A Monte Carlo model of detected singlet oxygen luminescence and photosensitizer fluorescence during ALA-PDT of skin

Singlet Oxygen (1O2) Luminescence Dosimetry (SOLD) and fluorescence photobleaching are being investigated as dosimetric tools for clinical PDT. Both have been applied during superficial ALA-PDT of normal skin and skin cancers. The interpretation of fluorescence and SOLD data is complicated by the non-uniform distribution and bleaching of PpIX and the absorption and scattering of light in the skin. The aim of the present work was to tackle these challenges using Monte Carlo (MC) simulations. Skin was modeled as a three-layer semi-infinite medium with uniform optical properties in each layer. The initial depth-dependent distribution of PpIX was an exponential decay and, after the delivery of each treatment fluence increment, standard photochemical reaction kinetics were used to update the distribution of sensitizer and reacted singlet oxygen. Oxygen depletion due to photochemical consumption or vascular shutdown was also incorporated in the model. The adjoint method was applied to calculate the PpIX fluorescence and 1270 nm singlet oxygen luminescence reaching the skin surface in each time increment. The time-resolved evolution of the fluorescence and cumulative SOLD signals during treatment were compared to the time-resolved volume-averaged distribution of reacted singlet oxygen in the dermis layer for typical clinical PDT conditions. Approximate linear relationships were observed over most of the treatment time.