ASK THE EXPERT: How can I develop melanoma on the bottom of my foot?

By Marcy Street, MD

Marcy Street, MD, is a Mayo Clinic-trained dermatologist. Dr. Street was the first fellowship-trained African American female Mohs surgeon in the United States. She is the Founder and Medical Director of the Doctor’s Approach family of services, including Doctor’s Approach Dermatology, a complete care medical clinic specializing in skin cancer, and Doctor’s Approach Medspa & Laser Center.

Q. I thought skin cancers were caused by sun exposure. But I have been diagnosed with a melanoma on the bottom of my foot, which is almost never exposed to the sun. Why did a melanoma develop there?

A. Skin cancers are most often caused by the cumulative ultraviolet (UV) damage from chronic and/or periodically intense sun exposure over one’s lifetime. But other factors can be involved as well — above all, genetic predisposition. Some key genetic factors include being fair-skinned and having light hair and eyes. People with these characteristics have a tendency to burn more easily because they produce less melanin, the protective pigment that absorbs some UV radiation, reducing sun damage to skin cells. 

Some environmental factors also increase skin cancer risk. For example, heavy drinkers and smokers are more likely to develop oral skin cancers. Chemical burns and environmental pollutants such as motor vehicle exhaust and arsenic (found in soil, urban air, and some water supplies) also cause skin damage that can lead to skin cancer, regardless of sun exposure history and skin type. Finally, immune deficiencies, organ transplants (with immune-suppressing drugs required), and viral infections such as human papillomavirus (HPV) or HIV can all reduce a person’s ability to eliminate damaged cells and hinder the body’s defenses against malignancies, increasing the risk of skin cancer and other cancers.

Two relatively uncommon types of skin cancer are usually unrelated to sun exposure — acral lentiginous melanoma (ALM) and mucosal melanoma. ALM is most often found on the palms of the hands, soles of the feet, and under the nails. Mucosal melanoma develops on mucosal surfaces in the body such as the mouth, sinuses, nasal passages, and genitals. Both ALM and mucosal melanoma are more common than other forms of the disease in people of color, and both can become fast-moving and dangerous because they are frequently discovered later, remaining undetected until the cancer has spread to other areas of the body. 

Be suspicious if you feel a mass in the mouth, or if you see a sore that won’t heal, a bruise that does not fade, or a change in a nail, under the nail, or in a mole. The best advice when you make such a discovery is to seek the opinion of a physician promptly.

Published on September 18, 2013