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 
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 .
Usually, in the studies on PDT dosimetry only the
photobleaching  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.