American Melanoma Institute · 5 min read
What actinic keratoses are, how often they turn into squamous cell carcinoma, and how spot treatments differ from field treatments.
An actinic keratosis (AK), also called a solar keratosis, is a small, rough, scaly spot on skin that has absorbed years of sun. Many people feel one before they see it. The patch catches a fingertip like fine sandpaper, flakes off, and then returns in the same place.
AKs are extremely common. The Skin Cancer Foundation estimates that 58 million Americans have at least one. They are usually smaller than a quarter inch and may be pink, red, tan, or skin colored. Favorite sites are the face, ears, bald scalp, neck, forearms, and backs of the hands. On the lower lip, the same process is called actinic cheilitis.
Ultraviolet light damages the DNA of keratinocytes, the main cells of the epidermis, the outer layer of skin. In an AK, some of those damaged cells have started to grow in a disordered way, yet they remain within the epidermis. If they break through into the dermis, the thicker layer below, the spot has become a squamous cell carcinoma (SCC).
Honest answers here come with a range. The American Cancer Society says only that a small percentage of AKs may become SCC and that most do not. The Skin Cancer Foundation cites 5% to 10%. Estimates differ because studies follow different groups of people for different lengths of time, and because AKs are usually diagnosed by eye, without a biopsy.
One of the most careful studies tracked individual spots with photographs. In the Veterans Affairs Topical Tretinoin Chemoprevention Trial (Criscione and colleagues, Cancer, 2009), researchers mapped 7,784 AKs on the face and ears of 169 high-risk veterans. The chance that a given AK became SCC, including SCC in situ, was 0.6% at 1 year and about 2.6% at 4 years. More than half of the AKs could no longer be found a year later.
So the risk from any one spot is low. The risk for the person is a different matter. Someone with 10 or 20 AKs carries that small risk many times over, year after year. In the same study, about 65% of the SCCs that developed arose in spots previously identified as AKs. DermNet puts the chance of an SCC at some point at roughly 10 to 15% for a person with more than 10 AKs.
No one can tell by looking which AK will progress, which is the main reason doctors treat them. Certain features do raise suspicion that a spot has already become SCC, and they usually lead to a biopsy:
When a person has a handful of separate AKs, doctors usually treat each one directly. Cryosurgery is the most common method. The dermatologist sprays or dabs liquid nitrogen on the spot for a few seconds. It stings, may blister, then crusts and peels. Thick AKs sometimes need a second freeze, and treated skin can heal lighter than the skin around it.
Skin around a visible AK often contains the same DNA damage, along with early AKs too small to see or feel. Doctors call this field cancerization. When many AKs crowd one area such as the forehead or scalp, treating the entire field clears both visible spots and the ones still forming. The American Academy of Dermatology lists these field treatments:
Most field treatments work by making damaged skin red, raw, and crusted for a period of days to weeks. With fluorouracil in particular, a strong reaction is expected and signals that the drug is reaching abnormal cells. Knowing the timeline in advance helps people finish the course. After photodynamic therapy, patients stay out of daylight for 48 hours. Fluorouracil must not be used during pregnancy.
Daily broad-spectrum sunscreen, a wide-brimmed hat, and shade during midday hours limit further damage, and DermNet notes that some existing AKs clear with consistent sunscreen use alone. Avoiding tanning beds matters at every age.
Nicotinamide, a form of vitamin B3, has been tested as a preventive. In the ONTRAC trial (Chen and colleagues, New England Journal of Medicine, 2015), 386 adults with a history of skin cancer who took 500 mg twice a day had 13% fewer AKs at 12 months than those on placebo, along with fewer new skin cancers. A later trial of the same dose in organ transplant recipients (Allen and colleagues, New England Journal of Medicine, 2023) found no benefit, although it enrolled only 158 people and closed early. More data is needed on who benefits.
AKs tend to return, because the sun damage behind them is permanent. The American Academy of Dermatology notes that people with many AKs are typically examined once or twice a year, and those with suppressed immune systems as often as every 8 to 12 weeks.
