photodynamic therapy
Meaning of photodynamic therapy: Three ingredients are required and none does anything alone: the light-sensitive drug, the light itself, and oxygen already present in the tissue.
Definition of photodynamic therapy
Photodynamic therapy is a treatment combining a light-sensitive drug with light of a specific wavelength. The drug is given first and collects preferentially in abnormal or rapidly dividing cells, staying inactive until light reaches it. The light then triggers a reaction with oxygen inside those cells, producing molecules that damage them from within while largely sparing the surrounding tissue.
How does it work?
The photosensitizer is applied to the skin, swallowed or injected, and time is allowed for it to concentrate in the target. Light of a matched wavelength is then delivered by lamp, laser or a fiber threaded through an endoscope. The drug absorbs that energy and passes it to oxygen molecules nearby, generating reactive forms of oxygen that oxidize cell membranes and structures within a very short distance. Cells die, small vessels feeding the area shut down, and an immune response follows into the treated zone. The selectivity comes from three things having to coincide: drug, light and oxygen. Where any one is missing, nothing happens, which is why abnormal tissue can be targeted without cutting.
What is it used to treat?
| Setting | Typical target |
|---|---|
| Skin | Actinic keratoses, some superficial basal cell carcinomas, acne in some protocols |
| Esophagus | Barrett esophagus with precancerous change, and some obstructing tumors |
| Airway | Early or obstructing lung tumors reachable by bronchoscope |
| Eye | Abnormal vessel growth in some forms of macular degeneration |
What limits it?
Light penetrates tissue only a few millimeters, so treatment suits surface and accessible lesions rather than deep or bulky tumors, and it does not address disease that has spread. Unlike targeted therapy aimed at a molecular feature, its selectivity is physical and depends on where light can be aimed. Treated skin becomes red, swollen and sore for days, similar to sunburn. With drugs given by injection rather than applied to the skin, the whole body remains light-sensitive for a period afterward, so bright light and direct sun cause burning until the drug clears. The approach is related to but distinct from light therapy, which uses light alone without a sensitizing drug.
Also known as
Used in a sentence
Photodynamic therapy was used to treat the superficial skin lesion identified during the dermatology visit.
Good to know
Photodynamic therapy depends on light reaching the treated tissue directly, which is why it is generally suited to superficial or accessible lesions rather than tumors located deep within the body.
Word origin
Where “photodynamic therapy” comes from
Formed from Greek photo-, meaning light, and dynamis, meaning power, combined with therapy, from Greek therapeia, meaning treatment.