Monte Carlo simulation of light fluence in tissue in a cylindrical diffusing fibre geometry

The propagation of light emitted by a linear light diffuser in a cylindrica l hollow organ was investigated by means of the Monte Carlo (MC) method. Th e height and radius of the cavity, scattering (mu(s)) (or reduced scatterin g, mu(s)') and absorption (mu(a)) coefficients, anisotropy (g), and refract ive indices of the media involved (nl, nr) are required as input data by th e MC code, as are characteristics of the light diffuser (length, delivered power and emission profile). Results of our MC model were tested by measuri ng the light fluence rate in a tissue-simulating phantom (mu(a) = 0.5 cm(-1 ), mu(s) = 23 cm(-1) and g = 0.75) irradiated at 633 nm with a cylindrical diffuser. Since geometric and optical parameters determine the behaviour of light propagation in tissue, MC simulations with different sets of input p arameters were carried out to provide qualitative as well as quantitative d ata useful in planning photodynamic therapy. Data are reported on light pen etration and fluence rate build-up at mu(a) and mu(s)' values ranging betwe en 0.1 and 5 cm(-1) and 2.5 and 50 cm(-1), respectively. Furthermore, resul ts suggest that a shift and spread could occur in the isofluence curves alo ng the symmetry axis, which depend on the diameter of the treated lumen as well as on the emission profile of the light diffuser. Using our data it is possible to estimate how inaccuracy in knowledge of the optical coefficien ts can affect (i.e. usually by increasing) the light dose scheduled at a ce rtain depth into tissue.