Mammary Tumors in Dogs
Many of us are familiar with mammary or breast cancer in humans. But many pet owners are unaware that their dogs are even more susceptible to mammary cancer with one in four unspayed female dogs developing tumors. Spaying a female puppy before she has her first heat cycle brings her likelihood of developing mammary tumors down to 0%. A dog spayed after experiencing one heat cycle has a 7% chance of developing mammary cancer. If she is allowed to experience more than one heat cycle, that puppy’s risk of developing mammary cancer goes up to 25%. The only way to prevent mammary tumor development is to have female dogs spayed before their fist heat cycle. This does not mean that a dog spayed later in life cannot enjoy the cancer prevention benefits.
Female hormones encourage mammary tumors. If your female dog is unsprayed, has had puppies, or was spayed later in life she fits into the high risk category. In order to recognize abnormal growths it is important to be familiar with normal female anatomy. The average female dog has nine mammary glands, but it is not unusual to have ten. Normal healthy mammary glands are soft and pliable especially as you get closer to the back legs. There should be no firm swellings or unusual discharge. If you find any lumps or swelling on or near the mammary glands call your veterinarian to set up an exam. Most tumors are found in the lower glands near the back legs.
On the upside, half of the tumors found in female dogs are benign. It is impossible to tell just by looking if a tumor is benign or malignant. A sample of the tumor must be looked at under a microscope. This can be done with a needle aspirate, a biopsy, or complete removal of the tumor. Based on the architecture of the mass cells, the laboratory can determine if the growth is benign or malignant. The growth of mammary tumors is influenced by the female hormones estrogen and progesterone. Therefore it is important to have your dog spayed even if tumors are already developing. Spaying a dog at the time of tumor removal can result in a 45% longer lifespan compared to an unspayed dog.
Types of Tumors
Fibroadenoma: This is a benign type of glandular tumor that usually requires no treatment.
“Mixed” Mammary Tumor: The “mixed” refers to the types of cells that make up the tumor. Epithelial cells, which line the glandular tissue, and mesenchymal cells, which make up the non glandular tissue are the two types of cells present in a mixed tumor. The tumor itself can be benign or malignant as determined by a biopsy.
Adenocarcinoma: These are malignant tumors described as “tubular” or “papillary” depending on which gland cells the tumor arose from. The pathologist will be able to determine how aggressive the tumor is based on cellular characteristics. It is advisable to have these tumors completely removed.
Inflammatory Carcinoma: These tumors are malignant and highly aggressive. They usually generate local inflammation and ulceration accompanied by pus and discomfort on the part of the pet. These tumors spread early and are difficult to treat. Fortunately less than 5% of mammary tumors are carcinomas.
Because 50% of malignant tumors will have spread by the time of surgery, it is a good idea to have xrays taken before surgery to determine if cancer is present in the lungs or lymphatic system. The other 50% are usually locally confined and successfully treated with surgical removal. After removing the tumor the pathologist will not only determine what type of tumor was present, he/she can tell weather or not the entire tumor was removed (“clean” vs. “dirty” margins).
Prognosis
Factors affecting prognosis include not only what type of tumor is present but also size, location, and history. Tumors with an overall diameter larger than 1.5 inches have a worse prognosis than smaller sized tumors. As mentioned above, a full exam to determine if the cancer has metastasized (spread) will aid in determining prognosis. Tumors felt deeper in the body or that are adhered to deeper tissue usually carry a poor prognosis as do any with an ulcerated surface. Owners of dogs with a history of masses or rapidly growing or changing lumps should contact their veterinarian to discuss their options. Cancer patients usually require special care and treatments. Please call your veterinarian if you find any strange or new lumps or bumps on your pet.