Erythematous reactions to two ultraviolet excimer therapy devices with different irradiance levels: Reconsidering the reciprocity law

BACKGROUND: Few studies have examined the effectiveness of the reciprocity law in ultraviolet excimer therapy. This study aimed to examine the difference in erythematous reaction in human skin when the irradiance of ultraviolet excimer treatment devices differed while the irradiation dose was constant. MATERIALS AND METHODS: This study, conducted at the Department of Dermatology, Chiba University, included 15 healthy adults aged 20–65 years (mean age, 46.3 years; seven men). Using ultraviolet excimer treatment devices with different irradiances (50 or 150 mW/cm(2)), the upper abdomen of each participant was irradiated with ultraviolet light at set irradiation doses (80, 100, 120, 140, 160, 180, and 200 mJ/cm(2)). The erythema index of each irradiated site was measured using a melanin‐ and erythema‐measuring device, and the difference in erythema index before and 24 h after irradiation was the primary endpoint. RESULTS: The change in erythema index was significantly higher for an irradiance of 150 mW/cm(2). Significant differences (p < 0.05) were observed between these irradiance levels at irradiation doses of 100‒200 mJ/cm(2). CONCLUSIONS: Even for the same irradiation dose, stronger erythematous reactions occurred at higher irradiances in ultraviolet excimer treatment. This suggests that the reciprocity law may not always hold true in excimer therapy.