Chapter 29 Mammary Gland Neoplasia
The mammary gland tumor (MGT) is the most common tumor in the female dog and the third most common neoplasm in the female cat. MGTs have been reported in the male dog and cat but are rare.
ETIOLOGY AND RISK FACTORS
• Predisposing risk factors proposed but incompletely established include dietary intake of red meat and obesity at 1 year of age.
• Although virus-like particles have been identified in feline and canine MGTs, their role as a causative agent has not been established.
• Routine administration of medroxyprogesterone acetate, proligestone, or other progestins for estrus prevention or dermatologic therapies increases the risk of MGTs in dogs and cats.
• About 50% of canine mammary carcinomas have estrogen and progesterone receptors at levels lower than in benign or normal mammary gland tissues. Only benign and well-differentiated adenocarcinomas appear to be hormonally sensitive in female dogs.
• Dogs with benign MGTs have more than a threefold risk of subsequently developing a mammary malignancy of a different cell type.
CLINICAL SIGNS
• A mass or swelling develops in the ventral thoracic or abdominal region. The mass is typically associated with the mammae but may appear distant to the mammary gland.
DIAGNOSIS
Establish a definitive diagnosis, determine local invasion, and determine the stage of the MGT.
Signalment
Dogs
• Most MGTs are reported in sporting breeds (pointers, English setters, and spaniels), poodles, Boston terriers, and dachshunds.
History
• The owner may have noticed the tumor or it may have been an incidental finding during routine examination.
Physical Examination
• Depending on the time of recognition, the tumors may be small and movable, lobular and firm, fixed to the body wall, and ulcerated.
• Dogs with inflammatory carcinoma have diffusely swollen glands with poor demarcation between normal and abnormal tissue, which may be confused with mastitis. In mastitis, the swelling is more localized and occurs after estrus, whelping, or false pregnancy.
• Hindlimb edema, popliteal lymphadenopathy, and ulceration may be noted in cats with IMC up to 4 months after mammary mass excision.
• In young, intact female cats, mammary hypertrophy can be mistaken for MGT. Mammary hypertrophy resulting from either endogenous or exogenous progesterone stimulation can be differentiated readily by case history and, if necessary, histologic examination.
• Carefully examine dogs for evidence of lameness and bony swelling. If these signs are present, radiograph the affected area and obtain a nuclear bone scan if needed. Mammary carcinoma is one of the most common primary tumors with metastases to the skeleton.
Diagnostic Imaging
• If caudal mammary glands are involved, use abdominal ultrasonography or abdominal radiographs to evaluate the iliac lymph nodes.
Cytologic Evaluation
• Varying degrees of epithelial atypia and inflamma-tion make definitive diagnosis and tumor grading difficult.
• Cytology may be useful to differentiate inflammatory carcinoma from mastitis in dogs and mammary hypertrophy from mammary carcinoma in cats.
Histologic Evaluation
• Tumors are graded by cell type (epithelial, mesenchymal, or mixed) and by degree of differentiation. High-grade tumors are typically poorly differentiated.
• Histopathologic evaluation is the preferred method of definitively diagnosing and grading mammary masses. The histopathologic diagnosis does not alter current treatment recommendations for MGT (except for IMC, as explained in “Contraindications”). Thus, surgical excision is performed for both diagnostic and therapeutic purposes. A wide excision is essential to ensure complete removal of tumor cells.
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