UVB radiation (280–320 nm) is the most biologically active radiation in sunlight and is mainly responsible for erythema after sun exposure.
According to multiple studies, the state-of-the-art starting dose is based on the patient's minimal erythema dose (MED). UVB is initially administered at 70% of the minimal erythema dose.
The mechanism of action of UVB light is multifaceted, including the ability to induce T-lymphocyte apoptosis, increase secretion of interleukin-10, and diminish antigen presentation. BB-UVB encompasses the most biologically active radiation in sunlight and guttate psoriasis responds best.
One of the greatest advances in phototherapy for psoriasis is the use of NB-UVB. Narrowband UVB (311 nm) is an advance made that has been shown to clear plaques at therapeutic doses, less erythemogenic than other wavelengths in the UVB range.
Selective UVB phototherapy proved to be an effective and safe alternative for the patients with moderate to extensive psoriasis involving 30–60% of the body surface. Patients with refractory plaque psoriasis usually require higher dose and a longer duration of treatment. UVB can cause acute phototoxicity, presenting as erythema and blistering after exposure. This can present in the first 4–6 hours after exposure and peaks 12–24 hours after.
Short-term side-effects of UVB phototherapy are usually mild and consist of xerosis, erythema, pruritus, and photoactivation of herpesvirus. Potential long-term effects include premature photoaging and cutaneous carcinogenesis.
Narrowband UVB therapy (311 nm) has largely replaced psoralens and UVA as initial choice in full-body phototherapy for psoriasis.
Pre-radiation with mineral oil application improves the efficacy of NBUVB in treatment of psoriasis. This beneficial effect of mineral oil has been attributed to its refractive index, similar to that of stratum corneum, and its ability to penetrate and fill the air-corneocyte interfaces thus decreasing backscattering or reemission of the incident light. UVB may be combined with topical treatment like calcipotriol, tazarotene and anthraline in order to maximize efficacy and keep the course of phototherapy as short as possible and to limit the cumulative UVB dose and thus and reduce the carcinogenic risk.