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The role of modeling in designing new treatment protocols and instruments is discussed. A computer program for modeling laser-tissue interaction named Latis is described. Interactions are divided into the processes of laser propagation, thermal effects, material effects, and hydrodynamics. Full coupling of the processes is taken into consideration. Applications in photothermal and photomechanical laser-tissue interactions are briefly discussed. A detailed description is given of a particular application of Latis to study the effects of dynamic optical properties on dosimetry in photothermal therapy. Optical properties are functions of tissue damage, as determined by previous measurements. Results are presented for the time variation of the light distribution and damage within the tissue as the optical properties of the tissue are altered. It is found that proper accounting of dynamical optical properties is important for accurate dosimetry modeling.

Optimum methods for calculating the effects of photon trapping are discussed. An efficient line-transfer algorithm that can calculate trapping when there are overlapping and interacting lines is described. Escape probability formulas are shown to be appropriate for calculating photon trapping for isolated lines and for the highest-energy line in a group of lines in many situations. Major computational savings are achieved by using cylindrical escape probabilities for recombination x-ray laser schemes. For collisional x-ray laser schemes it is shown that the calculation of line transfer in planar geometry is sufficiently fast that one only obtains substantial savings by exploiting the coarser spatial zoning that is possible when using escape probabilities in regions of steep velocity gradients. The use of escape probabilities is shown to be particularly well suited for single-zone parameter studies.