Several types of mammary tumor have been reported in the dog. The two most common are the benign fibroadenoma, or mixed mammary tumor, and the malignant adenocarcinoma. These occur with about equal frequency. The inflammatory carcinoma is a much less common type of tumor; one study reported inflammatory carcinoma to be 8% of mammary tumors reported. A given bitch may have more than one tumor type.
In humans, mammary neoplasia is hormone dependent, with estrogen the most common hormone involved in tumor development. In dogs, hormones play a role in mammary tumor development, but the exact pathogenesis is not clear. Bitches spayed before going through heat have a greatly reduced risk of developing mammary neoplasia when aged. The protective effect gradually declines as the bitch is allowed to cycle. Historically it was thought that once a bitch had cycled four times or had reached 2.5 years of age, all protective effect of OHE was gone. Recent work from Belgium suggests that some protective effect remains, even in bitches as old as 9 years.
Receptors for estrogen, progesterone, and other hormones have been identified in mammary tumor tissue, suggesting action of those hormones on the neoplastic tissue. Receptor type and number are not consistent in dogs and, as a general rule, the number of receptors declines as the malignancy of the tumor increases.
Body conformation and diet may contribute to mammary tumor development in dogs. In one study, dogs judged to have been thin when young were significantly less likely to develop mammary neoplasia when aged. Dogs fed homemade diets high in beef and pork were at increased risk; this may mimic the effect of diet high in animal fat or alcohol that has been implicated in mammary neoplasia in women.
Mammary Neoplasia: Signalment
Average age at the time of diagnosis of mammary neoplasia in dogs is 10 years. Mammary neoplasia is rare in animals younger than 5 years, regardless of whether the animal is spayed or neutered. There is no difference in age at diagnosis between benign and malignant tumors.
Predisposition of various breeds for mammary neoplasia is difficult to discern from the literature. Some studies show no effect of breed on incidence, and other studies do show an effect of breed or a general increased incidence in purebred versus crossbred dogs.
History and clinical signs
Dogs with mammary neoplasia most commonly present with a palpable or visible mass. The two caudal most glands on each side are most commonly affected. Tumors usually are palpable as very firm, round masses that may or may not be freely movable under the skin (). They usually are not painful to the bitch. Benign masses cannot be differentiated from malignant masses by palpation alone.
Bitches with benign masses usually exhibit no other clinical signs. Bitches with malignant masses are more likely to exhibit inflammation resulting from local spread of the tumor in the mammary tissue and may show clinical signs referable to spread of the tumor to other tissue, such as coughing in bitches with spread of the tumor to the lungs. The most common sites of spread of tumor are the lungs and lymph nodes. Lack of evidence of spread of the tumor on physical examination does not mean the tumor has not spread; more than half of the lung tissue must be affected before changes in lung sounds are likely to be heard.
Inflammatory carcinomas are very aggressive tumors. Bitches present with a mammary mass that often has arisen within days to weeks. The mammary area involved is swollen and painful and may be ulcerated. This tumor type is more commonly associated with systemic signs of disease, such as weakness, lack of appetite, increased thirst and urination, and edema of the limbs.
Tentative diagnosis is made on physical examination. Because tumor type cannot be differentiated by physical examination, a thorough physical examination should be performed to look for spread of the tumor elsewhere in the body. Bloodwork (complete blood count and serum chemistry profile) and urinalysis should be performed. Radiographs should be taken of the chest and possibly the abdomen. Ultrasound of the abdomen may allow identification of spread of the tumor to abdominal organs. Ultrasound of the mammary tumors themselves does not allow differentiation of benign from malignant tumors.
Collection of cells from the mass may be diagnostic. Fine-needle aspirate, in which a small needle is inserted into the mass and cells are removed, usually is not diagnostic. Mammary tumors do not shed cells easily, and lack of malignant cells in a sample retrieved in this manner cannot be equated with lack of malignancy of the tumor. Far better is submission of a portion of the mass or of the entire mass for histopathology, in which the tumor is preserved, sliced, stained, and examined by a pathologist. If the entire tumor was removed, the edges of the tissue removed also can be examined to ensure that the entire tumor was removed.
Treatment of Mammary Neoplasia
Surgical removal is the treatment of choice. If the tumor has not spread locally, surgery may permit complete removal of the mass. If the tumor has spread locally or to other tissues in the body, surgical removal of the bulk of the tumor enhances success of other therapies, such as radiation or chemotherapy. It has not been well defined in dogs whether more extensive surgery (e.g., removal of all the mammary glands on the side with one affected gland and all associated lymph nodes) is likely to provide better survival time than is less extensive surgery (e.g., removal of the mass only). The surgeon usually makes the decision as to the extent of surgery.
If the dog has not already been spayed, it may be beneficial to spay her at the time of surgical removal of the tumor. Published studies do not agree when this question is approached statistically. Because dogs with mammary neopla-sia usually are aged and beyond peak fertility, they should be spayed if only to prevent pyometra ().
Chemotherapy, radiation therapy, and immunotherapy all have been described for treatment of mammary neoplasia. Chemotherapeutic agents do not consistently reach clinically significant concentrations in tumor tissue. Chemotherapy and radiation therapy usually are used as secondary treatments after surgical removal of the mass.
Hormone therapy, either for treatment or prevention, is not commonly used in dogs. The antiestrogen tamoxifen has been used in dogs but with limited success. Because it can bind to and activate estrogen receptors, it may cause undesirable signs of heat in some treated bitches. Hair loss also may be seen. Tamoxifen cannot be used in intact bitches, in which it may cause vaginal prolapse () or pyometra ().
Prognosis is dependent on tumor type, tumor size, rate of tumor growth, and spread of the tumor ().