EP803 - Overall survival of Metastatic Appendiceal Mucinous Adenocarcinoma: A Contemporary Population-Based Study.

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Purpose/Background: The prognosis of patients with appendiceal mucinous adenocarcinoma varies widely based on the histological grade and stage of disease. Evidence is heterogeneous and outdated. Historic data from Japan and the USA demonstrate the 3-year overall survival (OS) to be ~50% for patients with metastatic disease from 4 previous studies of the 1990s to early 2000s. The purpose of this study was to assess outcomes of patients with metastatic appendiceal mucinous adenocarcinoma in a modern treatment era.

Methods/Interventions: All individuals with stage M1 (at diagnosis) appendiceal mucinous adenocarcinoma from 2018 to 2021 in the Surveillance, Epidemiology and End Results (SEER) Database were included. Histopathology details, patient characteristics, stage of disease, treatments and long-term outcomes were determined using linked administrative databases. The primary outcome was OS. Unadjusted and adjusted survival analyses were employed. Age, sex, grade of disease, receipt of chemotherapy and surgical procedure were controlled for in the multivariable analyses.

Results/Outcomes: Five hundred and ninety-three patients were identified. Sixty-one percent were female and roughly a quarter of patients were in each of the following age groups: < 50, 50-59, 60-69 and 70+ years. The distribution of grade of disease was 29% well-differentiated, 22% moderately-differentiated, 10% poorly-differentiated and 39% not assessed. Most patients had T4 (74%) and N0 (64%) stage of disease. The classification of M status was M1X (distant metastasis present, site not otherwise specified) 7%, M1A (intraperitoneal acellular mucin) 7%, M1B (intraperitoneal mucin containing tumor cells) 42%, and M1C (metastasis to non-peritoneal sites) 44%. Seventy-two percent of patients received systemic chemotherapy, and 80% underwent surgery, including 26% having cytoreductive surgery and intraperitoneal chemotherapy. The 3-year OS for the entire cohort was 62%. Based on metastatic site, three-year OS was 60%, 81%, 62% and 60% for M1X, M1A, M1B, and M1C disease (p=0.303), respectively. Poor differentiation was associated with decreased survival (HR 2.34 [95%CI 1.26-4.33] vs well differentiation, p=0.029). Patients who received chemotherapy or underwent surgery had improved survival (HR 0.68 [95%CI 0.48-0.97] for receipt of chemotherapy vs not, p=0.031 and HR 0.34 [95%CI 0.22-0.51] for surgery vs no surgical intervention, p< 0.001). Specifically, undergoing cytoreductive surgery and intraperitoneal chemotherapy was associated with increased survival (HR 0.42 [95%CI 0.25-0.70], p< 0.001).

Conclusion/Discussion: Compared to historical cohorts, we demonstrated an improved 3-year OS of 62% in this modern cohort, likely due to improved treatments such as cytoreductive surgery with intraperitoneal chemotherapy in selected patients. Acellular mucin was rare though acted protective when present. Grade of disease is an important prognostic factor and should be obtained. These data will help inform shared decision making and guide patient management.