Canine mammary gland tumors are a one of the most common tumors diagnosed in intact female dogs. Incidence of mammary tumors is dependent on when a dog is spayed. If it is before their first heat cycle, there is a 0.5% chance for development of a mammary tumor. The risk increases to 8 and 26% after they have gone through 1 and 2 heat cycles respectively. Spaying at the time of diagnosis has not been shown to decrease the risk for developing mammary tumors, but is often recommended for health reasons. These tumors most commonly occur in middle aged to older dogs.
Due to the risk for metastasis associated with mammary tumors, thorough staging is recommended prior to any definitive treatment. This would include bloodwork with a complete blood count, chemistry panel and urinalysis. Aspiration of the regional lymph nodes is performed when possible, to evaluate for any evidence that the cancer has spread. Chest radiographs are performed to evaluate for any evidence of metastasis (spread of disease) to the lungs. An abdominal ultrasound may also be recommended to look for any abnormalities in the abdomen (ie enlarged lymph nodes or evidence of spread to other organs). Aspiration of the mass is often performed but cannot always differentiate between a benign tumor and a malignant one, and a biopsy is typically needed. Prognosis for patients is dependent on the stage of disease and this is based on the size of the tumor (tumors less than 3cm have a better prognosis), evidence of metastasis (patients that have metastasis on average have a shorter survival time than those without) and evidence of any invasion into the vessels (this is determined from the biopsy and dogs that do not have any invasion have a better prognosis than those that do). There is a rare mammary gland tumor seen called inflammatory mammary carcinoma, which carries a very poor prognosis because of how aggressive it behaves and the more limited treatment options available.
Treatment options for mammary carcinomas include surgery for the primary tumor site. If the tumor is considered high grade, chemotherapy may also be warranted to combat the metastatic potential (spreading). Surgery is generally the first treatment option, and since local tumor recurrence is common if the tumor is incompletely or narrowly excised, wide surgical margins (3cms and one tissue plane deep) are recommended. This may mean that several mammary glands are removed at one time. Patients with low-grade mammary tumors who have a wide-complete excision can experience long term local control (possibly a cure).
High-grade tumors, tumors that have been narrow or incompletely excised, larger tumors, or tumors that have vessel invasion have a greater likelihood of metastasizing (spreading); therefore, we recommend chemotherapy in addition to local in these patients. The most commonly used chemotherapy treatments include agent doxorubicin (Adriamycin), a combination of doxorubicin and carboplatin, doxorubicin and cyclophosphamide or palladia. Side effects are generally self limiting but may include weakness/lethargy, poor appetite/anorexia, vomiting and/or diarrhea, decreased white blood cell count and rare bladder irritation with Cytoxan. Doxorubicin has a lifetime limit in most patients because if we exceed a certain dose, we run the risk of causing damage to their heart. Some patients require an echocardiogram prior to treatment and then again towards the end of treatment to ensure that their heart function is adequate to receive doxorubicin.
The goal with any treatment is to help to improve and maintain a patient’s quality of life. Your oncologist will be able to discuss the treatment options that are available and together you can determine what option is in your pets best interest.