The frequency of mammary neoplasia in different species varies tremendously. The dog is by far the most frequently affected domestic species, with a prevalence ~3 times that in women; ~50% of all tumors in the bitch are mammary tumors. Mammary tumors are rare in cows, mares, goats, ewes, and sows. There are differences in both biologic behavior and histology of mammary tumors in dogs and cats. About 45% of mammary tumors are malignant in dogs, whereas ~90% are malignant in cats, and dogs have a much higher number of complex and mixed tumors than do cats.

Etiology

The cause of mammary tumors is unknown in any species except mice, in which an oncornavirus is causative in certain inbred strains. Hormones play an important role in the hyperplasia and neoplasia of mammary tissue, but the exact mechanism is unknown. Estrogen or progesterone receptors (or both) have been reported on mammary tumor cells in animals; these may influence the pathogenesis of hormone-induced mammary neoplasia as well as the response to hormone therapy.

Genetic and nutritional effects on mammary neoplasia have been identified in mice and some people but are not as well understood in dogs or cats. It has been demonstrated that the consumption of red meat, obesity at 1 yr of age, and obesity a year prior to diagnosis are associated with an increased risk of mammary gland tumors in intact or ovariohysterectomized dogs. To date, investigations of oncogenes and tumor suppressor genes have not proved helpful at the clinical level. From a practical view, all mammary tumors should be regarded as potentially malignant regardless of the size or number of glands involved. Spread of mammary carcinomas in both dogs and cats is primarily to regional lymph nodes and lungs. In dogs, 5–10% of mammary carcinomas may produce skeletal metastases, primarily in the axial skeleton, but also in long bones.

Canine Mammary Tumors

Mammary tumors in dogs are most frequent in intact bitches; they are extremely rare in male dogs. Ovariectomy before the first estrus reduces the risk of mammary neoplasia to 0.5% of the risk in intact bitches; ovariectomy after 1 estrus reduces the risk to 8% of that in intact bitches. Bitches neutered after maturity have generally been considered to have the same risk as intact bitches. However, questions remain regarding the impact of ovariohysterectomy at the time of tumor excision. Questions also remain about the timing of such surgery relative to survival. In one study, dogs spayed <2 yr prior to tumor excision lived 45% longer than either intact dogs or those spayed >2 yr prior to tumor excision.

The 2 posterior mammary glands are involved more often than the 3 anterior glands. Grossly, tumors appear as single or multiple nodules (1–25 cm) in 1 or more glands. The cut surface is usually lobulated, gray-tan, and firm, often with fluid-filled cysts. Mixed mammary tumors may contain grossly recognizable bone or cartilage on the cut surface.

More than 50% of canine mammary tumors are benign mixed tumors; a smaller percentage of malignant mixed tumors are seen. In the latter, epithelial or mesenchymal components, either singly or in combination, may produce metastases. Histologically, canine mammary gland tumors have been classified by the World Health Organization as carcinomas (with 6 types and additional subtypes), sarcomas (4 types), carcinosarcomas (mixed mammary tumors), or benign adenomas. This classification scheme is based on the extent of the tumor, involvement of lymph nodes, and presence of metastatic lesions (TNM system); it includes unclassified tumors and apparently benign dysplasias. In addition to tumor size and the status and timing of neutering, special stains (including those for the KIT receptor and AgNOR) may have prognostic value.

Feline Mammary Tumors

Mammary tumors in cats are most common in older (average 11 yr) intact females. Spaying at an early age, especially before the first estrus, has a sparing effect and reduces the risk, but the degree of protection is less precisely documented than that for dogs. The 2 anterior or thoracic glands are more frequently involved than the posterior glands.

Histologically, 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 and account for only ~10% of these tumors. 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 (see p 1279). 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.

Diagnosis

A mammary tumor is usually suspected on detection of a mass during physical examination. The length of time the mass has been present is usually unknown, but the rate of growth may be helpful in determining prognosis. Palpation of the regional lymph nodes can help determine the extent of spread. Thoracic radiographs, preferably 3 views (a ventral-dorsal and 2 laterals), should be taken to detect pulmonary metastases. Fine-needle aspirates may differentiate between inflammatory and neoplastic lesions but may lead to erroneous conclusions and delay of surgery. The diagnosis is determined by histopathology and is important in defining treatment and prognosis.

Treatment and Prognosis

Mammary tumors are treated surgically, although there is no consensus as to the best procedure. Removal of the tumor alone (lumpectomy), simple mastectomy (removal of the affected gland only), modified radical mastectomy (removal of the affected gland and those that share lymphatic drainage and associated lymph nodes), and radical mastectomy (removal of the entire mammary chain and associated lymph nodes) all have their proponents. In dogs, the more involved procedures have not prolonged survival compared with the others, and the advantages of the simpler procedures are obvious. In cats, radical mastectomy has increased the disease-free interval but not survival time.

In theory, the use of anticancer drugs to combat micrometastatic disease (adjuvant chemotherapy) is a reasonable consideration. However, chemotherapy has not been proved an effective treatment for mammary tumors in dogs. Part of the difficulty of evaluating the response to adjuvant chemotherapy relates to the fact that only about half of the canine mammary tumors diagnosed as malignant on histopathologic examination actually behave that way. A combination of doxorubicin and cyclophosphamide has been used with limited efficacy in cats. Neither radiation therapy nor antiestrogenic compounds have been effective.

It has recently been demonstrated that use of the NSAID piroxicam as a single agent is beneficial in the treatment of inflammatory mammary carcinoma, a subtype of mammary tumor in dogs that has been very difficult to manage by surgical or medical means. This is in step with current interest in metronomic low-dose therapy in other malignancies for an anti-tumor and anti-angiogenic effect.

The prognosis is based on multiple factors. Most mammary tumors in dogs that are going to cause death do so within 1 yr. Sarcomas are associated with shorter survival times than carcinomas. Other factors, including size of tumor, lymph node involvement, and nuclear differentiation, also affect the prognosis. In cats, tumor size is important; cats with tumors >3 cm in diameter have a median survival time of 6 mo, but cats with tumors <2 cm in diameter have a median survival time of >4 yr.

Last full review/revision July 2011 by Robert C. Rosenthal, DVM, PhD, DACVIM (Small Animal, Oncology), DACVR (Radiation Oncology)