To give you an idea of how important spaying is, dogs who are spayed prior to their first heat cycle have a risk of malignant mammary tumors of less than 1 / 10th of 1 percent. (straightpoop.com)
The risk of malignant mammary tumors in dogs spayed after their first heat increases significantly, but if an owner waits to spay their dog until after their second heat, the risk increases to 25 %. (caninecancer.com)
The role of chemotherapy in cats and dogs with malignant mammary tumors has not been clearly defined for all tumor types but oncologist consultation after surgery is often recommended. (acvs.org)