Mammary Cancer

Cancerous Tumors that originate along the mammary chain. It is the canine equivalent of human breast cancer.

Click here to show/hide more detail

In 50% of cases mammary lumps are benign (not cancer). The remaining 50% are malignant tumors that are classified based on the embryonic origin of the cancerous cells they contain, determined by microscopic analysis of the tumor biopsy.

Benign tumors

  • Adenoma (glandular)

  • Benign mixed tumor

  • Duct papilloma

  • Lipoma

Malignant tumors

  • Carcinomas (including: Non infiltrating, Simple and Complex, Tubulopapillary, Anaplastic (dedifferentiated), Spindle cell, Squamous cell, Mucinous, and Lipid-rich)

  • Sarcoma

  • Carcinosarcoma

  • Adenocarcinoma

  • Malignant mixed tumor

Tumors can be mixed, having cells from different origins, and different tumors in the same animal can be different types.

Signs and Symptoms

Lumps arising along the mammary chain. Most commonly in the two sets of mammary glands closest to the hindquarters.


Specific causes of mammary cancers unknown. Generally cancers occur as a result of disruption of the normally highly regulated processes of cell division and differentiation.

Risk Factors

  • Bitches intact after the first or second estrus cycle.

  • Possibly obesity at 9-12 mo. of age but studies conflict.

Median age of onset is 10-11 years. Occurrence in males is less than 1% of all mammary cancers. 70% of bitches 15 years or older will have a clinically detectible mammary tumor and 100% will have microscopic tumor foci.

Diagnostic Tests

  • Palpation of the area along the mammary chain and associated lymph nodes.
  • Biopsy, sizing and histologic (microscopic) analysis of all lumps.
  • Staging of malignancy; i.e. chest X rays & abdominal ultrasound to look for spread to lungs and lymph nodes.
  • Cells removed from the lump with a needle may be examined (fine needle aspiration) but this is not as accurate as a biopsy.

Must rule out cysts, mastitis (infection of breast tissue), dermatologic (skin) diseases and tumors that are not mammary cancer but may be located over the mammary gland such as mast cell tumors and soft tissue sarcomas.

One report suggests that the most important assessment of mammary tumors may be their behavioral malignancy (will the tumor act malignant, i.e. spread and potentially be fatal). The report says that behavioral malignancy can be predicted with 80% accuracy by clinical examination (tumors that cross the mid line, are not freely movable or are obviously infiltrative eg. swollen lymph nodes) and confirmed by biopsy (evidence of local invasion). The report suggests 90% of canine mammary tumors are behaviorally benign.

Treatment Guidelines

Note: Treatment of animals should only be performed by a licensed veterinarian. Veterinarians should consult the current literature and current pharmacological formularies before initiating any treatment protocol.

Treatment depends on results of the diagnostic tests. Other factors to be considered are behavioral malignancy, prognosis, age and physical condition of the bitch.

Measures that can be taken:

  1. Surgery (lumpectomy, radical mastectomy, lymph node removal) except in the case of inflammatory carcinoma, where it does not increase survival.
  2. Anti inflammatory drugs (piroxicam) + radiation in the case of inflammatory carcinoma may afford palliative care.

Unlike in humans, radiation and chemotherapy are typically not very effective in dogs, although they are used in some cases.


Factors indicating a good prognosis:

  1. Small size of lump(s) (less that 3 cm) (highlights the importance of early detection)
  2. Absence of metastases (i.e. no spread to regional lymph nodes or lungs)
  3. Well differentiated (specialized) cells in tumor (lack of anaplasia: “primitive” cells, like stem cells)
  4. Few signs of cell division (mitotic figures) in tumor.
  5. Clean edges of tissue removed in lumpectomy (differentiated, non-dividing cells at margins)
  6. Absence of invasion of tumor into lymphatics or blood vessels or lymphoid invasion of tumor
  7. Absence of skin ulceration, swelling
  8. Tumors that are not attached to under or overlying tissues (suggests less rapid growth of tumor)

Factors irrelevant to prognosis:

  1. Spaying in adult hood
  2. Age
  3. Breed
  4. Weight
  5. Number of glands involved
  6. Number of tumors


Some recommend life time nutritional supplements for predisposed dogs: Inositol hexaphosphate, 1-3 beta glucan, fucose. These have some effect on cancer in isolated cells in the laboratory or in preliminary animal experiments, but there has been no definitive research proving their effectiveness.

Twenty-six percent of intact bitches are expected to eventually have mammary cancer. If a bitch will not be used for breeding, spaying prior to her first estrus cycle can reduce her risk of cancer to 0.5%. Spaying after one estrus cycle reduces the risk to 8%.


Hellmen E et al. Prognostic Factors in Canine Mammary Tumors: a Multivariate Study of 202 Consecutive Cases. Vet Pathol 1993. 30:20-27. three bones

Canine Mammary Tumors. Ohio State College of Veterinary Medicine. three bones

Hoskins JD. Prognosis, treatment of canine mammary tumors. 2008 two bones

Mammary Tumors in Dogs and Cats: What Do I Tell the Owner. Histovet Surgical Pathology. two bones

Mammary Cancer. one bone

Cancer and Tumors - Mammary Tumors. two bones

Overview of Mammary Tumors. Merck Veterinary Manual. four bones

Mammary Tumors in Cats and Dogs. American College of Veterinary Surgeons. three bones

Brooks WC. Mammary Tumors in Dogs. two bones

Mammary Gland (Breast) Cancer in Dogs. one bone

1 bone 2 bones 3 bones 4 bones (full description of ratings)

Support Groups

This support group is for all types of canine cancer.

Yahoo Group Canine Cancer one bone

1 bone 2 bones 3 bones 4 bones (full description of ratings)

“Living with a Disorder” Journal Entries