THE THEORY OF PHOTODYNAMIC THERAPY DOSIMETRY: CONSEQUENCES OF PHOTO‐DESTRUCTION OF SENSITIZER

Abstract Photodynamic dose is defined as the area under the curve of sensitizer level plotted as a function of light dose. This is a photochemical definition of dose. We will show that this definition is useful in predicting photobiological response. The photodestruction of sensitizer during photodynamic therapy is shown to result in an upper limit on the photodynamic dose which can be delivered by an unlimited light dose. This limit results in the opportunity to make total photodynamic dose uniform to considerable depths (one to two centimeters). The existence of thresholds for permanent tissue damage allows protection of normal tissue from the large light doses required to achieve this limiting dose deep in the tissue. Higher sensitizer levels in the tumor permit tumor destruction while the normal tissues are protected. A clinical trial to determine the proper level of injected dose necessary for these results is required.