Mammary tumors are the most common tumor in female dogs. Mammary tumors are more common in non-neutered female dogs and cats. The life-time risk of developing a malignant mammary tumor is over 25%. This risk is significantly reduced with early neutering with 0.5% of dogs developing mammary tumors if neutered before their first estrus, 8% between their first and second estrus, and 26% after their second estrus.

Approximately 50% of mammary tumors in female dogs are benign and 50% are malignant. In contrast, the vast majority of mammary tumors in cats and male dogs are malignant. A variety of malignancies are reported in cats and dogs including carcinomas, sarcomas, and carcinosarcomas. Multiple mammary tumors are relatively common in dogs and the fourth and fifth mammary glands are most frequently affected. Benign mammary tumors are often small, well-circumscribed and firm. Malignant mammary tumors are rapidly growing with ill-defined borders and frequent inflammation or ulceration.


Mammary tumors can be diagnosed with palpation alone. Fine-needle aspirates can be performed, but cytology rarely differentiates between benign and malignant mammary tumors and surgical treatment is the same regardless of whether the tumor is benign or malignant.


The two most common sites for metastasis are the regional lymph nodes and lungs. The regional lymph nodes should be palpated and perhaps aspirated. Chest radiographs or CT scans are done to check for metastasis to the lungs.


Surgical excision is recommended for treatment of mammary tumors in both cats and dogs. In dogs, the aggressiveness of surgery does not determine prognosis and depends on the size, location and number of tumors. Most tumors can be excised with 1 cm margins. In cats, however, the prognosis is significantly improved with a more aggressive bilateral mastectomy (or excision of both mammary chains). Neutering at the time of mastectomy is recommended in both cats and dogs, although the effect of this on prognosis is controversial.

The need for chemotherapy is determined by the tumor type. Chemotherapy, using agents such as doxorubicin, is recommended for high-grade malignant tumors, tumors with evidence of vascular or lymphatic emboli, sarcomas, and metastatic tumors.


The prognosis in dogs is dependent on clinical stage and tumor type. Benign mammary tumors are cured with surgical excision (although the development of new tumors is possible). The median survival time for dogs with malignant mammary tumors < 5 cm is approximately 2 years compared to 10-14 months for tumors > 5 cm. The use of desmopressin in the perioperative period significantly improves median survival times in dogs with grade II and III mammary carcinomas. Metastasis significantly decreases median survival time from 28 months for dogs with no metastasis to 5 months for dogs with metastasis. Tumor-related mortality varies from 15% for adenocarcinoma, 65% for ductular adenocarcinoma, and 100% for inflammatory carcinoma, carcinosarcoma, and sarcoma. The median survival times for various tumor types are 25 days for inflammatory carcinoma, 6 months for mammary sarcoma, and 21 months for adenocarcinoma.

The prognosis in cats is dependent on clinical stage, surgical aggressiveness, and degree of differentiation. Cats with small tumors (< 2 cm) can be cured with surgery alone with a median survival time of > 36 months compared to 24 months for tumors 2-3 in diameter and 6 months for tumors > 3 cm. The median survival time is significantly reduced in cats with mammary tumors that have metastasized to the regional lymph nodes (6 months compared to 18 months for non-metastatic mammary tumors). The median survival time for cats treated with subtotal mastectomy is 216 days compared to 566 days for unilateral mastectomy and 917 days for bilateral mastectomy.

Inflammatory carcinoma in a dog. This is an extremely aggressive tumor with no known effective treatment and a poor prognosis.

Benign mammary adenoma in a dog. Benign tumors are usually well-circumscribed and surgical excision is often curative.

Bilateral mastectomy in a cat with a malignant adenocarcinoma. Aggressive surgical resection is important in cats to improve survival times.