Photodynamic therapy for actinic keratoses and thin skin cancers

How a light-sensitizing drug plus a light source clears sun-damaged skin, what the visit feels like, the 48 hours of light avoidance, daylight PDT, and results.

Photodynamic therapy, or PDT, treats a whole area of sun-damaged skin in a single office visit. It pairs a drug that makes abnormal cells sensitive to light with a lamp that sets the drug off. Together they destroy precancerous cells and leave most normal skin intact.

A drug that turns light into a weapon

The drug is aminolevulinic acid (ALA) or its close relative methyl aminolevulinate (MAL), applied as a solution, gel, or cream. Skin cells absorb it and convert it into a light-sensitive pigment called protoporphyrin IX. Abnormal, fast-growing cells build up much more of this pigment than healthy cells do.

When light of the right color hits the pigment, the pigment passes that energy to oxygen inside the cell. The result is a burst of highly reactive oxygen molecules that destroy the cell from within. Blue light and red light both work. Red light reaches a little deeper into the skin.

What PDT treats

Its main use is actinic keratoses, the rough, scaly precancers that appear on the face, scalp, and arms after decades of sun. PDT fits best when there are many of them spread over an area, since the whole field gets treated at once. In the United States, the FDA has approved ALA with blue light or red light for actinic keratoses.

PDT can also treat squamous cell carcinoma in situ and thin, low-risk basal cell carcinomas. In September 2026 the FDA approved ALA gel with red light for superficial basal cell carcinoma in adults, and regulators in a number of other countries approved PDT for these uses earlier. The Skin Cancer Foundation states that PDT is not recommended for invasive basal cell carcinoma. The light cannot reach a deep tumor. Melanoma is never treated with PDT.

The visit, from start to finish

1. A nurse cleans the skin, often with acetone or alcohol, to strip surface oils. Thick scale may be gently scraped off so the drug can soak in.
2. The drug is applied to the entire treatment area. You then wait while your cells convert it. The wait runs from about 1 hour to 3 hours or more depending on the product, the body site, and the clinic's protocol. Arms need longer than the face.
3. You sit in front of the lamp wearing goggles. Blue light sessions last roughly 15 minutes, and red light sessions are often shorter.
4. Staff wash off any leftover drug, apply sunscreen, and send you home with a wide-brimmed hat.

Many treatment plans include a second session. The American Academy of Dermatology describes a repeat treatment about 3 weeks after the first.

What the light feels like

The light itself is not hot, yet most people feel stinging, prickling, or burning once it switches on. The sensation builds during the first few minutes and then levels off. Heavily sun-damaged skin hurts more, as do the scalp and forehead.

Clinics manage this with fans, cool air blowers, water spray, and short pauses. Some use a numbing spray. The discomfort fades fast when the lamp turns off.

The 48 hours afterward, then a week of healing

Your skin stays sensitive to light for about 48 hours, because some of the pigment lingers in your cells. During that time, daylight can set off the reaction all over again. The American Academy of Dermatology advises avoiding the outdoors during daylight for 48 hours, and points out that even a cloudy or snowy day can cause a serious skin reaction.

Sunscreen offers limited protection here. The pigment reacts to visible light, which most sunscreens do not block. Staying indoors and away from bright windows works. If you must go out, cover the area with clothing and a hat. Schedule the appointment for a time when 2 days at home are possible.

Treated skin looks and feels sunburned: red, swollen, and tight, most of all during the first 2 or 3 days. Peeling and crusting follow. On the face most of this settles within 1 to 2 weeks, and treated skin cancers can take several weeks to heal fully.

Cool compresses, a bland moisturizer or plain petrolatum, and acetaminophen handle most symptoms. Blisters, pigment changes, and infection are possible but uncommon, and scarring is usually minimal. Call the office for pus, spreading redness, or pain that gets worse after day 3.

Daylight PDT

In daylight PDT the sun replaces the lamp. After sunscreen goes on, the clinic applies the drug, and within about half an hour you go outdoors and stay there for 2 hours. The pigment gets used up continuously as it forms instead of all at once under a lamp, so the treatment is nearly free of pain.

An international consensus group (Wiegell and colleagues, Journal of the European Academy of Dermatology and Venereology, 2012) reviewed 3 randomized trials. The group concluded that daylight PDT is effective for thin actinic keratoses and far more comfortable than the lamp-based version. The method depends on weather and season, so cold or dark months rule it out. Daylight PDT is routine in Europe and Australia. In the United States the method is off-label, and fewer clinics offer it.

How well it works

PDT clears most thin actinic keratoses on the face and scalp in the short term, with a cosmetic result that doctors and patients rate highly. Thick, crusted keratoses and those on the arms and hands respond less well.

A head-to-head trial enrolled 624 patients (Jansen and colleagues, New England Journal of Medicine, 2019). About 38% of those treated with MAL-PDT remained successfully treated 12 months later, compared with about 75% of those who used fluorouracil cream for 4 weeks. PDT still appeals to many people because a single day replaces weeks of daily cream and visible irritation. Actinic keratoses tend to return whatever the method, so a repeat course a year or 2 later is common.