Q. What are the facts and figures for mammary tumors in dogs?
A. Dr. Antony S. Moore gave an excellent lecture on Diagnosis and Management of Dogs with Mammary Tumors at the 2007 American
College of Veterinary Medical Forum in Seattle. Here are some relevant points from that lecture:
Mammary tumors are the most common neoplasm in intact female dogs and account for approximately half of all neoplasms in them.
Mammary neoplasms are uncommon in male dogs, accounting for less than 1 percent of these types of tumors. The incidence of
mammary tumors is higher in dogs than in any other domesticated animal and is three times the incidence in humans.
Approximately half of the tumors are malignant, and half have metastasized by the time they are initially diagnosed.
Sex hormones certainly play a role in development of mammary tumors in the bitch. Intact females have a seven-fold increased
risk of developing mammary cancer compared to neutered females. The age when ovario-hysterectomy takes place is directly proportional
to the risk of developing mammary cancer. Data clearly indicate the preventive role of ovariectomy prior to the second estrus.
When only the dogs that had two or more estrus cycles before spaying are considered, there is still a benefit to earlier spaying.
If they were spayed before 2.5 years of age, then the risk was 6 percent of those dogs not spayed at all; (that approximates
to the 8 percent quoted for one estrus cycle). If they were older than 2.5 years, the risk was 40 percent of unspayed dogs,
but that difference was not significant. Essentially the greatest reduction in risk comes if you spay before the first estrus,
but then, as long as you spay before 2.5 years, the risk is very low.
Two studies showed that after a malignant mammary tumor develops, there is no effect of ovariectomy on tumor progression.
In contrast, a recent study showed increases in survival time for dogs that were ovariectomized within two years before mammary-tumor
Obesity may be a factor in mammary neoplasia in dogs. In one case-controlled study, the risk of mammary carcinoma among ovariectomized
dogs was significantly reduced (by 40 percent) in dogs that had been thin at 9 to 12 months of age. Among intact dogs that
were thin at that age, however, risk of mammary cancer was not reduced. In contrast, another study showed obesity at 1 year
of age almost trebled the incidence of mammary cancer in intact dogs. In that study, dogs with a higher intake of red meat
in their diet were at higher risk for developing mammary cancer.
Obesity and feeding a high-fat diet the year before being diagnosed with a mammary cancer had no influence of tumor development.
Caregivers should be counseled to ovariectomize their dogs before the first estrus, and to ensure weight control is practiced
in at least the first year of life. Ovariectomies performed later in life, while not as protective, should not be discouraged.
Staging and diagnosis
The two most common sites of metastases are lungs and regional lymph nodes. Therefore, staging should include a minimum database
of thoracic radiographs, complete blood count (CBC), biochemical profile, urinalysis and an evaluation of regional lymph nodes
(axillary and inguinal) by palpation, fine-needle aspiration cytology and (if indicated) biopsy.
The prognosis for dogs with mammary cancer is not influenced either by tumor location or number of tumors. Other factors that
are not prognostic are number of pregnancies, age at first pregnancy and occurrence of pseudopregnancies. The following are
prognostic factors that have been shown in studies to predict survival or the disease-free interval.
Stage of tumors
Dogs with Stage 1 tumors were more likely to survive longer than those with any other stage tumor. This effect of tumor stage
was similar in other studies and is detailed in the following tables.
Effect of tumor stage on survival
Effect of tumor stage on survival