Dog: Mammary Cancer

General information

Other common/scientific names: mammary tumor, breast cancer

Mammary cancer is the second most frequent type of cancer in dogs. Skin cancer is the most common. Mammary cancer most commonly affects females but on rare occasions, male dogs can be afflicted. Approximately 50% of the these tumors are benign and 50% are malignant. Spread of malignant mammary tumors is primarily to lymph nodes and lungs, however it can also spread to bone.

A female dog normally has five mammary glands on both sides of the abdomen, commonly referred to as the left and right mammary chains. Each gland has its own nipple. Lymph nodes, which drain cells from the glands, are located at each end of the mammary chain. The first, second and third glands drain forward to the axillary (armpit) lymph node. The fourth and fifth glands drain back toward the inguinal (groin) lymph node.

Mammary cancer commonly affects the fourth and fifth glands.


Since mammary cancer is virtually nonexistent in dogs which are spayed before their first heat, the female hormones (estrogen and progesterone) are thought to play an important role in the development of mammary tumors. Dogs spayed after their first heat have an 8% chance of developing mammary tumors while dogs spayed after their second heat increase their risk to 25%.

Cardinal symptom

Swelling of a mammary gland


Mammary tumors can be either single or multiple. They can be smooth, small and slow growing or they can be large, aggressive tumors which invade the skin and underlying tissue causing ulceration, infection and bleeding.

Abb. GG97U556
Abb. GG97U556: Mammary Cancer.
This is a photograph of a mammary tumor with a nodular appearance.


Diagnosis of mammary cancer is by physical examination. While most benign tumors are smooth, small and slow growing, it is very difficult to identify the type of tumor based on physical inspection. All mammary tumors should be regarded as potentially malignant until identified. A biopsy or tumor removal and analysis is needed to determine if the cancer is benign or malignant. The lymph nodes in the axillary (armpit) area and inguinal (groin) area should be examined for enlargement. Radiographs should be taken of the chest to check for any spread of the cancer.


Surgical removal of the tumors is recommended unless the dog is unable to undergo anesthesia. The amount of tissue excised will depend on the tumor and the individual dog. Many times, the entire mammary chain and lymph nodes will need to be removed. If both sides of the mammary glands are affected, one chain is removed and allowed to heal. Four to six weeks later, the other chain is removed. A sample of the tumor should be submitted to a laboratory for identification of the cells. If a malignant type of cancer is diagnosed or the cancer has spread, chemotherapy and/or radiation may be needed. Since the growth of some tumors are controlled by hormones, some veterinarians recommend spaying the intact female. Research to support this reasoning is controversial.

It is very important that the female does not scratch, lick or pull at her sutures. If the entire chain is removed, the dog will have a large incision to heal. It is imperative that the dog owner be educated regarding wound healing and understand the importance of not allowing the dog access to the sutures. A second surgery can be necessary if the dog removes the sutures too soon. A special collar (E-collar or Elizabethan collar) is worn by the dog to prevent this. While this collar may seem uncomfortable, most dogs tolerate it well. It may need to be removed to allow the dog to eat. Another option is placing a body suit on the dog which is specifically made to cover and protect the incision. Alternatively, some dogs benefit from wearing a tee shirt to achieve this purpose.


If mammary cancer is diagnosed early and complete excision is achieved, the prognosis is good for survival. Tumors larger than 1.5 inches have a worse prognosis. Malignant tumors which have spread to the lymph nodes, lungs or bone carry a grave prognosis.

Many owners associate surgical removal of canine mammary glands with breast removal in women. With the canine surgery, only the mammary tissue and possibly lymph nodes are removed. Unlike human surgery, no muscle is removed and recovery is quicker and less painful than with women.


Spaying female dogs before their first heat cycle prevents mammary cancer.

Update version: 4/24/2014, © Copyright by
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