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What is a Mammary Gland Tumor? - Hastings Veterinary Hospital

What is a Mammary Gland Tumor?

A mammary tumor is a tumor originating in the mammary gland; it is a common finding in older female dogs that are not spayed. Mammary glands in dogs are associated with their nipples and extend from the underside of the chest to the groin on both sides of the midline. Mammary tumors can be small, simple nodules or large, aggressive, metastatic growths. With early detection and prompt treatment, even some of the more serious tumors can be successfully treated.

Are Mammary Gland Tumors Common?

Mammary gland tumors are the most common tumors in dogs. Among unspayed females the risk of a mammary tumor is 26 percent, this is three times the risk of breast tumors in women. Most mammary tumors occur in females over the age of 6, the average being 10 years old. It is not uncommon for a female to have multiple tumors. If she has developed one, she is three times more likely to develop a second.

Symptoms and Risks of Mammary Gland Tumors:

The principal sign is a painless lump or mass. Most lumps occur in the larger glands closest to the groin. A mass may be large or small, with boundaries that are distinct or indefinite. Some lumps are freely movable, while others are fixed to the overlying skin or underlying muscle. Occasionally, the mass ulcerates the skin and bleeds.

There are few cancers that are as easily prevented as mammary cancer in dogs. The risk of breast cancer is almost completely eliminated in dogs that are spayed before their first heat. The risk of malignant mammary tumors in dogs spayed after their first heat increases significantly. Pregnancy and lactation appear to have no influence on mammary cancer risk.

Breeds at increased risk include:





Fox Terriers

Boston Terriers


Unfortunately, it is very difficult to determine the type of tumor based on its appearance. A biopsy or tumor removal and analysis are almost always needed to determine if the tumor is benign or malignant, and to identify what type it is. A needle aspirate can be performed to help preliminarily diagnose the illness. A needle aspirate may be a helpful pre-operative procedure in many cases, but it is the biopsy that will ultimately determine the extent of the disease.

Tumors which are more aggressive may metastasize and spread to the surrounding lymph nodes or to the lungs. A chest x-ray, abdominal ultrasound, and physical inspection of the lymph nodes will often help in determining this. Approximately 50% of malignant mammary tumors will have already spread by the time of surgery.


Prompt surgical removal of any mammary tumor is recommended, unless the dog is very old or has other medical conditions that would rule out this option. If a surgery is in the early stages of this disease, the cancer can be totally eliminated in over 50% of the cases having a malignant form of cancer. Sometimes, only the mass itself will be removed. This is usually only done when the tumor is localized. However, because of how this cancer spreads, it is generally advisable that radical surgery be performed which entails the removal of the mass and all the mammary tissue.

A radical mastectomy in dogs means all the breasts, the skin covering them, and the four lymph nodes are all removed at the same time. Although this is major surgery, normal activity will likely resume after the sutures have been removed (10-14 days post surgery). Many veterinarians will spay a dog having a mastectomy.


Name: Jo-Jo

Sex: F/I

Age: 7 years

Breed: Doberman


Pet was presented for lump near mammary gland in November 2012. Punch biopsy was advised and possible removal of mammary glands bilaterally.


Fine needle aspirate sample was collected from mass and sent to lab for analysis. Chest x-rays were performed to rule out possible spread of tumor, pre-surgical blood work was performed and was normal.


Pet was dropped off for mammary gland removal under general anaesthesia on March 18/13.

Left mammary caudal chain removed and pet was also spayed during procedure. Pet recovered from anaesthesia and home care instructions were discussed with owner. Mammary gland mass was sent to lab for histopathology.

Follow up:

7 day: Histopathology results revealed complete tumor excision. Results were discussed with owner and advised removal of right mammary chain in 1-2 months.

14 day: Suture removal was performed. Right side mammary gland removal scheduled for one month later.

Unilateral caudal mammary chain removal was performed in June 2013. The pet recovered from procedure as after first procedure and is doing well.


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