ABSTRACT
The rationale of the therapeutic efficacy of photodynamic therapy is based on the cytotoxic action of products generated by excited photosensitizers. The excited photosensitizer can react directly with tissue constituents through the so-called type I process, yielding radicals or radical ions or through the type II process in which energy is transferred to singlet oxygen. The relation between damage to the normal tissue and that to the malignant tissue is determined by the difference in photosensitizer concentration. In summary, the optimal use of photobleaching enables the selective depth to coincide with the therapeutic depth, whereas the value for the therapeutic depth itself is rather unaffected by bleaching. The retention ratio is of only indirect importance in a procedure optimized for bleaching; the optimization procedure, in principle, requires, only that the retention ratio is larger than 1.
