Actinic keratosis (also called "solar keratosis and "senile keratosis" is a premalignant condition of thick, scaly, or crusty patches of skin. It is more common in fair-skinned people and it is associated with those who are frequently exposed to the sun, as it is usually accompanied by solar damage. They are considered as potentially pre-cancerous, since some of them progress to squamous cell carcinoma, so treatment is recommended. Untreated lesions have up to twenty percent risk of progression to squamous cell carcinoma.
Progressive development of these lesions occurs when skin is exposed to the sun constantly and thick, scaly, or crusty areas appear. The scaly or crusty portion is dry and rough. The lesions start out as flat scaly areas and later grow into a tough, wart-like area.
An actinic keratosis site commonly ranges between 2 and 6 millimeters in size, and may be dark or light, tan, pink, red, a combination of all these, or have the same pigment as the surrounding skin. The lesion may appear on any sun-exposed area, such as the face, ears, neck, scalp, chest, backs of hands, forearms, or lips.
A seborrheic keratosis (also known as "Seborrheic verruca," and "Senile wart":767:637) is a noncancerous benign skin growth that originates in keratinocytes. Like liver spots, seborrheic keratoses are seen more often as people age. In fact, they are sometimes humorously referred to as the "barnacles of old age".
The lesions appear in various colors, from light tan to black. They are round or oval, feel flat or slightly elevated (like the scab from a healing wound), and range in size from very small to more than 2.5 centimetres (1.0 in) across. They can resemble warts, though they have no viral origins. They can also resemble melanoma skin cancer, though they are unrelated to melanoma as well. Because only the top layers of the epidermis are involved, seborrheic keratoses are often described as having a "pasted on" appearance. Some dermatologists refer to seborrheic keratoses as "seborrheic warts"; these lesions, however, are usually not associated with HPV, and therefore such nomenclature should be discouraged.