304 Suitable wavelength characteristics for applying deep ultraviolet light-emitting diodes to phototherapy

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The luminous efficiency of deep UV light-emitting diodes (DUV-LEDs) are now remarkably improved and expected to be applied to phototherapy. The full width at half maximum (FWHM) of DUV-LEDs differs from that of traditional light sources such as the narrowband UVB and excimer lamps. Because the absorption coefficient of DNA greatly changes at wavelengths around 300 nm, slight differences in the spectral distribution may cause large differences in the clinical effect. Therefore, we studied the optimum wavelength characteristics when using DUV-LEDs for phototherapy. Jurkat cells were used to investigate apoptosis and DNA damage as an in vitro model of T cell-mediated disease. The cells were irradiated with UVB light ranging from 280 nm to 320 nm in 5-nm steps and apoptosis and CPD were measured. The slopes of dose-apoptosis and dose-CPD curves sharply decreased at wavelengths above 295 nm. For verification, we calculated the action coefficient of apoptosis and CPD from the dose-apoptosis curves and the dose-CPD curves. We defined the slope in the linear region as the action coefficient and normalized it by each maximum value. The wavelength at which the apoptosis action coefficient exceeded the CPD action coefficient was 285 nm to 297 nm and longer than 312 nm. The wavelength range from 285 nm to 297 nm is not suitable for clinical use because it has a high erythema action coefficient. On the other hand, the longer the wavelength, the longer the irradiation time needed for treatment. Therefore, we concluded that the optimum peak wavelength when using DUV-LEDs with an approximately 15-nm FWHM for ultraviolet phototherapy is 312 nm. If the spectrum can be narrowed and the short wavelength can be cut off, the peak wavelength may be able to slide to 308 nm, which is the same as the excimer light.