What is PDT?
PDT is a light activated chemotherapy. A photon is absorbed by a photosenstive drug which moves the drug into an excited state. The excited drug can then pass its energy to oxygen to create a chemical radical called "singlet oxygen". Singlet oxygen attacks cellular structures by oxidation. Such oxidative damage might be oxidation of cell membranes or proteins. When the accumulation of oxidative damage exceeds a threshold level, the cell begins to die.
In summary, photon + drug + oxygen -> singlet oxygen -> oxidative injury -> cell death.