By the identification of more factors, which influence the efficacy of a treatment, the dosimetry in photodynamic therapy (PDT) is becoming increasingly complex. In the simplest way a dose in PDT have been described by specifying the administered photosensitizer dose (mg of photosensitizer per kg of body weight), the incident light dose (energy of light in Joules delivered to 1 cm^2 surface of cancerous tissue) and time interval between the administration of photosensitizer and light illumination. Most clinical protocols still utilize only these three parameters for the selection of effective dose, despite the fact that these parameters give only partial information about the real photodynamic dose.
    An excellent review of Wilson et al [1] on the dosimetry in PDT appeared recently. The interdependencies of PDT dosimetry factors, definitions of explicit and implicit dosimetry, their shortages and alternative dose metrics are presented so comprehensively that it is difficult to add anything not mentioned by the authors. The main challenge to improving PDT dosimetry identified by the authors of review article is to understand how, both in principle and in practice, dose-modifying factors fit together to determine the effective delivered dose. The paper introduces the concept of implicit and explicit PDT dosimetry and discusses the advantages and limitations of each. The traditional dosimetric approach, measuring each dose factor independently, termed explicit dosimetry, is contrasted with the recent trend to use photosensitizer photobleaching as an index of the effective delivered dose, termed implicit dosimetry. The need to understand the degree to which the photobleaching mechanism is linked, or “coupled” to the photosensitizing mechanizm is analyzed. The fundamental advantage of the implicit dosimetry approach is that it folds together the multiple photophysical/photochemical/photobiological factors involved in the PDT effect [1].
    Usually, in the studies on PDT dosimetry only the photobleaching [2] or photobleaching followed by the formation of photoproducts with new absorption bands [3,4] are discussed. There is a lack of discussions on the influence of other parameters such as localization, interaction with cellular compartments or pH of the environment on the photosensitizer photobleaching as an index of the effective delivered dose. In our study we will concentrate on some additional factors which might be important for the use of photosensitizer photomodification in implicit dosimetry.