Photodynamic therapy (PDT) is a relatively new and exciting approach for treating cancers and other diseases. Non-toxic dyes known as photosensitizers are administered systemically, locally or topically and accumulate in the tumor or other lesion. Illumination with (otherwise harmless) visible (usually red light, frequently from a laser) excites the sensitizer, which in the presence of oxygen, produces reactive oxygen species that mediate cytotoxic effects. Undesirable cells such as infectious microbial cells or malignant cancer cells can be selectively killed by this approach.
On the other hand, light therapy alone (red and near-infra-red light with no photosensitizer) can stimulate healing, prevent tissue death and relieve pain and inflammation. The mechanisms that underlie this effect are under investigation. Applications of phototherapy to healing and treatment of traumatic brain injury and brain diseases are being studied.