Skin Cancer is by far the most common malignant tumor in humans. The most common types of skin cancer are basal cell carcinoma, squamous cell carcinoma, and melanoma. Basal cell carcinoma and squamous cell carcinoma begin as a single point in the upper layers of the skin and slowly enlarge, spreading both along the surface and downward. These extensions cannot always be directly seen. The tumor often extends far beyond what is visible on the surface of the skin. If not completely removed, both types of skin cancer may invade and destroy structures in their path. Although these skin cancers are locally destructive, they do not tend to metastasize (spread) to distant parts of the body. Metastasis of basal cell carcinoma is extremely rare. Squamous cell carcinoma is slightly more dangerous, and patients must be observed for any spread of the tumor. Such spread is still infrequent. Melanoma is a very different and potentially more dangerous kind of skin cancer if it is not adequately treated in its early phases.

Excessive exposure to sunlight is the single most important factor associated with the development of skin cancers. In addition, the tendency to develop these cancers appears to be hereditary in certain ethnic groups, especially those with fair complexions and poor tanning abilities. Fair-skinned people develop skin cancers more frequently than dark-skinned people, and the more sun exposure they receive, the more likely they are to develop a skin cancer. Other factors, including exposure to radiation, trauma and exposure to certain chemicals may also be involved in the development of skin cancers.

The vast majority of skin cancers are present for more than a year before being diagnosed and their growth is rather slow. Skin cancers may be more aggressive in certain instances: patients whose immune system is compromised, patients with a medical history of leukemia or lymphoma, cancers in certain locations such as the ear, lips, lower nose, or around the eyes.