Comparison of age, sex, and incidence rates in human and canine breast cancer

A 5-year collection of cases of human breast cancer in males and females from the Alameda County Tumor Registry and a 5-year collection of canine cases from the Alameda-Contra Costa Counties Animal Neoplasm Registry were compared on age distribution, crude and age-adjusted incidence rates, and agespecific incidence rate pattern. To facilitate comparisons, ages of dogs were converted to human equivalents. The median ages and overall proportional distribution by age were in close agreement for the 2 sexes of both species. When canine and human incidence were adjusted to the same population distribution, canine age-adjusted incidence rates were 3 times larger in females and 16 times larger in males. Female age-specific rates increased at the same magnitude for both species until the start of the ages of natural menopause in women; then the human rates continued to increase, but slowly, while the canine rates continued to increase at approximately the same exponential value as in the younger age groups. Important similarities and differences between species that may affect breast cancer risk are discussed in the context of the natural history of human breast cancer.