Retrospective and histopathological study on epithelial tumors in the rectum and anal canal of dogs.

Epithelial components of the rectum and anal canal comprise the rectal mucosa, anal canal mucosa, and anal glands. The clinicopathological characteristics of 111 cases of canine epithelial tumors in these regions were reviewed. Histopathological examination was performed on normal rectal and anal canal tissues and on 64 tumors. All signet-ring cell carcinomas and anal gland adenomas, as well as approximately half of the adenomas, were classified as anal canal tumors and were located within 1.5 cm of the anus. Acinar adenocarcinomas, papillary adenocarcinomas, and mucinous adenocarcinomas were predominantly classified as rectal tumors. Immunohistochemically, rectal adenomas, acinar adenocarcinomas, papillary adenocarcinomas, and mucinous adenocarcinomas frequently exhibited a CDX2+CK20+SOX2-CK7- immunophenotype, consistent with that of the proximal rectal mucosa. In contrast, adenomas in the anal canal frequently showed a CDX2-CK20-SOX2+CK7± immunophenotype, consistent with that of the rectal mucosa at the recto-anal junction. Signet-ring cell carcinomas and anal gland adenomas exhibited a CDX2-CK20-SOX2+CK7+ immunophenotype, consistent with that of the anal glands. In a hierarchical cluster analysis of tumor immunophenotypes, Group 1 (mostly anal canal adenomas) and Group 2 (rectal adenomas, acinar adenocarcinomas, papillary adenocarcinomas, and mucinous adenocarcinomas) formed one cluster, whereas Group 3 (signet-ring cell carcinomas) and Group 4 (anal gland adenomas) formed another distinct cluster. Based on these results, canine epithelial tumors in the rectum and anal canal may be categorized into a rectal mucosa-like immunophenotype (including adenomas, acinar adenocarcinomas, papillary adenocarcinomas, and mucinous adenocarcinomas) and anal gland-like immunophenotype (including signet-ring cell carcinomas and anal gland adenomas).