Skin is constructed of two major layers: the epidermis (or surface layer), and the dermis (or interior layer). The thin epidermis layer is composed of constantly renewing layers of cells called keratinocytes which rise in layers from the interior of the epidermis only to get sloughed off at the surface, and other supporting cell types including melanocytes (pigmented cells responsible for skin color or freckles), dendritic cells (involved in skin immune function), and basal cells. The thicker, deeper dermis layer is composed of connective tissues and embedded blood vessels, nerve and sensory fiber endings, oil and sweat glands, body hair follicles and a variety of other structures.
Like any other organ in the body, the skin is subject to cancer. Skin cancer occurs when malignant (cancerous) growths or tumors form on or in the skin. Today, skin cancer is the most prevalent form of cancer, accounting for about 50% of all cancer cases reported annually, according to the American Cancer Society (ACS, 2010).
Skin cancers are divided into two major forms: Nonmelanomas and Melanomas. These cancer subtypes are largely differentiated based on where in the skin layers they form.
- Melanoma. Melanoma is a form of skin cancer that affects the melanocytes in the epidermis. Melanocytes are special skin pigment cells that give our skin color, and which allow our skin to “tan” when exposed to ultraviolet light from the sun. The darkening of the skin we call tanning provides the deeper body tissues extra protection from ultraviolet radiation.Melanoma skin cancer will effect roughly 70,000 Americans in 2010 and roughly 12,000 Americans will die from melanoma in 2010 (ACS, 2010). The danger posed by melanoma is largely due to the risk of metasteses; Melanoma is much more likely to spread to other parts of the body, and to do so faster, than are non-melanoma skin cancers. As is the general case, metastasized cancers are harder to successfully treat than are localized cancers. Melanoma skin cancer is quite treatable provided it is caught early on before significant metastasis has taken place.
- Non-melanoma. As the non-creative name suggests, non-melanoma skin cancer is a sort of "blanket" term used to group together the types of skin cancer that aren't melanoma. There are two primary forms of non-melanoma skin cancer, and a handful of other rare non-melanoma types which will not be covered here.
- Basal cell Carcinomas begin in the basal cell layer of the epidermis (the most interior part of the outer skin layer). Basal cell skin cancers are common, and typically appear on the head, neck, arms, and other body parts frequently exposed to the sun. Basal cell carcinomas tend to progress very slowly and usually do not spread to other parts of the body.
- Squamous Cell Carcinomas originate in the outer layers of the epidermis. Like basal cell carcinoma they most commonly appear on areas of the body most exposed to the sun, although they can appear on the genitals as well. Squamous cell carcinomas rapidly progress to involve deeper dermal layers of the skin tissue but (like basal cell carcinoma) are unlikely to spread to other parts of the body.