Melanoma and Skin Cancer

What it is

Skin cancer is the most common type of cancer. NCI describes three main kinds: basal cell, squamous cell, and melanoma.

Melanoma starts in melanocytes, the cells that make skin pigment. It is much less common than the other two, but much more likely to grow into nearby tissue and spread. NCI says most deaths from skin cancer are caused by melanoma.

What to watch for

For melanoma, the signs are visual. NCI uses the ABCDE rule for a mole or spot:

• A for asymmetry. One half does not match the other.
• B for border. The edge is ragged, notched, or blurred.
• C for color. It is uneven, or has more than one color in it.
• D for diameter. It is larger than about 6 millimeters, roughly a pencil eraser.
• E for evolving. It is changing.

NCI also lists moles that itch, bleed, or ulcerate.

Of these, evolving is the one to hold onto. A spot that looks odd but has looked exactly that way for twenty years is less concerning than a spot that is new, or that has changed in the last few months. If you are not sure, a photo with a date is a genuinely useful thing to bring to an appointment.

What raises the risk

Things nobody can change: fair skin that burns easily, freckles, and tans poorly. Light-colored eyes. Red or blond hair. Many moles, or unusual ones. A personal or family history of melanoma. A weakened immune system. Certain inherited gene changes.

Things that can change: long-term exposure to sunlight and to artificial sources including tanning beds, and, in NCI's words, having a history of many blistering sunburns, especially as a child or teenager.

What to do about the sun is its own page: sun and UV.

Is there a test before symptoms start?

Not one that has been shown to save lives, which surprises people.

NCI states that skin exams to screen for skin cancer have not been shown to decrease the number of deaths from the disease, and that there is not enough evidence to know whether screening the whole population lowers death rates.

The US Preventive Services Task Force reached the same place. In 2023 it gave a grade I for skin cancer screening in adolescents and adults who have no symptoms, meaning the evidence is insufficient to weigh benefits against harms.

Two things that are not in doubt: NCI says regular skin checks by a doctor are important for people who have already had skin cancer, and a spot that is changing should be looked at whatever the population-level evidence says about screening everybody.