Photodynamic therapy (PDT) is a two-stage treatment that combines light energy with a drug designed to destroy cancerous and precancerous cells after light activation. PDT may also be called photoradiation therapy, phototherapy, or photochemotherapy. The treatment uses special drugs, sometimes called photosensitizing agents. The drugs only work after they have been activated or “turned on” by certain kinds of light. Photosensitizers are activated by a specific wavelength of light energy, usually from a laser. The photosensitizer is nontoxic until it is activated by light. However, after light activation, the photosensitizer becomes toxic to the targeted tissue.
Depending on the part of the body being treated, the photosensitizing agent is either put into the bloodstream through a vein or put on the skin. Over a certain amount of time, the drug is absorbed by the cancer cells. Then the light is applied to the area to be treated. The light causes the drug to react and form a special kind of oxygen molecule that kills the cells, called the oxygen radical that kills the cancer cells. PDT might also help by destroying the blood vessels that feed the cancer cells and by alerting the immune system to attack cancer.
Several photosensitizer drugs are available today to treat a variety of diseases, including acne, psoriasis, age-related macular degeneration, and several cancers. PDT also helps treat bacterial, fungal and viral infections.
The FDA has approved photodynamic therapy to treat:
- Actinic keratosis
- Advanced cutaneous T- cell lymphoma
- Barrett esophagus
- Basal cell skin cancer
- Esophageal (throat) cancer
- Non-small cell lung cancer
- Squamous cell skin cancer
- Esophageal cancer when it blocks the throat
- Non-small cell lung cancer when it blocks the airways
How Photodynamic Therapy Is Given
Photodynamic therapy is a two-step process. First, you will first receive a photosensitizer. The drug may be taken by mouth, spread on the skin, or given through an IV, depending on where the tumour is in the body. After 24 to 72 hours, most of the drug will have left normal cells but remain in cancer or precancer cells. Then your tumour will be exposed to the light source.
The period of time between when the drug is given and when the light is applied is called the drug-to-light interval. It can be anywhere from a couple of hours to a couple of days, depending on the drug used. The light used in PDT comes from certain kinds of lasers or from light-emitting diodes (LEDs). The kind of light used depends on the type of cancer and where it is located in the body. PDT is usually done as an outpatient procedure but is sometimes combined with surgery, chemotherapy or other anti-cancer drugs, or radiation therapy.
How the light is applied depends on where the tumour is. For skin tumours, the light is aimed right at cancer. For tumours in the throat, airways, and lungs, your doctor will insert an endoscope down your throat. Once the endoscope is in place, the doctor threads a fibre optic cable that transmits light through it to reach the treatment areas.