Rough, scaly patches of skin that are considered precancerous and are due to sun exposure. Prevention is to cut sun exposure and wear sunscreen. Treatments include performing cryosurgery (freezing with liquid nitrogen), cutting the keratoses away, burning them, putting 5-fluo- rouracil on them, and using photodynamic therapy (injecting into the bloodstream a chemical that col- lects in actinic keratoses and makes them more sen- sitive to destruction by a specialized form of light). Also known as solar keratosis and senile keratosis.