Anal gland cancer in dogs is a type of cancer that starts in the cells of the dog’s anal glands or anal sacs. Anal sac cancer is rare and accounts for less than 1% of dog cancer cases.  The anal sacs are pouches on each side of the anus, between the internal and external anal sphincter muscles. These pouches are lined with apocrine glands that produce a strong-smelling secretion deposited along with a dog’s feces. Anal gland cancer in dogs usually affects one anal gland, but both sides can become cancerous. The most common tumor type is anal sac adenocarcinoma, a highly malignant tumor that can spread to other organs, including the sublumbar lymph nodes, lungs, and liver. Over 50% of apocrine gland adenocarcinoma cases have already undergone metastasis by the time of diagnosis.