The first published case of the behavior was in a 1989 Lancet letter describing a border collie–Doberman mix who sniffed and bit so aggressively at a mole on her owner’s leg (even through her trousers) that the owner was moved to check with her doctor, who diagnosed a malignant melanoma. [...] In the decades since, studies have supported the claim that dogs can identify cancers by their smells, which are caused by the volatile organic chemicals in the waste products the cells release.

— Read more at How Dogs’ Sensitive Noses Could Change Cancer Diagnosis at Slate.