Clinical characterization of mycosis fungoides with abnormal light reactions induced by narrow-band UVB therapy: A review of a single-center experience.

Narrow-band UVB (NB-UVB) therapy at a wavelength of 311 nm is often used to treat mycosis fungoides (MF). However, we occasionally encounter cases of erythema induced by low doses of NB-UVB, known as an abnormal light reaction (ALR). We investigated the incidence of ALR in patients with MF and the association between ALR and clinical and laboratory findings. Forty patients (30 men and 10 women) with MF (excluding patients treated with bexarotene or etretinate) who received NB-UVB therapy at the Department of Dermatology, University of Toyama, from January to December 2022 were analyzed. ALR was defined as erythema caused by NB-UVB irradiation of the nonlesional skin at a dose of ≤0.5 J/cm². ALR occurred in six of 40 patients (15%). The main symptoms of ALR are erythema, irritation, and itching. ALR was observed in patients with Fitzpatrick skin types II and III, and was more common in patients with T2 and T4 disease than in others. The mechanism underlying its occurrence is unknown; however, it has been suggested that ALR is more likely to occur in patients with extensive skin lesions. In addition, treatment can be continued by reducing the dose of NB-UVB or switching to a 308-nm excimer laser, even when ALR occurs. Accumulation of additional cases is required because there were only six patients with ALR in this study.