Numerical Light Dosimetry in Murine Tissue: Analysis of tumor curvature and angle of incidence effects upon fluence in the tissue

In order to better understand light dosimetry issues for photodynamic therapy (PDT), we have used various tumor and normal tissue geometries to develop a diffusion model of light transport in tissues. We hypothesize that tumor tissues with curved surfaces will have significantly different internal fluence distributions, as compared to tissues with flat surfaces. Using a mouse subcutaneous tumor and rear limb muscle model we compared the internal fluence values within the tissue. In addition, numerical simulations for these corresponding tissue geometries and laser light incidence angles were made. Assuming that the relative photon fluence in the tissue can be accurately modeled by the diffusion equation, we used a finite element approach to approximate the distribution inside the tissue. Meshes with different geometries (flat and curved with different curvatures) were used in this study to mimic the tumor and leg geometries of the murine tumors treated in the lab. Results suggest that tissues surface geometries and incidence angle of light can significantly alter the photon fluence inside the tissue. The photon fluence difference for an 8 mm diameter, curved surface mouse tumor vs. flat muscle tissue can be as high as 20%. In general, the greater the tissues curvature, the greater the potential loss in light fluence is. In summary, our data demonstrates the importance of tissue surface geometry and the incidence angle of light in determining optimal PDT light dosimetry, and indicates that comparisons between tissue geometries must be carried out with attention to differences in the internal optical distribution.