Just one of the many articles on this topic you can find online:
Mammary Cancer
Race Foster, DVM
Holly Nash, DVM, MS
Mammary tumors are the most common tumors in female dogs who have not been spayed. Mammary tumors can be small, simple nodules or large, aggressive, metastatic growths. With early detection and prompt treatment, even some of the more serious tumors can be successfully treated. Cats also suffer from mammary tumors and they have their own unique set of problems that are discussed in a separate article.
Which dogs are at risk for developing mammary tumors?
Mammary tumors are more common in unspayed, middle-aged female dogs (those between 5 and 10 years of age), although they can, on rare occasions, be found in dogs as young as 2 years. These tumors are rare in dogs that were spayed under 2 years of age. Occasionally, mammary tumors will develop in male dogs and these are usually very aggressive and have a poor prognosis.
The risk of breast cancer is almost eliminated in dogs that are spayed before their first heat.
Spaying greatly reduces the chances of a female dog developing this condition. In those females spayed prior to their first heat cycle, breast cancer is very, very rare. The risk of malignant mammary tumors in dogs spayed prior to their first heat is 0.05%. It is 8% for dog spayed after one heat, and 26% in dogs spayed after their second heat.It is believed that the elimination or reduction of certain hormonal factors causes the lowering of incidence of the disease in dogs that have been spayed. These factors would probably be estrogen, progesterone, a similar hormone or possibly a combination of two or more of these
