Mammary cancer is the most common neoplasm in intact female dogs. Dogs spayed before their first estrous (heat) cycle have a greatly reduced risk of developing breast cancer, with the risks rising to 26% of dogs that are spayed after their second estrous. Breast cancer primarily affects late middle-aged (9-11 years) female intact dogs with an increased incidence beginning at approximately 6 years of age.

 

Dogs spayed prior to their first estrus have a 0.5% risk of developing mammary tumors in their lifetime. The protective effect being spayed decreases quickly over the first few estrus cycles, and most studies have not found a significant benefit after 4 years of age. Other risk factors for breast cancer include obesity and a diet high in red meat.

 

Withrow and MacEwan’s Small Animal Clinical Oncology 5th edition. Withrow, Vail, Page (Ch. 27)