Canine Mammary Tumors

Mammary cancer is, by far, the most frequent occurring neoplasm of the female dog. Mammary cancer accounts for 24-45% of all cancer in female dogs. The average age of affected dogs is 10-11 years of age. Fifty to sixty percent of canine mammary tumors are malignant, meaning they have the potential to spread to other portions of the body. The most common sites of spread are the local lymph nodes and lungs. Forty to fifty percent of canine mammary tumors are benign, meaning they will no spread except by local growth. To date, surgical excision at the earliest possible opportunity is the most effective therapy for any mammary tumor. If the tumor is benign, complete surgical excision is curative. If the tumor is malignant, post-surgical treatment with chemotherapy, immunotherapy or radiation therapy may be warranted. These options will be discussed with you by the medical oncologist following surgery, if appropriate.

Spaying at the time of mammary tumor resection has no measurable effect on the rate of tumor reoccurrence. Spaying prior to the first or second heat period significantly reduces the likelihood of mammary tumors. Spaying anytime up to 2.5 years of age (but prior to five heats) will have some protective effect, but after 2.5 years of age, spaying is not effective in the prevention of mammary tumors.

Biopsying mammary tumors prior to removing them is not recommended for the following reasons:

  1. initially benign tumors may transform at a later date to malignant forms.
  2. multiple tumors must be individually biopsied, as each may well be a different subtype of mammary tumor.
  3. within the same mass, a mixture of benign and malignant tissue may be present.

Instead, we recommend a single surgical procedure in which all affected tissue or potentially affected tissue is excised.

Depending upon the extent of the mammary tumors, different surgical procedures are recommended. In some cases only a single mammary gland will be removed. More commonly, we will excise a group of the glands or all five glands on one side plus the associated lymph nodes. When mammary glands on both sides are involved, two separate surgeries done four weeks apart may be required to remove each side. In general, we recommend more extensive excisions to insure as complete removal of affected tissue as possible.

The suture line will range in length from an inch or so to the full length of the body, from the axilla to the vulva. On occasion, small rubber drains will be inserted in the inquinal (groin) region to prevent fluid accumulation. These drains are removed 3-4 days after surgery. All dogs are bandaged after surgery. These bandages are usually removed 3-4 days after surgery. Some dogs may require an elizabethan collar to discourage them from chewing at the sutures.

The dog's activity must be curtailed post-surgically to avoid fluid build up along the suture line or dehiscence (wound gaping). Stairs should be avoided and only short leash walks allowed. Pathologic diagnosis of the tumor will be available 5-7 days after surgery and we will telephone you with these results. Ten to fourteen days after surgery, the sutures will be removed.