October is Breast Cancer Awareness Month and many pet owners may not know that dogs and cats can actually get breast cancer. Unfortunately breast (mammary) cancer is common; the good news is that it can be treated if caught early and almost prevent if the animal is spayed. In fact the most common type of tumor in the female dogs is the mammary tumor- especially in unsprayed dogs between the ages of five to ten years old. Male dogs can also develop breast cancer although this is much less common and, sadly, their prognosis is not good because this type of cancer is very aggressive.
Similar to the human form of breast cancer, mammary tumors in dogs can range in size. Breast tumors often grow quickly with irregular shape. Mammary tumors develop as a result or abnormal replication of the cells that make up the breast tissue. Aggressive, malignant tumors can also cause bleeding and ulceration. If a tumor does not exhibit these signs that does not mean that it is not breast cancer. Mammary tumors can be benign (non-cancerous) or malignant (cancerous). Small tumors that have been there unchanged for some time can suddenly grow aggressively as well. As with most all types of cancer, once malignant tumors start to grow the cancerous cells can spread to the other parts of the body.
If you find a lump on your dog, do not wait to go to the veterinarian. It is always best to play it safe and have your pet examined by a licensed veterinarian who may perform a biopsy. Half of all mammary tumors are benign. The other half that are malignant about 50% of them have spread to the rest of the body by the time that they are diagnosed. We don’t want to play a guessing game when it comes to your pet’s health. Benign tumors typically do not progress to malignant tumors but cause great concern to the pet owner and veterinarian because they are indistinguishable from malignant tumors. A biopsy of the tumor is the only test that can differentiate which type of tumor is present. In addition, a dog that has multiple mammary tumors may have some that are benign and others that are malignant; therefore, a pathologist must evaluate all masses microscopically.
The exact causes for the development of mammary tumors in dogs are not fully understood. It is known that exposure to specific hormones increases the risk for developing mammary cancer in dogs. The incidence of these tumors is related to weather a dog is intact or has had an ovariohysterectomy (i.e., has been spayed). Dogs spayed prior to their first heat have only 0.5% risk of developing mammary tumors. However, after a first or second heat, this dramatically increases the risk to 8% and 26%, respectively. Spaying your companion after the third heat does not have a sparing effect on mammary cancer but minimizes the risk for development of a life-threatening infection in the uterus called pyometra. Another risk factor for development of mammary tumors is obesity. Age also appears to play a role; the risk of tumor development significantly increases once a dog reached 7 years of age and continues increasing until 11-13 years of age.
This increased risk is breed dependent indicating that there is a genetic component. Certain breeds appears to be more commonly affected, including Chihuahuas, Poodles, Dachshunds, Yorkshire Terriers, Cocker Spaniels, Brittany Spaniels, English Setters, Boxers and Doberman Pinchers. Research in veterinary medicine continues, but to-date we still know little about which genes and mutations can lead to the development of mammary tumors. In human medicine, research has established a definite relationship between the BRCA gene (breast cancer gene) and the development of breast cancer.
Treatment of a malignant tumor usually involves surgery. Similar to breast cancer in humans, dogs will either have just the tumor removed or the entire mammary tissue chain along with lymph nodes. Dogs’ mammary glands are different than humans in that they are outside of the muscle, to the surgery is not as radical. If your dog is intact an ovariohysterectomy (spay) is recommended and may be done at the same time as tumor removal. For larger tumors or when there is evidence of spread to other areas of the body, chemotherapy maybe recommended. The options that are considered depend on each dog’s individual situation. The best way to prevent breast cancer in female dogs is to spay them before they go into heat for the first time- just another benefit of spaying. By doing this we can practically eliminate the chances of their dog developing mammary cancer.