Incidence:  This is the most common tumor of rats.  Tumors may occur in males as well as in females.

Clinical Signs:  Because of the extensive mammary tissue present in rats, tumors may be found practically anywhere – in the cervical region, behind the shoulder blades, at the base of the tail, etc.   The tumors, which appear as subcutaneous lumps, may be quite extensive (A.).  Ulceration and subsequent dermatitis occur frequently.

Pathology:  Mammary tumors are usually encapsulated and benign. Histologically, both fibrous and glandular components are hyperplastic (B.).  The progression of either the glandular or fibrous component to malignancy is rare.

Diagnosis:  Mammary fibroadenomas can be diagnosed from excisional biopsies.