Canine Mammary Tumors

Clinical Oncology Service
Veterinary Hospital of the University of Pennsylvania
Last Modified: November 1, 2001

Mammary gland ("breast") tumors are the most common type of tumor in the unspayed female dog. Breeds at risk for developing mammary gland tumors include toy and miniature Poodles, Spaniels, and German Shepherds. The average age of dogs at diagnosis is 10-11 years. There can be a single or several tumors, and they can occur in one or more glands. The last two sets of glands (the 4th and 5th glands) are most commonly affected. The tumors can be firm or soft, well-defined lumps or diffuse swellings. Tumors can be attached to underlying tissues or moveable, skin-covered or ulcerated. They can be different sizes, and they may grow slowly or quite fast. Most dogs are seen by the veterinarian for signs associated with the primary tumor and are otherwise feeling well. A few dogs are diagnosed with advanced metastasis (tumors that have spread to elsewhere in the body, such as the lungs and lymph nodes) and might be feeling ill from their tumors when they come for treatment.

The risk for developing mammary gland tumors is closely associated with exposure to the female sex hormone estrogen in the early years of development. This is a disease of female dogs and is extremely rare in males. Estrogen is necessary for normal mammary gland development. However, it may also be involved in the initial stages of cancer development that leads to tumors many years later. Estrogens may also provide continued stimulation to tumors and therefore contribute to tumor progression. Hormonal therapy is a common treatment in women with breast cancer and may also be helpful in the treatment of canine mammary gland tumors. Early spaying (ovariohysterectomy: removal of the ovaries and uterus which removes the source of estrogen) may significantly decrease the risk for tumor development. Studies have shown that spaying a dog before her first, second, or third heat cycle can significantly decrease the risk for developing mammary gland tumors later in life.

Mammary gland tumors can be either malignant (cancerous) or benign (non-cancerous) and arise from the different types of tissues (epithelial or glandular tissues, and mesenchymal or connective tissues) in the mammary gland. The most common types are tumors from the glandular tissues and include adenoma, carcinoma, and adenocarcinoma. Half of all mammary gland tumors are benign and can be treated successfully with surgery alone. The other half are malignant and have the potential for metastasis. The outcome for patients with malignant mammary gland tumors depends on several factors including tumor type, histologic grade (appearance of the tumor cells under the microscope and how similar or dissimilar they are to normal tissues), tumor size, and tumor stage (presence of regional and distant metastasis).

We recommend that all mammary gland masses are surgically removed and biopsied to determine the tumor type. Dogs with benign tumors usually do not require further treatment, but cases with malignant tumors should be staged (evaluated for metastasis by tests such as chest X-rays and abdominal ultrasound). Dogs with small (less than about 1 inch diameter) low histologic grade carcinomas and adenocarcinomas with no evidence of metastasis may be treated effectively with surgery alone. Dogs with large or invasive tumors, high histologic grade, sarcomas (tumors of mesenchymal origin), lymph node involvement and/or other sites of spread are at risk for both recurrence of the original tumor and metastasis.

Hormonal therapy in the form of ovariohysterectomy may be beneficial in unspayed dogs with carcinomas or adenocarcinomas. A recent study at the Veterinary Hospital of the University of Pennsylvania showed that dogs spayed either at the same time of their tumor removal or within two years prior to the tumor surgery lived significantly longer than dogs that remained unspayed after their tumors were removed. Chemotherapy may also be indicated in dogs with aggressive tumors. Chemotherapy has been shown to be effective in individual dogs with metastatic adenocarcinomas, but there are no large studies that prove the benefit of chemotherapy in dogs suspected of being at risk for metastasis.

Owners can play an important role in their dog's health. The protective effect of early ovariohysterectomy is substantial, and dogs that are not intended for breeding should be spayed before their first or second heat. Obesity and a high fat diet in the first year may also increase the risk for tumor development, so not overfeeding young growing dogs could be beneficial. Owners should examine their dogs at regular intervals for any lumps, bumps, or swellings and take them for yearly veterinary checkups. All lumps should be surgically removed and biopsied. Early diagnosis and treatment are crucial for a good outcome.

Canine mammary tumors have many similarities to breast cancer in women. In both, it is a disease that affects the middle-age to older patient, and the most common tumor types are the same. The treatments are similar, and patients with small tumors and early, localized disease can be cured. However, for patients with tumors that have spread elsewhere in the body, the prognosis is not good. Canine mammary gland tumors are excellent models for breast cancer in women, and clinical research studying mammary gland tumors in dogs has the potential to benefit both dogs and women.