Prognosis of anal canal adenocarcinoma versus lower rectal adenocarcinoma in Japan: a propensity score matching study.

PURPOSE Anal canal adenocarcinoma (AC) is rare and its surgical outcomes and prognostic factors (PFs) are not well understood. The aim of this retrospective study was to identify the characteristics and PFs of AC, using population-based data in Japan. METHODS Patients with AC (n = 390) or lower rectal adenocarcinoma (LR) (n = 12,477) diagnosed between1991 and 2006 were enrolled in this study. We compared the clinical- and patient-related factors of the two diseases and then examined propensity score matching, overall survival (OS), and PFs. RESULTS AC tended to develop more often in women and in patients of advanced age. Macroscopically, AC was of an unclassified type and microscopically, it was of high-grade histological types such as mucinous adenocarcinoma, poorly differentiated adenocarcinoma (por), or signet-ring cell carcinoma (sig), with a high frequency of inguinal node metastasis (P < 0.05). The 5 year OS rates were 56.9% for AC and 67.9% for LR (P = 0.002). The PFs of AC were a high-grade histological type (por/sig), T, N, and M. CONCLUSIONS AC has a significantly worse prognosis than LR. Moreover, the AC lymph node metastatic sites for AC, especially the inguinal nodes, are different from those for LR.