Diagnosis
Mammary gland ("breast") tumors are the most common type of tumor in the unspayed female dog, representing 52% of the tumors that dogs develop. Luckily, over the past several years the number of mammary tumors has declined as a result of most animals being spayed at younger ages. Dogs spayed prior to their first heat have a 0.05% chance of developing mammary tumors. In contrast, dogs spayed after their first heat have a 26 % chance of developing the tumors.
Between 41% and 53% of mammary tumors are malignant, meaning that the tumor has spread to different parts of the body. Single mammary tumors outnumber multiple tumors 3 to 1. It's interesting to note that the size of the tumor has no relationship to the aggressiveness of the tumor. Small, dime-sized tumors may well have spread throughout the body.
Breeds at risk for developing mammary gland tumors include toy and miniature Poodles, Spaniels, and German Shepherds. The average age of dogs at diagnosis is 10-11 years. There can be a single or several tumors, and they can occur in one or more glands. The last two sets of glands (the 4th and 5th glands) are most commonly affected. The tumors can be firm or soft, well-defined lumps or diffuse swellings. Tumors can be attached to underlying tissues or moveable, skin-covered or ulcerated. They can be different sizes, and they may grow slowly or quite fast. Most dogs are seen by the veterinarian for signs associated with the primary tumor and are otherwise feeling well. A few dogs are diagnosed with advanced metastasis (tumors that have spread to elsewhere in the body, such as the lungs and lymph nodes) and might be feeling ill from their tumors when they come for treatment.

Early spaying
The risk for developing mammary gland tumors is closely associated with exposure to the female sex hormone estrogen in the early years of development. This is a disease of female dogs and is extremely rare in males. Estrogen is necessary for normal mammary gland development. However, it may also be involved in the initial stages of cancer development that leads to tumors many years later. Estrogens may also provide continued stimulation to tumors and therefore contribute to tumor progression. Hormonal therapy is a common treatment in women with breast cancer and may also be helpful in the treatment of canine mammary gland tumors. Early spaying (ovariohysterectomy: removal of the ovaries and uterus which removes the source of estrogen) may significantly decrease the risk for tumor development. Studies have shown that spaying a dog before her first, second, or third heat cycle can significantly decrease the risk for developing mammary gland tumors later in life, and even if there were no mammary tumor, spaying is encouraged to prevent pyometra, a severe bacterial infection of the uterus. Pyometra affects about 50% of all unspayed females at some time during their lives. Each heat increases the dog’s chances of developing the disease, because of the surge and drop in hormonal activity.

Malignant or not?
Mammary gland tumors can be either malignant (cancerous) or benign (non-cancerous) and arise from the different types of tissues (epithelial or glandular tissues, and mesenchymal or connective tissues) in the mammary gland. The most common types are tumors from the glandular tissues and include adenoma, carcinoma, and adenocarcinoma. Half of all mammary gland tumors are benign and can be treated successfully with surgery alone. The other half are malignant and have the potential for metastasis. The outcome for patients with malignant mammary gland tumors depends on several factors including tumor type, histologic grade (appearance of the tumor cells under the microscope and how similar or dissimilar they are to normal tissues), tumor size, and tumor stage (presence of regional and distant metastasis).

Treatment
Veterinarians recommend that all mammary gland masses are surgically removed and biopsied to determine the tumor type (there is no way to tell if a tumor is malignant or not, just by the look of it). Dogs with benign tumors usually do not require further treatment, but cases with malignant tumors should be staged (evaluated for metastasis by tests such as chest X-rays and abdominal ultrasound). Dogs with small (less than about 1 inch diameter) low histologic grade carcinomas and adenocarcinomas with no evidence of metastasis may be treated effectively with surgery alone. Dogs with large or invasive tumors, high histologic grade, sarcomas (tumors of mesenchymal origin), lymph node involvement and/or other sites of spread are at risk for both recurrence of the original tumor and metastasis. Surgery is not inexpensive, so it's understandable if an owner chooses not to have the surgery performed. If it isn't performed, though, the tumor will continue to grow and eventually will open and drain blood-tinged fluid. Or, if the tumor is malignant, the animals may develop breathing difficulties as the tumors grow in the lungs.

Older Animals Handle Surgery Well
With the safety of present day anesthetic agents, anesthetic complications are minimal. Older animals handle recovery from surgery very well. If the biopsy of the tumor indicates that it is malignant, then complete removal of one or both sets of the glands (i.e., the right-side and/or the left-side glands) is the best and most effective course of action.
Even though this is a relatively intensive surgery, if the surgery is performed before the tumor has had a chance to spread, the disease can be cured. Early, aggressive removal is advised. Better too aggressive than not aggressive enough.

Hormonal therapy and chemotherapy
Hormonal therapy in the form of ovariohysterectomy may be beneficial in unspayed dogs with carcinomas or adenocarcinomas. A recent study at the Veterinary Hospital of the University of Pennsylvania showed that dogs spayed either at the same time of their tumor removal or within two years prior to the tumor surgery lived significantly longer than dogs that remained unspayed after their tumors were removed. Chemotherapy may also be indicated in dogs with aggressive tumors. Chemotherapy has been shown to be effective in individual dogs with metastatic adenocarcinomas, but there are no large studies that prove the long term benefit of chemotherapy in dogs suspected of being at risk for metastasis.

Prevention
Owners can play an important role in their dog's health. The protective effect of early ovariohysterectomy is substantial, and dogs that are not intended for breeding should be spayed before their first or second heat. Obesity and a high fat diet in the first year may also increase the risk for tumor development, so not overfeeding young growing dogs could be beneficial. High fat diets have been associated with the development of malignant mammary tumors in humans and possibly in dogs. Studies have shown that dogs that are thin at nine to twelve months of age are at a reduced risk compared to other spayed dogs. So feeding your dog a good dog food, not over-feeding it, and not feeding it table scraps could help to prevent your dog from getting a malignant mammary tumor.Owners should examine their dogs at regular intervals for any lumps, bumps, or swellings and take them for yearly veterinary checkups. All lumps should be surgically removed and biopsied. Early diagnosis and treatment are crucial for a good outcome.

Clinical research benefits both dogs and women
Canine mammary tumors have many similarities to breast cancer in women. In both, it is a disease that affects the middle-age to older patient, and the most common tumor types are the same. The treatments are similar, and patients with small tumors and early, localized disease can be cured. However, for patients with tumors that have spread elsewhere in the body, the prognosis is not good. Canine mammary gland tumors are excellent models for breast cancer in women, and clinical research studying mammary gland tumors in dogs has the potential to benefit both dogs and women.