What is photodynamic therapy (PDT)? a) Photodynamic therapy, a new disease treatment based on the interaction of light, photosensitizers and oxygen. b) The basis of the action of photodynamic therapy: photodynamic effect It is a photosensitization reaction accompanied by biological effects with the participation of oxygen molecules. The process is: the light irradiation of specific wavelength causes the tissue absorbing photosensitizer to be excited, and the excited state of the photosensitizer in turn transfers energy to the surrounding oxygen, generating highly reactive monomorphic oxygen, which reacts with adjacent biological macromolecules to produce cytotoxic effects, leading to cell damage or even death. The three elements of the photodynamic effect are: photosensitizer: its photodynamic activity, light absorption properties and targeting properties determine its clinical availability and scope of application. Irradiated light: its correct wavelength, output stability and cast reliability are also important controllable factors determining the therapeutic effect. Oxygen: In the excited state, the photosensitizer transmits energy to oxygen in the surrounding tissue, which generates highly reactive monomorphic oxygen and stimulates the production of a large number of free radicals, which in turn mediates cytotoxic effects. Since photodynamics is performed on living bodies and living tissues are oxygenated, no additional oxygen needs to be introduced to initiate photodynamic therapy. c) Why can photodynamic effects be used to treat diseases? There are two prerequisites for the photodynamic effect to be used for disease treatment: 1) the specific diseased tissue is able to take up and retain more of the photosensitizer, and 2) the target site is more susceptible to light irradiation. Only if the above two prerequisites can be achieved, the strong photodynamic effect can occur and the diseased tissue can be adequately destroyed. In addition, in photodynamic therapy, in addition to the monomorphic oxygen and free radicals generated during the conversion of light energy that can directly kill lesion cells, local microcirculatory disorders caused by capillary endothelial damage and vascular embolism triggered by this process further lead to ischemic necrosis of the lesion tissue.