Keratosis, actinic: A small rough spot on skin chronically exposed to the sun, precancerous, can develop into a skin cancer called squamous cell carcinoma, a process that typically takes years. Actinic keratoses occur most frequently in fair-skinned people. Common locations are the face, scalp, back of the neck, upper chest, forearm and back of the hand.

Prevention is by minimizing sun exposure. Treatments include cryosurgery (freezing them with liquid nitrogen), cutting them away, burning them, putting 5-fluorouracil (5-FU) on them, and photodynamic therapy (which involves injecting into the bloodstream a chemical that collects in actinic keratoses and makes them more sensitive to exposure to a specialized form of light).