PDT stands for photodynamic therapy, a medical treatment that uses a combination of light and a light-sensitive drug to destroy abnormal cells. It’s FDA-approved for several skin conditions and certain cancers, and it works by creating a chemical reaction inside targeted cells that kills them while leaving surrounding healthy tissue largely intact. The treatment is most commonly used for precancerous skin patches called actinic keratoses, but it also treats certain skin cancers, esophageal cancer, and lung cancer.
How Photodynamic Therapy Works
PDT requires three components working together: a light-sensitive drug (called a photosensitizer), a specific wavelength of light, and oxygen in the tissue. None of these three elements destroys cells on its own. The damage only happens when all three are present at the same time.
For skin treatments, a photosensitizing cream is applied directly to the affected area and left to absorb, typically for about one hour. The drug concentrates in abnormal cells, which absorb it more readily than healthy ones. A clinician then shines a specific wavelength of light onto the area, activating the drug. Once activated, the drug reacts with oxygen inside the cells to produce highly reactive molecules that tear apart cell structures from the inside, killing the targeted tissue.
For internal cancers like esophageal or lung cancer, the photosensitizer is injected into a vein instead. The drug clears from most normal tissues within 40 to 72 hours but lingers significantly longer in tumors. A fiber-optic light is then threaded to the tumor site to activate the drug.
Conditions PDT Can Treat
The FDA has approved photodynamic therapy for a specific set of conditions:
- Actinic keratosis: rough, scaly patches on sun-damaged skin that can become cancerous
- Basal cell skin cancer
- Squamous cell skin cancer (stage 0)
- Barrett’s esophagus: a precancerous change in the lining of the throat, specifically high-grade dysplasia in patients who aren’t candidates for surgery
- Esophageal cancer
- Non-small cell lung cancer
- Advanced cutaneous T-cell lymphoma: treated through a specialized version called extracorporeal photopheresis, where blood is drawn, treated with light outside the body, and returned
PDT is also used to relieve symptoms when esophageal cancer blocks the throat or lung cancer blocks the airways, even when the goal isn’t to cure the cancer itself.
Blue Light vs. Red Light
Two wavelengths of light dominate PDT. In the United States, blue light (400 nm) is more widely used, while red light (635 nm) is the standard across Europe. The key difference is penetration depth: red light reaches deeper into tissue, which makes it better suited for thicker lesions. Blue light is absorbed more strongly at the surface.
For basal cell skin cancer, both wavelengths produce comparable results. One study comparing the two found clearance rates of 98% with blue light and 93% with red light, with blue light shown to be statistically non-inferior. Patients also reported that blue light treatments were less painful.
What the Treatment Feels Like
The most common complaint during PDT is a burning or stinging sensation at the treatment site. This can range from mild to severe and may continue for 24 to 48 hours after the session. The treated area typically turns red and swollen for up to a week, then develops scaling or crusting that can last up to four weeks as the skin heals.
Pain levels vary depending on the approach. Conventional in-office PDT, which uses a concentrated lamp for about 16 to 17 minutes, tends to be the most uncomfortable. A newer option called daylight PDT skips the lamp entirely. Instead, the photosensitizing cream is applied for just 15 minutes before you spend an hour outdoors in natural daylight. Studies comparing the two approaches found that daylight PDT reduced peak pain scores by roughly 3.5 points on a 10-point scale compared to conventional treatment, with similar effectiveness. At 12 weeks, conventional PDT cleared about 64% of actinic keratoses while daylight PDT cleared about 62%.
How Effective Is PDT for Skin Lesions?
For actinic keratosis, which is the most common reason people get PDT, the treatment clears about 81% of lesions after up to two sessions, compared to 45% clearance with a placebo application. Smaller lesions respond slightly better (83% clearance) than larger ones (76%), but both outperform placebo by a wide margin. Overall, about 82% of total diseased skin area was cleared at 12 weeks.
These numbers mean most people see significant improvement, but PDT rarely eliminates every lesion in a single round. Some patients need repeat treatments, and new lesions can develop over time in sun-damaged skin.
Recovery and Sun Sensitivity
The photosensitizing drug doesn’t leave your body immediately after treatment. For topical skin PDT, you’ll need to stay indoors as much as possible for the first 48 hours. Even indoor light near windows can trigger a reaction on treated skin during this window. After that initial period, you should continue protecting the area from direct sunlight while it heals.
For injectable photosensitizers used in internal cancers, the sensitivity period is longer and more serious. The drug can remain in your skin for weeks, making you vulnerable to severe sunburn from even brief sun exposure. Your care team will give you a specific timeline for light avoidance based on the drug used.
PDT for Internal Cancers
When used inside the body, PDT looks quite different from a skin treatment. For esophageal cancer or Barrett’s esophagus with precancerous changes, the photosensitizer is injected days before the procedure. During the treatment itself, an endoscope delivers light directly to the tumor or abnormal tissue lining the esophagus. The light activates the drug at 630 nm, triggering destruction of the targeted cells.
This approach works best for small, early-stage tumors or precancerous tissue. The FDA specifically recommends it for esophageal cancer lesions that are limited to the inner layers of the esophageal wall and are less than 2 cm in size. For larger or more advanced cancers, PDT is more often used as a palliative treatment to reopen a blocked esophagus and restore the ability to swallow, rather than as a cure.
Advantages Over Other Treatments
PDT has a few distinct advantages that explain why it remains in use alongside surgery, freezing, and topical chemotherapy creams. It targets abnormal cells selectively, since those cells absorb more of the photosensitizing drug than healthy tissue. It can treat broad areas of skin in a single session rather than lesion by lesion. It doesn’t cause the scarring that surgery or aggressive freezing can. And for internal tumors blocking the airways or esophagus, it can relieve obstruction without major surgery.
The main limitations are that light can only penetrate so deep, which restricts PDT to surface-level or accessible cancers, and the temporary sun sensitivity requires real lifestyle adjustments during recovery. For thick or deeply invasive tumors, PDT alone is not sufficient.