Low-grade appendiceal mucinous neoplasm with heterotopic ossification and pseudomyxoma peritonei: a case report

Heterotopic ossification (HO) within the gastrointestinal tract is extremely rare, and its association with low-grade appendiceal mucinous neoplasm (LAMN) and pseudomyxoma peritonei (PMP) is exceptional. We report a rare case of LAMN with visceral HO and PMP, highlighting its clinical features, pathological findings, and postoperative outcomes. A 55-year-old female presented with progressive abdominal distension over 4 months. Imaging suggested an ovarian mass with ascites. Intraoperatively, gelatinous material filled the peritoneal cavity, with findings consistent with PMP. The patient underwent total abdominal hysterectomy, bilateral salpingo-oophorectomy, omentectomy, appendectomy, and right hemicolectomy. Histopathology confirmed LAMN with ovarian involvement, dystrophic calcification, and foci of mature ossification within the appendix. The patient declined adjuvant chemotherapy but remained under surveillance. At 12 months follow-up, she remains asymptomatic with no radiological evidence of recurrence. Visceral HO is an uncommon phenomenon, possibly involving epithelial–mesenchymal transition (EMT) and bone morphogenetic protein pathways. While EMT is theoretically linked to aggressive tumor biology, the clinical course in this case has been indolent. The coexistence of LAMN, PMP, and HO underscores the complex biology of mucinous appendiceal tumors. While HO represents a rare pathological differentiation, long-term surveillance is required to determine its true prognostic significance.