins. This case illustrates the diagnostic complexity of LAMN in elderly women, where mucinous collections can accurately simulate intrauterine or pelvic inflammatory processes such as massive pyo/hydrometra. It highlights the necessity of including appendiceal neoplasms in the differential diagnosis of complex pelvic masses to avoid iatrogenic rupture and optimize surgical outcomes. Precise surgical technique and long-term oncological surveillance remain critical, as the risk of late recurrence and progression to pseudomyxoma peritonei persists even in cases with acellular mucin and negative margins.