Cancer mortality varies across breeds, from under 10 percent to higher than 60 percent. In general, small dogs weighing less than 20 pounds are at very low risk. For instance, the chance that a Chihuahua, Maltese, Miniature Pinscher and Pomeranian will get cancer is less than 18 percent.
Breeds with the highest risk include the Bernese Mountain Dog and Golden Retriever - upwards of 60%. (The average cancer risk in dogs is 27%. In humans it's 23%.)
Only 10% of Chihuahuas get cancer.
Cancer rate in Golden Retrievers bred
in North America is higher than 60%.
Why do miniaturized dogs have a lower incidence of cancer? Scientists suspect that one reason might be low levels of IGF-1, a hormone that, along with growth hormone, affects bone and tissue growth. Dogs under 20 pounds are small because of a mutation that puts the brakes on IGF-1 production.
Although there are many causes of cancer, each type starts with alterations in genes that tell cells how to function, which triggers
accelerated and uncontrolled cell growth. A lower level of IGF-1 is related to shunting growth in small dogs, so maybe it does the same to cancer cells.
This journal article addresses the possibility of the link between size and cancer in dogs: The Size-Life Span Trade-Off Decomposed: Why Large Dogs Die Young.