Mammary tumors are extremely common in dogs; approximately 50% of them are malignant. Mammary tumors are more common in intact than in spayed females; in fact spaying before the first or second heat cycle significantly reduces the risk of developing mammary tumors. Median age on presentation is 10 to 11 years. Dogs fed a high-fat diet or overweight at 1 year age are at increase risk of developing mammary tumors. Appropriate early treatment, even if the tumor is malignant, is often curative.
If you are petting your dog and you notice a lump along the mammary chain, please have your vet examine her. In intact female dogs you may notice lumps that come and go after the heat cycle; they are typically due to mammary gland hyperplasia (proliferation of normal mammary tissue).
Before any diagnostic or therapeutic steps are taken, the health status of the dog must be fully assessed. Bloodwork and urinalysis should be done to identify any presurgical abnormalities. Thoracic radiographs (both right and left lateral and ventrodorsal planes) should be obtained to search for pulmonary metastases. A fine needle aspiration cytology of the mass is usually not recommended because its diagnostic value to discern between malignant or benign tumors is very low. Regional lymph nodes (lymph glands) should be palpated carefully; fine-needle aspiration or surgical removal are necessary to determine the presence of metastases.
Unfortunately the only way to know if mammary tumors are benign or malignant is to surgically remove them and do a biopsy. It doesn't matter how many mammary tumors a dog has: because all of them can be different, every mass should be submitted to the lab and analyzed. Depending of the size and the number of tumors conservative surgery (lumpectomy/ single mastectomy) or a more aggressive surgery with removal of a whole mammary chain may be recommended.
As of now, there is no proven efficacy of any chemotherapeutic protocol for the treatment of malignant mammary tumors in the dog. Certain drugs used for the treatment of carcinomas or sarcomas like gemcitabine/carboplatin/ doxorubicin may be helpful in delaying recurrence or metastases, but their efficacy is unknown.
The prognosis for dogs with malignant mammary tumors depends on the following factors: tumor type, size, regional lymph node (lymph gland) involvement, presence or absence of distant metastases, completeness of resection, local behavior, vascular or lymphatic invasion, and tumor differentiation.
As a general rule with this type of cancer, the best prognosis is directly related to early detection and treatment.