Malignant Mammary Tumors of Female Dogs

Mammary tumors constitute the second most common form of neoplasia in dogs, exceeded only by tumors of the skin, and constitute the second most common form of neoplasia in dogs, exceeded only by tumors of the skin. The risk for development of mammary tumors in female dogs increases significantly with age with sharp increase in incidence at approximately six years of age. Female dogs spayed before the first estrus have 200 times less risk of mammary cancer than do intact ones. Those spayed after the first cycle have 12 times less risk of mammary cancer than do intact ones, whereas those permitted to have two or more cycles have 4 times less risk of cancer than do intact ones.2

The WHO classification divides canine mammary tumors into three groups: epithelial neoplasms, mesenchymal neoplsms and mixed neoplasms.4

Epithelial Malignant Neoplsms

  • Adenocarcinoma (simple or complex); tubular, papillary or papillary cystadenocarcinoma
  • Solid carcinoma (simple or complex)
  • Spindle cell carcinoma (simple or complex)
  • Anaplastic carcinoma
  • Mucinous carcinoma
  • Squamous cell carcinoma

Mesenchymal Malignant Neoplsms

  • Fibrosarcoma
  • Osteosarcoma
  • Chondrosarcoma
  • Liposarcoma
  • Combined sarcoma

Mixed Neoplasms

  • Malignant mixed tumor (carcinosarcoma)

Clinical signs include rapid growth, poorly defined boundaries, fixation to the skin or underlying tissues, and ulceration or inflammation. The presence of one or more of these signs may indicate an increased risk of underlying malignant growth.5 Surgery remains the treatment of choice for all dogs with mammary tumors except those with inflammatory carcinomas or distant metastasis. The type of surgery depends on the extent of the disease. A variety of procedures can be used to remove canine mammary tumors, and the choice of procedure is determined by the size of the tumor, whether it is fixed to surrounding tissue, the number of lesions, and the probability that the local cure can be achieved.5

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The exact causes of malignant mammary tumors in dogs are not known. It is likely, however, that multiple factors influence the environment of susceptible cells in mammary tissue causing mammary tumors, and these influences are mediated by oncogenes and tumor suppressor genes. Mammary tumors of female dogs have greatly increased in recent years, thus demanding rapid diagnosis and effective treatment in order to determine the animal survival. There is considerable scientific interest in the possible role of environmental contaminants in the etiology of mammary tumors, specifically in relation to synthetic chemical substances released into the environment to which living beings are either directly or indirectly exposed. The International Agency for Research on Cancer (IARC) has listed several chemicals as carcinogens. Some chemicals known to cause cancer are benzene, asbestos, vynil chloride, arsenic and aflatoxins.

The following contaminants have been identified in adipose (consisting of fat cells) tissue adjacent to the mammary tumors: allethrin, cyalothrin, cypermethrin, deltamethrin and tetramethrin. Once absorbed they are distributed to various tissues, but they are concentrated especially in the adipose tissue. The contamination level was observed in more aggressive tumors.1

Breed Predisposition

Several spaniel breeds and, according to some studies, the poodle and dachshunds seem to be predisposed to this condition. A recent report, however, suggested that the incidence of malignant mammary tumors was different in small breeds compared with large breed dogs. In this study, of 101 tumors (60 small breeds, 40 other), 25% of the small breed dogs had malignant tumors compared with 58% of the large breed patients. 5

Tumors may be associated with the nipple or, more often, with the glandular tissue itself. The dog has 5 pairs of glands, all of which can develop one or more benign or malignant tumors. Roughly 65% to 70% of canine tumors occur in glands 4 and 5, probably because of the greater volume of mammary tissue in these glands.


  1. Malignant mammary tumor in female dogs: environmental contaminants. Fabio H E Andrade , Fernanda C Figueiroa , Paulo R O Bersano , Denise Z Bissacot and Noeme S Rocha. In: Diagnostic Pathology 2010, 5:45doi:10.1186/1746-1596-5-45
  2. Veterinary Pathology. Thomas Carlyle Jones, Ronald Duncan Hunt
  3. Tumors in domestic animals. Jack E. Moulton
  4. Cancer in dogs and cats: medical and surgical management. Wallace B. Morrison
  5. Withrow and MacEwen's small animal clinical oncology. Stephen J. Withrow, David M. Vail

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