Did you know that half of all the tumors in female dogs are preventable breast tumors? Dogs develop breast cancer because they were not spayed before their first or second heat period. Intact female dogs are highly prone to developing breast tumors. In fact, they are seven times more likely to get breast cancer than a spayed dog. One out of four intact female dogs over 4 years of age will probably develop one or more breast tumors along the mammary gland chains. Half of all tumors are malignant and unfortunately, half to 75% of them will kill the dog by recurrence or spreading (metastasizing) to the lungs within one to two years. German Shepherds, Dobies and Nordic Breeds seem to do poorly, while poodles, terriers and cockers seem to have greater incidence. Sex hormones produced by canine ovaries during their six-month cycle cause a harmful sensitization or pre-programming of the breast tissue. This hormonal influence ultimately causes point mutations in the genes of the breast tissue cells that dictate tumor growth. Progesterone and estrogen are the hormones that cause this “field cancerization” effect. Progesterone therapy may cause breast tumors in dogs.
The sex hormone receptors in canine mammary tumors have not been good indicators for therapeutic response due to their very low level of activity. If the tumor has positive estrogen or progesterone receptors, it tends to be benign. An intact female dog may develop a tumor in any one of her ten mammary glands and over half will present with more than one tumor.