Investigators for years have questioned whether diabetes is a risk factor for the development of cancer. There are theoretical reasons to suspect that there might be a link: insulin levels are increased in Type 2 (adult onset, non-insulin-requiring) diabetes because the body cannot utilize insulin properly to control blood sugar levels.
Insulin acts as a growth factor in certain situations and can theoretically stimulate tumor growth. Obesity and so-called oxidative stress, increased in diabetes, could also theoretically promote the development of cancer. In medicine things that should make sense don’t always, so there needs to be clinical confirmation of an association before one can be assumed based on scientific theory.