Mammary gland tumors (MGT) are some of the most common tumors seen in veterinary clinical practice. They are the most common tumor seen in the female dog and the second most common tumor of the female cat. The risk of development of a MGT is well known to be increased in dogs that have not undergone ovariohysterectomy at an early age (Schneider etal, JNCI 1969). When compared to an intact female, the risk of development of a MGT in dogs spayed before their first heat, after their first heat and after their second heat is 0.05%, 8% and 26%, respectively. Importantly, this study did not find any reduction in MGT incidence in dogs spayed after their third heat. Early spaying of cats results in only a 50% reduction in MGT incidence, however a recent study suggests the protection is more like four-fold. The culmination of these studies strongly suggests that hormones influence the development of MGT in dogs and cats. In keeping with this tenet, the use of hormones such as synthetic progestins and progesterone has also been found to increase the risk of benign MGT formation in dogs. Though little work has been done to delineate additional risk factors, obesity and home cooked meals has been found to increase the risk of development of MGT in dogs. There are likely genetic influences concerning the etiopathogenesis of MGT in dogs and cats, however, very little research has been done in this area to date.

Pathology

The pathology of MGT in dogs and cats can be remarkably different. In dogs, approximately 50% are benign and 50% malignant, whereas in cats, 90% or more are malignant. Most MGT in the dog and cat are epithelial in origin (ie adenoma or carcinoma), however, carcinosarcomas and sarcomas are occasionally noted. Mixed mammary tumors are benign tumors of epithelial and mesenchymal origin that extremely common in the dog. A number of histologic classification schemes have been reported, however, the various histologic categories do not generally make a difference in prognosis clinically. That said, ductular carcinomas and carcino-sarcomas are well known to behave in a more malignant and metastatic fashion. In addition, grade and degree of differentiation have been found to be of prognostic importance in multiple studies. A rarely seen canine MGT subcategory termed Inflammatory Mammary carcinoma (IMC) is diagnosed by histologic and clinical criteria including the presence of a MGT, erythema and/or bruising of the overlying skin, as well as potential blockage of lymphatics in the local area with possible pitting edema of one or both hindlimbs. IMC is an extremely malignant MGT of the dog that routinely has a grave prognosis as most dogs have overt metastasis, or develop overt metastasis, within a very short period of time from presentation.

Dogs have 5 mammary glands, whereas cats have only 4 mammary glands. Interestingly, approximately 60-70% of MGT in dogs and cats occurs in the caudal most 2 mammary glands. In addition, approximately 50% of dogs as well as cats will have a solitary MGT, and the other half will present with multiple MGT. Dogs with multiple MGT on presentation will generally have tumors in a variety of locations, whereas cats with multiple MGT will have multiple tumors within a mammary chain. Though exceptions will always occur, most benign MGT in dogs are small, firm, and well circumscribed lesions that are not adherent to underlying tissues. In cats, because benign tumors are extremely rare, it is important to differentiate the typical malignant MGT from benign mammary fibroadenomatosis (BMF). Cats with BMF are generally younger cats that are intact females, and pregnancy is commonly noted. Administration of progestins can also be a common historical feature of cats with BMF. While multiple mammary glands in feline BMF are generally swollen and many times painful on palpation, single glands can occasionally be affected in older cats. BMF will generally resolve with cessation of progestin administration and/or spaying. Occasionally, surgical removal of cats with single mammae BMF must be performed if spaying does not resolve the condition.