Causes of Breast Cancer (Mammary Gland Tumors) in Dogs - Biological Progression
Mammary gland tumors always start with abnormal growth of mammary tissue. They are classified by the type of mammary tissue involved. Roughly 30% to 50% of breast tumors in dogs are malignant. Most (but not all) of these tend to be highly aggressive and invasive, spreading to the lungs, to nearby lymph nodes and possibly to other places, such as the liver and kidneys.
Malignant mammary tumors include:
- Inflammatory carcinoma
- Malignant mixed mammary tumor
Benign mammary tumors include:
- Benign mixed mammary tumor
Causes of Breast Cancer (Mammary Gland Tumors) in Dogs – In Real Time
What causes breast cancer to develop in dogs remains a mystery. However, it is extremely likely that hormones and genetics play a critical role.
- Hormonal Involvement. Bitches that are spayed before their first heat cycle have an extremely low chance of developing breast cancer (0.5% risk compared to intact females). Females spayed after their first but before their second cycle have an 8% chance of getting breast cancer compared to those that aren’t spayed. Those spayed after their second heat cycle but before their third have a 26% chance of developing breast cancer compared to intact bitches. Spaying after the third heat cycle or after 2 years of age has no salvaging effect on the risk of breast cancer compared to female dogs that remain intact throughout their lives. Treating females with progesterone, estrogen and/or growth hormones reportedly elevates the incidence of breast cancer.
- Genetic Involvement. Genetics probably are involved in the development of breast cancer. Mutations of certain genes have been located in come canine mammary gland tumors, although a specific, widely spread common mutation has not been identified.
- Diet. Dietary intake and body condition may play a role in canine breast cancer. Obese young dogs seem to more prone to this disease, while young fit animals seem to be at less risk.
Prevention of Breast Cancer (Mammary Gland Tumors)
There currently is no sure-fire way to actually prevent breast cancer in dogs. However, several things can help to reduce that risk. Most importantly, spaying a bitch before her first heat cycle – which basically means spaying her before 6 or 7 months of age – is strongly protective against breast cancer. The surgical procedure, ovariohysterctomy (OVH), involves removing both ovaries and the entire uterus, which effectively eliminates production of many of the hormones thought to contribute to breast cancer. However, owners should recognize that spaying a bitch can increase her chances of becoming incontinent. This is mainly due to hormonal changes that affect the function of the urethral sphincter. Finally, given the possible hereditary component, some authorities recommend that females with a history of breast cancer not be bred.
Many people think that tumors are defined by where they are located – in other words, that a mass found in a dog’s lung is “lung cancer.” Technically, that often is incorrect. Tumors (and cancer) are defined by the type of cells that they are made of. Frequently, malignant tumors (actually, the cells of those tumors) spread from their original location to nearby tissues or far-away areas of the dog’s body, where they lodge and continue to multiply. Breast cancer (cancer made of mammary gland cells) often spreads to the lungs, lymph nodes and other organs; it still is considered to be breast cancer, no matter where the malignant cells eventually stick.