There are conflicting findings regarding whether or not pilots have an increased risk of cancer in general. But there are number of studies that have shown that pilots are at increased risk of melanoma in particular (and it doesn't seem to just be due to the sun-bathing opportunities that can sometimes go with the job).
Take a look at this...
Rafnsson et al (2000). Risk factors for cutaneous malignant melanoma among aircrews and a random sample of the population