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Mammary Gland Tumor in Dogs


Benign and malignant tumors of the mammary glands occur fairly frequently in unspayed female dogs, in fact they are the most common type of tumor in the group. Spaying can largely reduce the risk of developing this type of cancer, especially if the dog is spayed before it has an opportunity to go into heat. There are two main types of mammary gland breast tumors, each with several subtypes of tumor growth.


The mammary glands' function is to produce milk to feed newborn puppies. They are located in two rows that extend from the chest to the lower abdominal area; the nipples indicate their location on the trunk of the body. While this condition is more likely to occur in the female population, it does also affect male dogs, albeit rarely. When a male dog is affected by a tumor of the breast, the prognosis is much more guarded and grave.


A genetic basis is possible in some breeds, and there are frequently some genes that can be identified in dogs that are predisposed to cancer of the mammary glands. For example, toy and miniature poodles, English springer spaniels, Brittanys, cocker spaniels, English setters, pointers, German shepherd dogs, Maltese, and Yorkshire terriers have been reported to have an increased risk of developing breast or mammary tumors compared to other breeds. Median age is about 10.5 years (range, 1 to 15 years of age); it is less common in dogs younger than five.


Symptoms and Types


  • Usually slow-growing single or multiple masses in the mammary glands - about half of patients have multiple tumors
  • May have superficial loss of tissue on the surface of the skin over the mammary tissue, frequently with inflammation
  • Mass may be freely movable, which implies benign behavior
  • May be fixed to skin or body wall, which implies malignant behavior or cancer


About half of affected dogs will be diagnosed with the benign form of mammary tumors, which may be classified as complex adenomas, simple adenomas, fibroadenomas, and duct papillomas. The approximate other half of dogs to be diagnosed with mammary tumors will have a malignant form of tumor, which may be osteosarcomas, fibrosarcomas, solid carcinomas, and papillary cystic adenocarcinomas, amongst others.




Unknown, although likely hormonal or genetic.



Several diseases could account for the symptoms, so your veterinarian will want to rule them out before arriving at a conclusion. You will need to give a thorough history of your dog's health and onset of symptoms.


A complete blood profile will be conducted, including a chemical blood profile, a complete blood count, and a urinalysis. Diagnostics will include X-rays of the chest and abdomen, which may detect metastasis. It will be necessary to conduct a biopsy of the mass to fully determine its nature, whether benign or malignant. In addition, the lymph nodes will be examined, and a sample taken from them for laboratory analysis.





Surgery is the primary mode of treatment. The tumor(s) will need to be removed. Depending on the age of your dog, the type of tumor is present, and the rate at which the tumors have metastasized, your doctor may remove the tumor only, or all of the tumor along with the surrounding tissue, lymph nodes and mammary glands.


Some types of tumor are more invasive, rotting deeper into the tissue or bone, making then very difficult to remove. In these cases, partial removal of the cancerous mass and surrounding tissue may be performed, and chemotherapy may be an option, but use of chemotherapy for breast or mammary tumors is not typically used. Surgical removal of the tumor may be as effective in terms of disease-free interval as radical bilateral mastectomy is for humans.


Spaying intact bitches at the time of surgical removal of the breast or affected mammary tissue is often the preferred method, and this may enhance survival as it decreases the possibility of recurrence. However, this depends on the age of your dog as well. If your dog is older, this method will not be as beneficial. Your veterinarian will consult a veterinary oncologist (cancer specialist) for additional or updated information regarding chemotherapy in dogs.




Spaying before the first heat or estrous cycle will greatly reduce your dog's risk for developing breast or mammary tumors compared to an intact bitch; spaying before the first heat or estrus is also suggested to markedly decrease the likelihood of developing mammary tumors. Early spaying is the best method for prevention of this form of cancer. If spaying is delayed until before the second heat or estrous cycle the risk of developing breast or mammary tumors compared to intact bitches increases to 8 percent.


If spaying is delayed until after the second heat or estrous cycle, there is a 26 percent risk of developing breast or mammary tumors compared to an intact bitch, and if spaying takes place after the dog has reached 2.5 years of age, there is no sparing effect on the risk of developing breast or mammary tumors.


Living and Management


Your veterinarian or a consulting veterinary oncologist will recommend a treatment plan that will include managing your dog's health at home, with follow-up progress visits to the veterinarian or oncologist. Physical examinations and chest X-rays will be required one, three, six, nine and twelve months following the initial treatment to check for recurrence or changes in the tissue.


The prognosis and course the disease will take varies with the type of breast or mammary tumor your dog has (for example, whether the tumor is benign or malignant), as well as the size of the tumor, and the presence or absence of metastasis.


Surgery for tumors that have not spread may be curative. For example, median survival after surgical removal of the breast or mammary tissue (mastectomy) with tubular adenocarcinoma is 24.6 months. Median survival after surgical removal of the breast or mammary tissue (mastectomy) with a solid carcinoma is 6.5 months. Benign tumors have an excellent prognosis after mastectomy. Carcinomas that are less than five centimeters in diameter also usually have a good prognosis for remission, if the excision is complete. However, regional lymph-node involvement, which can be confirmed by microscopic evaluation, makes the prognosis worse and full recovery may not be possible.


You should never ignore a breast or mammary nodule, or adopt a wait and see attitude towards it. A breast or mammary lump should never be left in place and observed, as it can quickly metastasize when it is the malignant form, spreading throughout the body and becoming untreatable before its severity is recognized. Early detection and thorough surgical intervention is best.


Always make a plan for evaluation and possible surgical removal of any lump in the mammary gland(s), if you have a female dog that you are leaving intact for possible breeding. On the other hand, if breeding is not essential for your dog, early spaying before the first heat or estrus will markedly decreases the likelihood of her developing breast or mammary gland tumors.



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