Monte Carlo simulations of the use of isotropic light dosimetry probes to monitor energy fluence in biological tissues

The use of isotropic light dosimetry probes is being increasingly advocated to monitor the light dose during photodynamic therapy. In this paper Monte Carlo simulations were used to study the potential error resulting from the disturbance of the local fluence due to presence of the probe. External irradiation of a plane slab of tissue was modelled for two sets of tissue optical properties. A highly scattering, nearly spherical detector probe, approximately 1 mm in diameter, positioned just above the tissue surface and at several depths below the surface, was incorporated into the simulation. Tissue fluence distributions were generated in the absence of the detector probe, and with the probe in situ. The presence of the probe caused negligible disturbance to the ambient fluence distribution when embedded in the tissue. The fluence measured at the centre of the detector probe showed a good correlation with the tissue fluence measured in the absence of the probe. With the detector probe positioned just above the tissue surface some reduction in surface tissue fluence was observed, although this effect was negligible at greater tissue depths. It was concluded that, when embedded in tissue, isotropic detector probes produce an accurate measure of tissue fluence or fluence rate.