Mammary tumors in cats are most common in older (average 11 years), intact females. Cats spayed before 6 months or 12 months of age have a 91% or an 86% reduction, respectively, in the risk of mammary carcinoma development compared to intact cats. Parity does not affect feline mammary carcinoma development. Unlike in dogs, in cats the two anterior or thoracic glands are more frequently involved than the posterior glands.
There are also differences in both biologic behavior and histology of mammary tumors in dogs and cats. Approximately 90% of mammary tumors are malignant in cats, and cats have fewer complex and mixed tumors than dogs. Most feline mammary tumors are adenocarcinomas, with tubular or papillary types more common than solid or mucoid types. Mixed mammary tumors and sarcomas are less commonly diagnosed than carcinomas. Benign tumors of the feline mammary gland are relatively infrequent. The TNM clinical staging system is used for mammary tumors in cats as well as in dogs.
A distinct entity called feline mammary hypertrophy has been noted in cats. It affects primarily young, actively cycling, or pregnant cats. It also has been seen in neutered cats, including older males given exogenous progestational drugs (megestrol acetate). The disorder is marked clinically by the rapid growth of one or more mammary glands.
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