Prognostic Factors for Canine and Feline Mammary Cancer

Rodney L. Page MS, DVM; Diplomate ACVIM (Internal Medicine/Oncology)
College of Veterinary Medicine, Cornell University



1.  Incidence - Second most common neoplasm in dogs. Most common malignant tumor in dogs.

162-198/100,000 dogs at risk or 260/100,000 sexually intact dogs at risk.

25 - 50 % of all canine neoplasms.

Breeds: High Risk: poodle, English Spaniel, English Setter, Terriers; Low risk: Boxer, Chihuahua

2.  Etiology and Risk Factors

Hormonal Influence:

Age at ovariohysterectomy.

Body Conformation - Lean conformation at 9-12 months correlated with reduced risk.

BRCA1 gene mutations - presently unknown but suspected in ~10% of Human breast cancer.

3.  Biologic behavior:

Location: About 65 % of the tumors occur in the caudal 2 glands (#4,5). 75% are solitary.

Hormonal Receptor Status

ER (estrogen receoptor) and PR (progesterone receptor) are found in canine mammary tumors.

~ 40-60% ER (+) and predicts for favorable outcome compared to ER (-).

General clinical behavior

Approximately 50-60 % of mammary neoplasms are benign.

Approximately 25 % of the malignant tumors may result in mortality.

4.  Histopathology

90% epithelial origin:

70% adenocarcinoma (solid, tubular, papillary)

20% ductal (inter or intralobular)

Mammary tumors may have one cell type present (simple) or have ductular and secretory cells

(complex) or have both elements of carcinoma and sarcoma (mixed).

Further subdivisions include:

Infiltrating or noninfiltrating

Inflammatory - 4% of all mammary gland tumors

5.  Treatment

Surgery - Lumpectomy, mammectomy, regional mastectomy, unilateral or bilateral radical mastectomy. Concurrent OHE is of benefit.

Other - Chemotherapy, Immunotherapy, Radiotherapy, Hormonal - No long-term survival benefit identified lthough high-risk patients should be considered for adjuvant therapy trials.

Tamoxifen should be evaluated in ER (+) tumors.

Inflammatory type mammary cancer associated with coagulopathy. Therefore, surgical manipulation not recommended.

6.  Prognostic factors


Age, breed and reproductive

Location or Multiplicity

Duration of tumor

Type of surgery (simple, regional, radical)


Recurrence @ 1 yr

Recurrence @ 2 yr


Size (Gilbertson, et al)



<3 cm diam



> 3cm



Lymph node Inv(Gilbertson, et al)



Histologic Type (Benjamin et al)




Lobular (AdenoCa, Myo)








All others




Histologic Stage all types (Gilbertson, et al)

0 Locally confined (in situ)


20 %

I Local Invasion

Well Diff









II Vascular/Lymphatic Invasion





Mammary gland neoplasia is the third most common form of cancer in cats. It represents approximately 10% of all non-lymphoid forms of cancer. It is more common than in women but less common than dogs. The average age of occurrence is approximately 10 yrs. Cats that are neutered have a significantly reduced risk of developing mammary cancer compared to sexually intact cats. Siamese cats are reported to be at increased risk of mammary cancer.


The etiology is unknown although retroviral particles have been identified within breast tissue it is not associated with development. The feline BRCA1 gene has not been identified to date so the contribution of mutations to this gene to the overall incidence of breast cancer in cats is unknown. Hormonal status influences the incidence of breast cancer as mentioned above. Feline breast tissue does not routinely express high estrogen receptor levels. Progesterone receptors are expressed at significant levels in feline mammary cancer. The reason and significance of this is unknown but itdoes influence treatment options (tamoxifen, an antiestrogen is not likely to be effective in cats). Cats treated with progesterone compounds such as ovaban for estrus control have a much higher rate of cancer development.

Clinical Behavior

Approximately 85% of feline mammary neoplasms are histologically malignant. Cancer may arise from either glandular or ductal tissue and tumors are categorized as either solid (35%), tubular (50%) or papillary (15%). Fifty percent of cats may have multiple glands develop tumors simultaneously. Local extension may be significant with rapid spread to regional lymph nodes and a relatively high rate of distant metastasis if not detected and treated early.

Diagnosis and Staging

Diagnosis is generally trivial with a mass arising in the glandular tissue associated with the mammary gland surrounding the nipple. Cats can develop a mammary gland hyperplasia that may resemble multiple mammary tumors but this condition generally involves multiple glands, is uniform and is hormonally responsive.

Staging evaluation of a cat with mammary cancer should include survey radiographs of the chest, laboratory evaluation and any other assessments as dictated by a thorough physical exam. Staging criteria based on prognostic information is described below.


Surgery - Radical, unilateral or staged bilateral mastectomy is the procedure often recommended due to malignant nature of most mammary tumors in cats and the likelihood of multiple tumors within different glands. Radical resection produced better local control but was not better than regional or simple mastectomy in terms of overall survival. Therefore, local or regional mastectomy may be considered if the tumor involves the most caudal or cranial glands on one side, if there are favorable prognostic factors (see below) or if follow-up therapy (chemotherapy) will be instituted. Draining lymph nodes should be excised. The axillary node may be difficult to identify but the inguinal lymph node should always be removed and submitted for histopathologic evaluation.

Chemotherapy - Adria mycin alone or Adriamycin/Cytoxan combined have been associated with significant reductions in tumor volume in the majority of cats treated. This implies the potential for pre-operative treatment of feline tumors to significantly downstage the local disease. Other chemotherapeutic compounds have not been evaluated thoroughly. Improved survival following adjuvant chemotherapy in cats with mammary cancer has also not be definitively confirmed.

Hormonal therapy - since the majority of cats do not possess significant estrogen receptors, antiestrogenic therapy is not likely to be of value.

Radiation therapy - little information exists to determine the effectiveness of radiation therapy for feline mammary cancer. Response of human breast cancer to irradiation, however, indicates a potential role in feline mammary cancer.


A large body of new evidence has been accumulated in the last 3-5 years indicating several new prognostic features for feline mammary cancer outcome. The known prognostic indicators are listed below.

1.  Size is prognostic

Median Survival

< 2 cm diam

36+ months

2 - 3 cm

24 months

> 3 cm

  6 months

2.  Lymph Node involvement (ln (+) - 6 months vs. LN (-) - 18 months median survival)

3.  Histologic parameters (MI (mitotic index), cell pleomorphism, tubule formation)



1 yr survival

  (following simple/regional resection)

Well differentiated









4.  Biologic Parameters

Proliferation status (Ki-67 ag expression)

High proliferation - 12% cats survived 1 yr after nonradical resection

Low proliferation - 88% survived 1 yr.


Small, well differentiated mammary tumors without lymph node invasion should have prolonged survival after a regional or simple but complete resection. Adjuvant chemotherapy is not indicated in this group. Larger, more poorly differentiated or high grade and invasive tumors with lymph node extension will require significant, multimodality management regimens including surgery and chemotherapy. Pre-operative chemotherapy may be of benefit for large, invasive tumors. The prognosis for the latter category remains guarded to poor.