| Abstract: | Abstract The development of a methodology which optimizes the light dosage in tissue and improves the tailoring of the light with consequent sparing of normal adjacent tissue, will enhance the possibility for routine clinical photodynamic therapy. Specific, important goals for the clinical use of PDT are (a) efficient distribution of light flux to all parts of the tumor at sufficient level to effect eradication. (b) avoidance of the destruction of adjacent normal tissue, and (c) ability to tailor the treatment field, taking into account the varied shapes of tumors. Dividing the available power among several fibers is a promising method of achieving these goals. This is accomplished by (1) extending the volume, and by (2) increasing the flux spatial uniformity. This latter defocussing of the flux field is especially important because it may help to avoid concentrating a high intensity field from an implanted fiber near an essential structure of the normal tissue. The question arises how best to orient these multiple fibers for maximum coverage and uniformity. Hence, theoretical and experimental investigations were made to determine optimal fiber placements. A series of intensity distributions were generated using two and three fibers positioned at various separations within a postulated tumor volume. A criterion for uniformity was defined. Iterative computation produced optimal fiber separation for the given constraints. In the two fiber case, for small values of attenuation coefficient (μ. ≦ 0.2 mm?1), optimal fiber separation ranged from 0.6 to 0.7 times the diameter of the defined volume. For large values of attenuation coefficient (μ. ≧ 0.8), fiber separation was about 0.5 to 0.55 times the region diameter. The effects of fiber separation on volume of treatment were also determined. Maximal treatment volume was found to be dependent on the attenuation coefficient. With μ, = 0.50, a 40% increase in treatment volume over single fiber insertion of equivalent energy input was shown to be obtainable with a dual fiber configuration of 24 mm separation. Experiments using two fibers in vitro in mammalian tissue were performed to substantiate these results. The multiple fiber system is a promising method for delivering optimum light dosage to targeted PDT tissue. |
