Factors Influencing Canine Mammary Cancer Development and Postsurgical Survival 2

Abstract

A retrospective study was conducted involving canine mammary cancer cases and age- and breed-matched controls. Only histologically confirmed malignant mammary cases were used. Of these, 71 were diagnosed as adenocarcinomas and 22 as malignant mixed mammary tumors. There were 87 case-controlled matches. Factors studied were classified within the categories of neutering, estrus, pseudopregnancy, parity, fecundity, and postsurgical survival. Within the scope of these variables, there were no differences in effects on the occurrence of adenocarcinomas or malignant mixed mammary tumors. Among the variables, neutered bitches had 12% of the mammary cancer risk as compared to intact animals. Bitches spayed before any estrous cycles had approximately 0.5% of the mammary cancer risk; those that had only 1 estrous cycle had 8%, and animals that had 2 or more estrous cycles before neutering, 26%. Within the group having 2 or more estrous cycles before being spayed, those neutered before 2½ years of age exhibited a marked sparing effect on mammary cancer risk not shown for bitches neutered after 2½ years of age. Tests of various factors within the categories of pseudopregnancy, parity, and fecundity indicated no significant effects on mammary cancer risk. Survival data, based on observation for a minimum of 42 months after surgical removal of the cancer, indicated excess mortality due to mammary cancer was concentrated in the first year; 2½ times more dogs died, either naturally or by euthanasia, than expected. Apparently the younger an animal at the time of cancer surgery, the more apt it was to survive. Neutering after cancer diagnosis did not affect either survival or cause of death. The age distribution of mammary cancer in female dogs was compared to the age distribution of breast cancer in women. Mammary cancer in the bitch mimics that of human breast cancer, except that the dog, after maturity, ages at 4 times the rate of humans. This age similarity and the common sparing effect of neutering in the 2 species provide strong support for the use of canine mammary cancer as a research model for breast cancer in women.