BACKGROUND: Appendiceal tumor has recently been treated differently from colorectal cancer. However, obtaining knowledge of this disease is difficult because of its rareness. OBJECTIVE: This study aimed to investigate the clinicopathological characteristics of appendiceal tumors in a Japanese cohort. DESIGN: This was a nationwide multi-institution retrospective observational study. SETTING: This study was conducted at the participating tertiary referral hospitals. PATIENTS: Patients with appendiceal tumor who were diagnosed between 2000 and 2017 were included. MAIN OUTCOME MEASURES: The primary outcome measured was the total survivability. RESULTS: A total of 922 patients from the 43 facilities that participated were identified. Of these, 114 patients were diagnosed with cancer other than adenocarcinomas. The remaining 760 patients, with the exception of 48 patients with unknown survival data, were eligible for the final cohort analysis. Of these 760 patients, 515 (67.8%) had mucinous adenocarcinomas and 245 (32.2%) had nonmucinous adenocarcinomas. Within the mucinous adenocarcinoma group, 267 patients (35.1%) were diagnosed as having low-grade appendiceal mucinous neoplasm. The 5-year survival rate of mucinous adenocarcinoma (83.1%) was superior to that of nonmucinous adenocarcinoma (62.0%; p < 0.01). Regarding mucinous adenocarcinoma, multivariate analysis revealed that high differentiation grade (American Joint Committee on Cancer grade 2/3), distant metastases, and R2 operation were significantly associated with a higher 5-year mortality rate. The 5-year survival rate was significantly better for low-grade appendiceal mucinous neoplasms (93.3%) than for other mucinous adenocarcinomas (72.1%; p < 0.01). LIMITATIONS: This study was limited by its retrospective study design. CONCLUSIONS: We cumulatively investigated appendiceal tumors in a multicenter retrospective study; this is the first such report from Asia. Grouping the grades as per the American Joint Committee on Cancer was useful as a prognostic indicator of appendiceal mucinous adenocarcinomas, including low-grade appendiceal mucinous neoplasm. See Video Abstract at http://links.lww.com/DCR/B282.
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