Medullary carcinoma in the colorectum: a systematic review and meta-analysis.

Medullary carcinoma (MC) is a very rare variant of colorectal carcinoma (CRC). Its clinicopathologic findings are not fully elucidated. The aim of this study was to investigate the clinicopathological characteristics of MC in the colorectum through a systematic review and meta-analysis. The meta-analysis examined the incidence, age, sex, site, mismatch repair deficiency (MMRd), MMR protein expression, ARID1A expression, BRAF(V600E) mutation, KRAS mutation, and survival rate of MC. The 21469 CRCs included 462 MCs in 16 eligible studies, representing an estimated incidence of MC of 0.027 (95% confidence interval [CI] 26 0.016-0.045). MC frequently occurred in female patients and in the right colon. Lymph node metastasis of MC was significantly lower than that of poorly differentiated adenocarcinoma/undifferentiated adenocarcinoma (PDA/UDA). In addition, MC had a higher MMRd rate (0.892, 95% CI 0.758-0.956), higher BRAF(V600E) mutation rate (0.652, 95% CI 0.143-0.954) and lower KRAS mutation rate (0.171, 95% CI 0.065-0.378) than PDA/UDA and conventional adenocarcinoma. Patients with MC had significantly better overall survival rate compared to patients with PDA/UDA (hazard ratio 0.441, 95% CI 0.262-0.742). However, there was no significant difference of overall survival rate between MC and conventional adenocarcinoma patients. MC predominantly occurred in females and in the right colon, and had different molecular characteristics and behaviors compared to PDA/UDA and conventional adenocarcinoma.