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An important medical laser application is in the emerging field of photoradiation therapy (PRT). PRT is the process in which malignant tissue is destroyed by administration of light to a specific photosensitized site. Filtered arcs, incandescents and dye lasers have been used as sources of activating light. We have carried out light experiments in tissue to study such PRT light distributions. The results of this research have shown that a number of important optical phenomena occurring within illuminated tissue must be accounted for in order to make good predictions of tumor light dosage. Among these are; tissue type, interface effects and anomalies due to composition. These effects substantially influence light levels in PRT and, thus, the therapeutic effect. The uniqueness of tissue as a medium for light transport presents special problems for optics research and instrumentation. Successful solutions necessarily will involve collaboration between the life sciences and optic specialists. Attempts at treatment of human disease using non-ionizing radiation have a history archaeologically traceable to archaic societies (in which the sun's photons were used and often worshipped).1 Western medicine in the past, has used visible light beneficially, albeit empirically, on a few ailments. However, in this century, a significant development in the understanding and in the therapeutic use of this electromagnetic radiation in the UV, visible and IR has occurred, based on scientific study. This utilization of radiation in the visible and ultraviolet can be by two distinct processes. One is through the direct action of the photons which serve as the sole treatment agent. In this case the photon interacts with the cell, or its components, in a single step, to produce a desired effect. An example is the successful use of blue light for treatment of bilirubinemia in newborns. The second process is a biological effect produced through the combination of electromagnetic radiation with a photosensitive drug or naturally occurring compounds. The photo-sensitive molecule (ideally concentrated in the target tissue) serves as a kind of reaction catalyst in the living subject by transferring the photon's energy for the initiation of a biochemical reaction. An example is the use of the photosensitive compound hematoporphyrin derivative (HpD) combined with red light used to initiate a lethal oxidation of tumor tissue in cancer therapy. Dougherty has introduced this process and termed it photoradiation therapy (PRT)2. The current understanding of the PRT mechanism is schematized in Figure 1. A photosensitive compound (PC) at groundstate, concentrated in target tissue, is acted on by a photon of appropriate energy (hv) resulting in PC being raised from its ground state to the short lived, excited singlet state (PCs). In step two, PCs decays to its longer lived triplet state (PCt). In the third step, the excited compound, PCt in the presence of ground state molecular oxygen (02) transfers its energy to the oxygen, thus raising oxygen to a highly reactive singlet oxygen state (02s). PC, through the foregoing transition, is returned to its ground state and may repeat its role on encounter with a second photon. In the last step, 02s reacts with an available tissue substrate (M). Some of this toxic oxidation is thought to occur in membrane structures, with resulting cellular disruption. The damage to this cell component is presumed to be the key to the observed tumor destruction by the
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