Can photodynamic therapy treat deep tumors?

Not well. The National Cancer Institute gives a specific reason and a specific number. The light used in photodynamic therapy cannot pass through more than about a third of an inch, or 1 centimeter, of tissue. Because the light cannot reach far into a large tumor, PDT is less helpful for treating one.

Why the depth limit exists

Photodynamic therapy is a two-step treatment, and both steps have to reach the same cells.

First a photosensitizer is given. That is a drug activated by light. It may be swallowed, spread on the skin, or given through an IV, depending on where the tumor is. After 24 to 72 hours, most of the drug has left normal cells but remains in cancer or precancer cells.

Then light is aimed at the tumor. When treated cells absorb a specific wavelength, the photosensitizer produces a form of oxygen called an oxygen radical, which kills them. NCI adds two other effects: the treatment may damage blood vessels feeding the tumor, and it may trigger an immune attack on tumor cells elsewhere in the body.

The drug can travel anywhere the blood goes. The light cannot. That mismatch is the whole limitation.

What PDT is approved to treat

The list follows directly from the depth limit. NCI says the FDA has approved photodynamic therapy for actinic keratosis, advanced cutaneous T-cell lymphoma, Barrett esophagus, basal cell skin cancer, esophageal cancer, non-small cell lung cancer, and stage 0 squamous cell skin cancer.

Every one of those is on a surface or a lining.

PDT is also used to relieve symptoms rather than to cure. NCI names two situations: esophageal cancer blocking the throat, and non-small cell lung cancer blocking the airways. In both, the goal is opening a passage, not clearing the disease.

One form works differently. Extracorporeal photopheresis, or ECP, treats abnormal white blood cells that cause skin symptoms in cutaneous T-cell lymphoma. A machine collects blood cells, treats them with a photosensitizer, exposes them to light outside the body, and returns them through a vein.

An endoscope moves the light, not the limit

For tumors in the throat, airways, and lungs, a doctor threads an endoscope down the throat. That is a thin, lighted tube that lets the doctor see inside. A fiber optic cable passes through it to carry light to the treatment area.

This solves access, not penetration. Once the light reaches the tissue, the same 1 centimeter applies. So an endoscope makes a lining reachable. It does not make a bulky tumor treatable.

Side effects, and one that lasts weeks

PDT can burn, swell, cause pain, and scar in the treated area. Other effects depend on the site and may include cough, trouble swallowing, stomach pain, painful breathing, shortness of breath, and skin redness, stinging, swelling, or itching.

One deserves planning for. A photosensitizer called porfimer sodium makes skin and eyes sensitive to light for about 6 weeks. During that time NCI advises avoiding direct sunlight and bright indoor light. That is a six-week change to daily life, not an afternoon.

ECP can cause brief periods of low blood pressure, a faster than normal heart rate, anemia, and a low platelet count. NCI says side effects improve once treatment is over.

What might change this

Researchers are working on the exact problem. NCI says studies are looking for photosensitizers triggered by light that can travel through tissue, in order to treat deep or large tumors, along with agents that are more powerful, more precise, and less toxic.

A newer approach called photoimmunotherapy pairs a photosensitizer with an immune protein that delivers it to cancer cells. Studies are running in head and neck cancers.

Only a small number of hospitals and cancer centers have the trained doctors and equipment for PDT, so availability is a real question to ask locally. Our guide to questions to ask about a clinical trial covers how to approach that.

NCI's page was last updated on May 15, 2025.

Sources

• https://www.cancer.gov/about-cancer/treatment/types/photodynamic-therapy