Definition of Phototherapy

Phototherapy is treatment with ultraviolet light, which suppresses the action of immune cells in the skin (t-cells). Ultraviolet light also slows the growth rate of keratinocytes, the cells that make up much of the dermis and nearly all of the epidermis, helping reduce symptoms such as plaque formation and scaling. Phototherapy is effective for numerous chronic dermatologic conditions, notably psoriasisvitiligo, and atopic dermatitis.

Many people require therapeutic phototherapy to bring symptoms under control, with ongoing maintenance treatments to help prevent recurrent outbreaks or to continue subduing the immune response. With appropriate protection to prevent ultraviolet damage to the eyes and skin, phototherapy has few short-term side effects.

Questions remain, however, about long-term consequences such as the same problems that accompany extended sun exposure (notably skin cancer). Dermatologists use three types of phototherapy: ultraviolet B (UVB) phototherapy, psoralen plus ultraviolet A (PUVA) phototherapy, and excimer laser phototherapy.


UVB lightwaves are less intense than ultraviolet A (UVA) lightwaves, achieving a therapeutic benefit with low risk of sunburn and other complications. UVB phototherapy was the first therapeutic application of ultraviolet light and remains the most common one in use today. Dermatologists use two forms of UVB phototherapy: broadband, the conventional UVB phototherapy, and narrowband, which employs a narrower width of ultraviolet light.

Narrowband UVB phototherapy is often more effective for treating psoriasis, though has a higher risk of sunburn than broadband UVB phototherapy. For UVB phototherapy treatments, the person stands inside a small room called a light box with the areas of skin exposed that are to receive treatment. Other skin surfaces remain clothed or covered, though treatment may be appropriate for nearly the entire body.


PUVA lightwaves provides a stronger, more focused therapeutic effect. UVA lightwaves are more intense than UVB lightwaves. Psoralen is a photosensitive substance taken orally as a pill or applied as a lotion to lesions and desired skin surfaces. Short exposures to UVA lightwaves activate the psoralen, which intensifies the effect. This combination reduces the risk for complications such as sunburn during treatment.

However, the skin remains photosensitive for up to 36 hours after treatment, requiring the person to avoid all sun exposure for 12 to 36 hours following a PUVA phototherapy session. The individual must also wear sunglasses after taking oral psoralen because the psoralen tends to accumulate in the retinal tissues of the eyes. Some people have an adverse reaction to psoralen including nausea, vomiting, itching (pruritus), and heightened sensitivity to sun exposure even after PUVA phototherapy ends.

Excimer Laser

Another type of phototherapy is the excimer laser, which emits high-intensity UVB lightwaves that the dermatologist focuses on specific lesions or defined areas of the skin’s surface. Such laser phototherapy allows targeted treatment and limits the risk for sunburn, though targeted lesions often acquire deeper pigmentation than the surrounding skin and may scar after healing. Often the hyperpigmentation fades over time. Dermatologists generally reserve excimer laser phototherapy for conditions that do not respond to other treatments.


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