Image analysis of erythema for Quantitative Evaluation of Photosensitivity in Photodynamic Therapy

We studied quantitative evaluation of skin erythema using dorsum of hand images of photodynamic therapy (PDT) patients before and after the exposure to sunlight. While PDT is used as a minimally invasive cancer treatment, there is a problem of photosensitivity caused by residual photosensitizer in the skin. To prevent this problem, a dark hospitalization of two weeks in a room with less than 500 lx is prescribed in the case of Npe6 in Japan. After 2 weeks of a dark hospitalization, dorsum of hand is exposed to sunlight for 5 minutes, and the photosensitivity is visually judged to determine the end of a dark hospitalization, but this method has the problem of being qualitative and invasive. The purpose of this study was to quantitatively evaluate cutaneous photosensitivity by analyzing dorsum of hand images of PDT patients before and after the exposure to sunlight using three colorimetric systems. Dorsum of hand images before and after sunlight exposure of nine PDT patients were taken along with color scales. Image processing software was used to extract the sRGB values of the sunlight-exposed area and color scale from the acquired images as histograms. The sRGB values of the color scale of the images were used to correct the imaging conditions and to compare the images before and after sunlight exposure in three color systems: sRGB, CIELAB, and HSV color system. We suggest that the G values of sRGB has the possibility of quantitative evaluation of erythema and photosensitivity.