Cancer is the name given to a collection of related diseases. In all types of cancer, abnormal cells in the body begin to divide uncontrollably, and in most, the cells eventually form a malignant (cancerous) mass or tumor. Cancer cells are unique in that they can invade the tissue surrounding them and spread to other parts of the body in a process called “metastasis.”
Growths on the skin are relatively common. Most are benign (not cancerous), and these become more prevalent during the aging process. Skin cancers are growths that have become malignant. Abnormal cells in the skin divide in an unregulated manner, resulting in a bump or patch-like rash that may bleed, scab or fail to heal. In some cases, skin cancers are unusually colored, or may even appear variegated with a variety of colors.
Most skin cancers arise when skin cells and their DNA are damaged by ultraviolet light (UV) during periods of sun exposure, or while using tanning beds and sunlamps. The top or outermost layer of the skin (the “epidermis”) consists of densely packed cells that are easily penetrated by UV. This helps to explain why epidermal skin cells are especially vulnerable to damage during sun exposure. DNA damage can occur as a result of brief exposure to sunshine (such as mowing your lawn or unpacking your car), while chronic exposure increases the risk for cumulative DNA damage. Certain wavelengths of UV (UVA) can penetrate glass, potentially leading to increased risk for drivers and passengers. Long-term exposure to UV can also lead to the development of wrinkles and age spots in the skin, damage which is typically associated with aging but in reality is generally due to sun exposure. Skin cancers primarily occur on sun-exposed skin but can form anywhere, including areas of skin that are rarely or never exposed to sunlight. Some people are more likely than others to develop skin cancer, but it is important to understand everybody’s skin, regardless of ethnic background or skin color, can be damaged by UV exposure.
Skin cancer is the most common type of cancer in the United States, with over five million cases annually. Looking at this from another perspective, it is thought that at least 1 out of every 5 adults may develop skin cancer within his or her lifespan. Skin cancers are often classified as either non-melanoma or melanoma. Non-melanoma skin cancers, including basal cell carcinomas and squamous cell carcinomas, are the most common skin cancers and are named after the type of skin cell which gives rise to the cancer. While melanomas are less common than non-melanoma skin cancers, they are typically much more aggressive and more likely to invade nearby tissues and metastasize (spread to different parts of the body), making it one of the skin cancers with the highest mortality rates. Other cancers which can develop in the skin include Merkel cell carcinoma, sebaceous cancers, skin lymphoma and certain sarcomas, but these are rare.
The information presented is intended to be a brief overview of common as well as some uncommon cancers that occur in the skin. Consistent with our mission, our goal is to educate the community on the demographics (population factors such as race, age, sex) and characteristics of skin cancer in a comprehensible manner. Additional information can be sought from the references and links provided. The National Cancer Institute is an excellent resource for most types of skin cancers and can be contacted by phone (1-800-4-CANCER). All unusual, rapidly growing, bleeding or scabbed (ulcerated) skin growths, or those that fail to heal, should be brought to the attention of your dermatologist or medical provider for assessment and treatment.
All photographs and diagrams are intended for informational purposes only and are not intended to be used for diagnosis. A dermatologist should be consulted for examination and diagnosis of skin lesions.
Diagram 1. Starting at the skin surface, the epidermis is composed of a keratin layer, a much thicker layer of squamous cells, and a thin layer of basal cells. Scattered between the basal cells are a few other cells, such as melanocytes (pigment cells) and Merkel cells. The dermis, a dense layer of connective tissue and proteins which support the skin, contains blood vessels, nerves and hair follicles. The layer of fat below the dermis (hypodermis) allows the skin to move without tearing underlying structures.
Diagram 2. Ultraviolet radiation from the sun is made up of UVA and UVB rays. UVB primarily penetrates the epidermis, where it can damage the DNA in basal, squamous and other cells, and over time, potentially lead to skin cancer development. Because of its deeper penetration into the dermis, UVA can also damage skin proteins, resulting in wrinkles, weakened blood vessels and sun-damaged skin. Use of sunscreen or protective clothing can help to block UV.
Diagram 3. This photo demonstrates the effects of chronic sun exposure on skin. The upper arm of this person was protected from UV damage by the sleeve of a T-shirt. In contrast, the lower arm has extensive UV damage resulting from lifelong sun exposure.
Diagram 4. UV exposure can damage DNA in cells. The genetic changes in the DNA can lead to uncontrolled cell growth (cell division) and the formation of a cancerous (malignant) tumor.
Diagram 5. Most types of skin cancer arise from cells in the epidermis (upper layer of skin). Over time, the rapidly dividing cancer cells form a mass of tumor cells which can extend both upwards onto the skin surface, and deeper into the dermis.