Relationship between intensity of reflected light and temperature increase: assessment of fundus pigmentation for transpupillary thermotherapy

To develop a new device for measuring reflected light during diode-laser irradiation in transpupillary thermotherapy (TTT) and to assess the correlation between fundus pigmentation, the measured intensity of reflected light, and the increase in fundus temperature. We developed a device to measure reflected light by modifying a slit-lamp-mounted 810-nm diode-laser delivery system used clinically for TTT. The diode laser was used to irradiate test charts with various degrees of reflectance in in vitro experiments, and the fundus of nonpigmented or pigmented rabbits in in vivo experiments; then, the intensity of the reflected light and the temperature increase at the target were measured. The retinal sections were also examined histologically. There was a significant negative correlation between the intensity of the reflected light and the temperature increase in the target that depended on the degree of reflectance of the charts or the pigmentation of the rabbit eyes. On histopathologic examination, the extent of the changes in the irradiated retina after TTT was clearly different between pigmented and nonpigmented rabbits. Correlations between fundus pigmentation, the intensity of reflected light during diode-laser irradiation, and the temperature increase in TTT were demonstrated in vitro and in vivo. Our results suggest that measurement of the intensity of reflected light should be helpful for modulating the laser power in TTT according to the degree of fundus pigmentation.