Photodynamic therapy (PDT) is a treatment modality, which is at present widely used on an experimental basis for the treatment of cancer patients. It is based on the light induced excitation of light sensitive chemical compounds localized in malignant tissue. These so-called photosensitizers are capable of absorbing photons due to their extended conjugated ring systems. As a result the compound is excited to the singlet state. By decay of the excited electrons to the ground state electromagnetic energy is released in the form of fluorescence, but some singlet state molecules will undergo intersystem crossover to the triplet state. The energy of these triplet state molecules is conveyed in either of two ways. In the absence of oxygen electrons are transferred to other molecules of the photosensitizer or to biological substrates which will lead to negatively or positively charged radicals (Type I reaction). In the presence of oxygen the energy of the photoexcited sensitizer is directly transferred to oxygen, leading to the formation of singlet oxygen (Type IT reaction). As a result of these reactions the excited photosensitizer will return to its ground state ready to absorb photons again.

, , , , ,
J.F. Koster (Johan)
Erasmus University Rotterdam
Dutch Cancer Society
Erasmus MC: University Medical Center Rotterdam

de Vree, W.J.A. (2000, February 9). Identification of neutrophils as important effector cells in photodynamic therapy. Erasmus University Rotterdam. Retrieved from