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These skin cancers are among the most common types. They are typically caused when ultraviolet (UV) rays from the sun or tanning beds turn some normal skin cells into cancer cells.
Basal and squamous cell skin carcinomas start in the top layer of skin—the epidermis—and are often related to sun exposure.
The epidermis is made of three types of cells—basal cells, squamous cells, and melanocytes. Skin cancer occurs when these cells grow out of control and invade the deeper layers of tissue below the epidermis.
Squamous cells are flat cells in the outer part of the epidermis, which are constantly shed as new ones form. Roughly two out of ten skin cancers are squamous cell carcinomas, also called squamous cell cancers. These cancers usually develop on sun-exposed areas of the body such as the face, ears, neck, lips and backs of the hands. Less often, they form in the skin of the genital area. Squamous cell cancers can usually be removed completely, although they are more likely than basal cell cancers to grow into deeper layers of skin and spread to other parts of the body.
Basal cells are in the lower part of the epidermis. About eight out of ten skin cancers are basal cell carcinomas, also called basal cell cancers, making it the most common type of skin cancer. These cancers usually develop on sun-exposed areas, especially the face, head, and neck. Even though basal cell carcinoma grows slowly and rarely spreads, it can grow into nearby areas and invade the bone or other tissues beneath the skin if not treated.
Actinic keratosis (AK), also known as solar keratosis, is a pre-cancerous skin condition caused by too much exposure to the sun. AKs are small, rough or scaly spots that may be pink-red or flesh-colored. Usually they start on the face, ears, backs of the hands, and arms of middle-aged or older people with fair skin, although they can occur on other sun-exposed areas. People who have them usually develop more than one.
Squamous cell carcinoma in situ, also called Bowen disease, is the earliest form of squamous cell skin cancer. The cells of these cancers are still only in the epidermis and have not invaded into deeper layers. Bowen disease appears as reddish patches. Like most other skin cancers and AKs, these patches most often appear in sun-exposed areas.
Bowen disease can also occur in the skin of the anal and genital areas, where it is known as erythroplasia of Queyrat or Bowenoid papulosis.
Keratoacanthomas are dome-shaped tumors that are found on sun-exposed skin. They may start out growing quickly, but their growth usually slows down, and many keratoacanthomas shrink or even go away on their own over time without any treatment.
While we understand the common risk factors for basal cell and squamous cell cancer, it’s still not completely clear how these factors cause cancer. We do know that most of these skin cancers are caused by repeated and unprotected exposure to UV rays from sunlight and tanning beds. UV rays can damage the DNA inside skin cells, the chemical that makes up our genes and that control how our cells function.
The gene most often altered in squamous cell cancers is the TP53 tumor suppressor gene. The gene often mutated in basal cell cancers are the PTCH1 or PTCH2 genes.
People with xeroderma pigmentosum (XP) have a high risk for skin cancer—XP is a rare, inherited condition resulting from a change in one of the XP (ERCC) genes.
Source: American Cancer Society