Dosimetry Methods in Photoradiation Therapy

Abstract

It is important to be able to specify the effective dose in photoradiation therapy (PRT) or photodynamic therapy (PDT) in order to compare results and to assure that the dose is sufficient to achieve the desired therapeutic effect while not damaging nonmalignant tissues excessively. The effective absorbed dose depends on the energy flux density or “space irradiance” at the dose point, the concentration of photosensitizing drug, and the concentration of molecular oxygen. Up to now, the clinician has not been able to specify the true, effective absorbed dose, although such terms as “light dose” (which is really an energy flux density, not a dose) have been used. The energy flux density at the dose point within the tumor is seldom known; instead the source power or irradiance is given. The dosage (mg drug per kg body weight) and the time delay after intravenous injection are specified, but the concentrations of the drug in tumor and normal tissue are not measured or even reproducible with the same dosage and time delay. The concentration of molecular oxygen is usually unknown, although a hypoxic region of a tumor would be protected from photodynamic cytotoxicity. Methods have to be developed to measure or calculate the contributions to the effective absorbed dose.