Only the thin line of hair suggests the pic above is not an asteroid-pocked Martian landscape. But that was my forehead just two weeks ago. Three skin cancers over 20 years have made me an habitué of dermatologists who look for precancerous hard spots on my skin. The last time, she took a look, ran her fingers over my face, neck and arms, and gave up. "You have too many actinic keratoses for me to freeze. I want to prescribe a cream, Fluorouracil, that...