Photodynamic therapy (PDT) is a medical treatment that uses a light-sensitive drug and a specific wavelength of light to destroy abnormal cells. The drug is applied to the skin or injected into a vein, allowed to absorb into targeted tissue, and then activated by light, which triggers a chemical reaction that kills the cells. It’s FDA-approved for several types of cancer, precancerous skin lesions, and certain eye conditions, and it’s valued for producing less scarring than surgery.
How PDT Works
The treatment relies on three ingredients working together: a photosensitizing drug, light, and oxygen already present in your tissue. None of these three components is harmful on its own. The drug sits inactive in targeted cells until light of a precise wavelength hits it, bumping it into a high-energy state. In that excited state, the drug reacts with oxygen in the surrounding tissue to produce reactive oxygen species, which are chemically aggressive molecules that damage and kill the cells from the inside.
There are two pathways this can happen. In the most common one, the activated drug transfers energy directly to oxygen molecules, converting them into a highly destructive form. In the second pathway, the drug reacts with nearby molecules first, creating free radicals that then cause damage. Most current photosensitizing drugs work primarily through the first pathway, but both can occur during a single treatment session.
Because the drug concentrates in abnormal or rapidly growing cells and the light is aimed precisely at the treatment area, healthy surrounding tissue is largely spared. This selectivity is one of PDT’s main advantages over treatments that affect the whole body.
What PDT Treats
The FDA has approved photodynamic therapy for a range of conditions across several medical specialties:
- Precancerous skin lesions (actinic keratoses): The most common dermatologic use. A topical photosensitizer called aminolevulinic acid (ALA) is approved for treating these rough, scaly patches on the face and scalp. A systematic review in JAMA Dermatology found that ALA-based PDT had the highest long-term clearance rates among all treatments studied for actinic keratoses, outperforming cryotherapy (freezing), imiquimod cream, and other options.
- Skin cancers: Basal cell carcinoma, early-stage squamous cell carcinoma, and advanced cutaneous T-cell lymphoma are all approved indications. In Europe, approvals extend to Bowen disease, an early form of squamous cell carcinoma.
- Esophageal cancer and Barrett esophagus: A photosensitizer called porfimer (brand name Photofrin) is injected intravenously and activated with light delivered through an endoscope. This treats cancerous or precancerous tissue lining the esophagus.
- Non-small cell lung cancer: PDT can treat tumors blocking the airways, both as a curative approach and to relieve symptoms when a tumor obstructs breathing.
- Wet macular degeneration: A different photosensitizer is injected into a vein and activated by laser light directed into the eye. The drug collects in abnormal blood vessels beneath the retina and, when activated, creates blood clots that seal those vessels off. This can slow vision loss, though it cannot restore vision already lost.
What Happens During Treatment
The procedure varies depending on what’s being treated, but it follows the same basic sequence: apply the drug, wait, then apply light.
For skin conditions, a topical photosensitizer is spread directly onto the affected area. Current protocols call for a waiting period of one to three hours while the drug absorbs into the target cells, though older protocols used incubation times as long as 14 to 18 hours. Newer research has explored eliminating this waiting period entirely, using back-to-back light cycles instead. After incubation, the area is exposed to blue-violet light (around 400 to 430 nanometers) for about 16 minutes and 40 seconds, or to red light at a longer wavelength. Red light penetrates deeper into tissue, so it’s used when the target cells sit below the skin’s surface. The main treatment window for most PDT falls between 600 and 950 nanometers.
For internal cancers, the photosensitizer is injected into a vein and given time to accumulate in tumor tissue, sometimes over one to three days. Light is then delivered through a fiber-optic cable threaded to the treatment site via an endoscope or similar device.
For wet macular degeneration, the injection-to-light sequence is much faster. The drug is infused intravenously, and within minutes, a low-power laser is directed into the eye through a special contact lens. The whole procedure takes place in an eye doctor’s office.
Recovery and Side Effects
The most important thing to know about recovery is that your skin (or eyes, depending on the treatment) will be extremely sensitive to light afterward. For skin treatments, you need to protect the treated area from sun exposure and bright lights for at least 40 hours. That means wearing a wide-brimmed hat and keeping treated skin covered. For the eye treatment with verteporfin, the sensitivity window is 48 hours, and patients are advised to avoid sunlight entirely and cover all exposed skin during that period.
Common short-term side effects of skin PDT include pain during and after the light exposure, redness, swelling, itching, and sometimes blistering. The treated area can look inflamed and feel uncomfortable for several days. For some people, especially older or frail patients, this inflammatory response can be functionally limiting.
For the eye treatment, the most common concern is worsening vision from bleeding beneath the retina. Sensitivity reactions to sunlight after injection are also frequent. In rare cases, if the injected drug leaks out of the vein during infusion, it can damage surrounding skin tissue.
Long-term, skin PDT can cause changes in pigmentation (lighter or darker patches) at the treatment site. There have been rare reports of new skin growths appearing after treatment, though whether PDT directly causes these remains unclear.
PDT Compared to Surgery
For skin cancers and precancerous lesions, the choice between PDT and surgical removal often comes down to location, tumor type, and cosmetic priorities. Surgery generally has higher cure rates for aggressive or deep tumors. But PDT holds a significant cosmetic advantage, particularly on the face and neck.
Studies comparing the two approaches have found that PDT produces less scarring, better preservation of skin texture, and fewer pigmentation changes than surgical excision. Patient satisfaction with cosmetic outcomes is consistently higher in PDT groups. Surgery, even when performed carefully, carries inherent risks of scarring and contour changes in cosmetically sensitive areas.
The tradeoff is that PDT often requires multiple sessions, and the inflammatory healing process between sessions can be uncomfortable and visually noticeable. For patients who can’t undergo surgery due to health conditions, or who have lesions in areas where scarring would be especially problematic, PDT offers an effective alternative. For the eye condition, PDT was once a primary treatment for wet macular degeneration but is now used less often as a standalone therapy, since newer injectable drugs that block abnormal blood vessel growth have become the standard. Some eye doctors still use PDT in combination with these newer drugs.
Who Should Not Have PDT
PDT is not appropriate for everyone. People with conditions that make them abnormally sensitive to light, such as porphyria or lupus, should not undergo the treatment. Anyone with a known allergy to the photosensitizing drug’s components is also excluded. The treatment isn’t recommended for tumors that haven’t responded to previous PDT sessions. And because the photosensitizing drugs haven’t been proven safe during pregnancy, they carry a precautionary risk category for pregnant patients.
If the photosensitizer is applied over a large skin area, there’s a risk of it absorbing into the bloodstream and causing light sensitivity throughout the body rather than just at the treatment site. This is why dermatologists typically treat defined, limited areas in each session.