Quick Facts at a Glance

• Most common tumor of female dogs
• Third most common tumor of female cats
• Very uncommon in male dogs or cats
• 50% of canine and greater than 80% of feline mammary gland tumors are malignant (cancerous)
• Adenocarcinoma (ADC) is the most common malignant mammary tumor in dogs and cats
• Early ovariohysterectomy (spay), prior to the first heat cycle, is protective in both dogs and cats, but has greater benefit in dogs
• Mammary gland tumors can occur anywhere along the mammary chain
• Multiple tumors occur in more than 50% of patients
• Ulcerated tumors or those adherent to the underlying body wall are likely to be malignant
• 25% of feline mammary tumors are ulcerated

Are there different types of mammary gland tumors in dogs?

Approximately 50% of mammary gland tumors in dogs are benign (non-cancerous). These tumors include primarily the benign mixed tumor (fibroadenoma), and to a lesser degree simple adenomas and benign mesenchymal (type of cell) tumors (fibroma). These benign tumors may be solitary or occur as multiple primary tumors. They may also coexist with malignant mammary tumors.

Most malignant mammary tumors are classified as epithelial (glandular). Solid carcinoma and ductular carcinoma are most common. Inflammatory or anaplastic carcinoma comprises between 5-10% of mammary gland tumors. These are the most difficult to treat as there is generally widespread lymphatic invasion in the overlying skin resulting in marked edema (fluid swelling) and inflammation and they are often associated with local and systemic clotting abnormalities.

Mammary gland sarcomas comprise only 3% of mammary gland tumors, but tend to have a high metastatic (spread) rate, primarily to lungs. These tumors include osteosarcoma, fibrosarcoma and chondrosarcoma.

Are there different types of mammary gland tumors in cats?

Between 80% and 90% of mammary gland tumors in cats are malignant. ADC of the mammary gland is most common in cats, with solid carcinoma and sarcoma being less common.

Lobular hyperplasia (rapid growth of cells) is a benign condition that can occur in intact female cats of any age (average age 8 years). This condition is due to the development of cysts or dilated ductuals within the mammary gland and which takes on the form of obvious masses in the affected gland. Fibroepithelial hyperplasia is a benign condition of mammary glands of young, intact, cycling or pregnant cats. This condition can also develop in cats treated with megestrol acetate (ovaban). The glands can become extremely enlarged and edematous, with edema affecting the skin of the back legs. Spay after the swelling has been controlled is the treatment of choice for both of these benign conditions of mammary gland tissue.

What diagnostic tests should be performed?

Complete excisional biopsy and histopathologic analysis of mammary gland tumors is the method of choice to confirm the diagnosis. Unlike other neoplasms, cytologic evaluation of mammary gland tumors is not recommended as it generally yields equivocal results.

Before excisional biopsy, chest x-rays should be done to evaluate for metastasis (spread). Three views of the chest are required. In addition, the overall health of the patient should be assessed with a complete blood count (cbc), platelet count, biochemical profile and urinalysis.

Radical mastectomy or lumpectomy?

The treatment of choice consists of wide and deep surgical excision of all abnormal appearing tissue. If there is only one small circumscribed tumor present, simple lumpectomy is adequate, whereas more extensive or multiple tumors require radical mastectomy for complete excision. Bilateral (both chains) radical mastectomy does not offer any survival advantage over simple lumpectomy for small, solitary tumors in dogs. Bilateral, radical mastectomy is preferred in all cats because feline mammary gland tumors have a high likelihood of recurrence and development of satellite lesions (spread) via regional lymphatic vessels.

What about lymph nodes?

Careful palpation of the inguinal (groin) and axillary (arm pit) regions should be performed prior to surgery to determine if lymph node enlargement exists. Wide resection of the draining lymph nodes (nodes that are close in proximity) and surrounding tissue should be performed and also submitted for histopathologic analysis. The presence of metastasis to lymph nodes significantly worsens the prognosis even when adjuvant chemotherapy is administered.

Histopathology: more is better

Whenever possible, the entire mammary gland tissue sample should be submitted for histopathologic analysis. In order to adequately assess prognosis, one needs to know the tumor type, the presence or absence of lymphatic invasion, if the tumor extends to the surgical margins and if there is lymph node involvement. Tumor borders can only be assessed if all tissue is submitted. If multiple tissue pieces must be submitted, each sample should be carefully labeled and the borders tagged so the pathologist can determine tumor orientation. This will facilitate a second surgery if cancer cells are found to extend to the tissue edges. If indicated, a second surgery should be performed as soon as possible to reduce the likelihood of tumor metastasis.

Will this patient develop metastatic disease or recurrence?

Tumor size is the single most important prognostic indicator for mammary ADC. Canine mammary gland ADC measuring greater than 3 cm in diameter has an 80% chance of recurrence. Feline mammary ADC measuring greater than 2 cm in diameter also has a high likelihood of recurrence and metastasis.

Chemotherapy: When to treat and what to use

Chemotherapy should be considered post-operatively for all canine mammary sarcomas, and for all high-grade ADC with or without lymphatic invasion or lymph node involvement. Chemotherapy may also be beneficial for palliation of inflammatory carcinomas with or without surgery. Chemotherapy should be considered post-operatively for all malignant feline mammary tumors.