Mammary Gland Tumors

Carcinomas are the most common type of mammary gland cancer in both dogs and cats, and approximately 50% of all canine mammary tumors are malignant. Most tumors appear in dogs between 8-10 years of age (similar to cats). The exact etiology of mammary cancer is unknown. It is known that spaying a cat or dog before her first estrus (heat) cycle dramatically decreases the risk of mammary tumors (0-5% risk vs. 26%).

It is general consensus among veterinary oncologists that no mammary lump should ever be left in place and “observed.” Needle biopsies are frequently inconclusive and misleading in this case. Biopsy with wide excision is the primary recommendation. These results help determine the best treatment plan for the patient.

Mammary tumors can develop either as a single mass or as multiple masses within the mammary gland.  Dogs and cats have multiple mammary glands, all of which can develop one or more benign or malignant tumors.  Benign tumors are typically well-defined and firm upon examination whereas the clinical symptoms of malignant tumors include rapid growth, ill-defined boundaries, ulceration, and/or inflammation.  Inflammatory carcinomas are typically rapidly growing, affect multiple mammary glands, and are characterized by firmness, warmth, observable swelling, redness/rash on the skin, thickening, and signs of pain.  Extensive swelling (lymphedema) of a limb next to the affected gland(s) can also be observed.
Tumor size in both dogs and cats has been found to be prognostic. Dogs with tumors less than 5 cm in diameter (cats less than 2 cm) have significantly longer survival times than animals with larger tumors.


As with other cancers, it is important to assess the extent of cancer in a particular patient. Complete staging usually requires:

  1. Complete blood count
  2. Serum chemistry panel
  3. Urinalysis
  4. Chest radiographs (x-rays)
  5. Abdominal radiographs
  6. +/- Ultrasonography
  7. Cytology of regional lymph nodes
  8. Review and assessment of biopsy results. This includes complete vs. incomplete resection borders as well as cell types and degree of tumor invasiveness.

The traditional method of treatment for mammary tumors in dogs and cats is surgery. Treatment by local excision with wide and deep (at least 2 cm) margins is advised. This means that the surgeon needs to take some healthy tissue from around the tumor to ensure the best chance for complete removal. Remember, even though the incision may be quite large, it heals in the same amount of time as a “short one.” There appears to be no advantage of a bilateral chain mastectomy (over local excision) unless multiple mammary glands are involved. Local and wide excision for tumors that are less than 5 cm (2 cm for cats) may be curative.

As in human medicine, chemotherapy can be considered for patients that have a high risk for recurrent or metastatic disease. These patients would include:

  1. those in which complete tumor resection is not possible
  2. those having sarcomas vs. carcinomas
  3. those having large and/or multiple tumors
  4. those whose biopsy results indicate aggressive disease
  5. those with local or distant metastasis

Chemotherapeutic agents that have been used as adjunct therapy include doxorubicin, cyclophosphamide, mitoxantrone, and carboplatin. Most dogs can be treated without serious side effects and owner acceptance of costs and potential for complications is generally good. Overall, anti-estrogen drugs (e.g. Tamoxifen) have failed to show clinical benefit in veterinary patients. Radiation therapy has not yet been evaluated as a treatment modality in animals but may hold promise as it does in humans.

Survival times for patients with invasive carcinomas are difficult to predict. Published medical reports generally state ranges of 6.5-30 months for patients on adjuvant chemotherapy.


Good Prognostic Factors Poor Prognostic Factors Indifferent Prognostic Factors
Tumor is less than 3 cm in diameter Tumor is greater than 3 cm in diameter Age
Well defined tumor boundaries Poorly defined tumor boundaries, ulceration Breed
Negative lymph nodes Positive lymph nodes Weight
Carcinoma-well differentiated, complex, tubular/papillary Carcinoma-poorly differentiated, simple, solid, anaplastic, inflammatory carcinoma, sarcoma Type of surgery (simple or radical)
Tumor grade I Tumor grade III Number of tumors
Positive estrogen and progesterone receptors Negative estrogen receptors Glands involved
Index of proliferation


Low Ki-67


Index of proliferation

High AgNOR

High Ki-67


P53 gene mutation