I'm convinced that the high cancer incidence in dogs is partly due to several factors. Neutering and castration predisposes
for Osteosarcoma and Hemangiosarcoma. Furthermore, overmedication and overvaccination (yearly shots) as well as
lack of sufficient physical activity, sunlight, lack of healthy, natural food contribute to the development of canine cancer
in our modern society...
I found this article...
http://www.caninesports.com/SpayNeuter.html