It is a fact that both dogs and cats get breast cancer. Breast cancer, also known as mammary cancer, can be seen in both intact (not spayed or neutered) and spayed/neutered dogs and cats. It is most common in female pets but can also occur in males.
Some research suggests that breast cancer in dogs accounts for over >50% of all canine cancers. Is the third most common type of cancer in cats.
The timing of the spay significantly impacts development of mammary gland tumors in dogs. Dogs spayed prior to their heat cycle have less than a 1% risk, those spayed between the first and second estrus have an 8% risk, and dogs spayed after their second estrus cycle have a more than 25 X lifetime risk of development.