Neuroendocrine carcinoma of the apocrine glands of the anal sac in a dog.

A perianal subcutaneous tumor involving the anal sac developed in an 8-year-old male mixed Labrador Retriever dog. Histologically, this tumor showed typical features of the solid-type carcinoma of the apocrine glands of the anal sac. However, neoplastic cells were immunoreactive for cytokeratin 8, chromogranin A, vasoactive intestinal peptide, neuron-specific enolase, and synaptophysin, and negative for S-100 protein, α-smooth muscle actin, vimentin, glucagon, insulin, somatostatin, carcinoembryonic antigen, serotonin, and parathyroid hormone-related protein. Considering the distribution of chromogranin A-positive cells within the anal sac apocrine glands, this tumor was diagnosed as neuroendocrine carcinoma originating from the apocrine glands of the anal sac.