Reclassification of Appendiceal Mucinous Neoplasms and Associated Pseudomyxoma Peritonei According to the Peritoneal Surface Oncology Group International Consensus: Clinicopathological Reflections of a Two-Center Cohort Study

Background International consensus on classifications of appendiceal mucinous neoplasms (AMNs) and associated pseudomyxoma peritonei (PMP) have been carefully made but clinicopathological associations supporting decision making remain scarce. Objective This study aimed to assess interdependence between AMNs and PMP and provide directions for clinical management. Methods This two-center retrospective cohort study reviewed patients with PMP treated with cytoreductive surgery and hyperthermic intraperitoneal chemotherapy between 2005 and 2021. The primary objective was to reassess histopathologic grade of AMNs and PMP according to the Peritoneal Surface Oncology Group International classification and to establish its interdependence. Secondary outcomes were recurrence rate, PMP grade progression, ovarian involvement, and overall survival (OS). Results Of 105 patients included, 78 (74.3%) had low-grade AMNs as the primary tumor, 8 (7.6%) had high-grade AMNs, 7 (6.7%) had mucinous adenocarcinoma (MAC), 1 (0.9%) had MAC with signet ring cells (SRC), and 11 (10.5%) had unidentified tumors. Overall, 11 patients (10.5%) had no PMP, 21 (20.0%) had acellular mucin, 56 (53.3%) had low-grade PMP, 12 (11.4%) had high-grade PMP, and 5 (4.8%) had PMP-SRC. In 11 cases (13.3%), AMNs and matching PMP grade differed. Over a 16-year follow-up, recurrence occurred in 31.8%, with three cases showing histopathologically changed PMP. Ovarian involvement was observed in 43/65 females (66.2%). Median OS was 13.8 years, and 5-year OS rates were 100%, 74.4%, 44.4%, and 20% for acellular mucin, low-grade PMP, high-grade PMP and PMP-SRC, respectively ( p