Phototherapy for Skin Conditions: UV Treatment in Dermatology

Phototherapy uses controlled ultraviolet (UV) radiation to treat a range of inflammatory, pigmentary, and proliferative skin conditions. Administered in clinical settings under established protocols, it represents one of dermatology's oldest evidence-based interventional modalities, with documented use spanning more than six decades. This page covers the mechanism of action, the principal UV modalities, the conditions most commonly treated, and the clinical factors that determine when phototherapy is appropriate versus when alternative approaches are indicated.

Definition and scope

Phototherapy, in the dermatological context, refers to the therapeutic application of non-ionizing electromagnetic radiation — primarily in the ultraviolet A (UVA, 315–400 nm) and ultraviolet B (UVB, 280–315 nm) spectral ranges — to skin tissue to achieve a measurable clinical response. It is distinct from photodynamic therapy (PDT), which combines a photosensitizing agent with light activation, and from laser or intense pulsed light (IPL) treatments covered separately under Laser Treatments in Dermatology.

The American Academy of Dermatology (AAD) recognizes phototherapy as a first- or second-line treatment option for psoriasis, vitiligo, atopic dermatitis, and mycosis fungoides, among other conditions. The field operates within a regulatory framework that includes U.S. Food and Drug Administration (FDA) device classification for phototherapy equipment — UV treatment cabinets and targeted phototherapy devices are regulated as Class II medical devices under 21 CFR Part 880 — and clinical practice is further shaped by guidance documented at the regulatory context for dermatology level.

The scope of phototherapy extends to at least 30 distinct dermatological diagnoses in peer-reviewed literature, though clinical consensus is strongest for the five or six most prevalent inflammatory skin diseases.

How it works

UV radiation exerts its therapeutic effect through three primary mechanisms operating at the cellular and molecular level:

  1. Immunomodulation — UVB radiation (particularly the 311 nm narrowband peak) suppresses the activity of T-lymphocytes in the dermis and epidermis, reduces pro-inflammatory cytokines including interleukin-12 (IL-12) and tumor necrosis factor-alpha (TNF-α), and promotes apoptosis of pathologically activated T-cells. This is the dominant mechanism in psoriasis and atopic dermatitis treatment.

  2. Melanogenesis stimulation — UVA and UVB both stimulate melanocyte proliferation and migration from hair follicle reservoirs into depigmented epidermis. This mechanism underlies phototherapy's role in vitiligo, where repigmentation rates with narrowband UVB (NB-UVB) reach 50–75% in published clinical trials for facial lesions, though truncal repigmentation rates are typically lower.

  3. DNA damage to rapidly proliferating cells — At therapeutic doses, UV radiation induces pyrimidine dimer formation in keratinocytes, slowing the hyperproliferative cell cycling characteristic of plaque psoriasis. At the 311 nm NB-UVB wavelength, therapeutic DNA effects are achieved at lower overall radiation doses compared to broadband UVB, reducing cumulative phototoxicity risk.

Principal modalities compared:

Modality Wavelength Primary targets Typical session frequency
Narrowband UVB (NB-UVB) 311–313 nm Psoriasis, vitiligo, atopic dermatitis 3× per week
Broadband UVB (BB-UVB) 280–320 nm Psoriasis, eczema 3× per week
PUVA (UVA + psoralen) 320–400 nm Severe psoriasis, mycosis fungoides, CTCL 2–3× per week
Targeted NB-UVB / excimer (308 nm) 308 nm Localized psoriasis, vitiligo patches 2–3× per week

NB-UVB has largely displaced broadband UVB in clinical practice because it delivers equivalent or superior efficacy with a more favorable side-effect profile, as summarized in systematic reviews published by the Cochrane Collaboration. PUVA carries a documented elevated risk of squamous cell carcinoma with cumulative exposure exceeding 200 treatments, a threshold cited in the AAD's psoriasis treatment guidelines.

Common scenarios

Phototherapy is applied across a defined set of dermatological presentations:

Phototherapy is also relevant for pediatric presentations. Pediatric dermatology conditions covers age-specific considerations, including the lower minimum erythema dose (MED) thresholds used in younger patients.

Decision boundaries

Not every patient with a phototherapy-eligible diagnosis is an appropriate candidate. Clinicians apply a structured set of inclusion and exclusion criteria before initiating treatment:

Factors that support phototherapy selection:
- Moderate-to-severe disease burden affecting quality of life, not controlled by topical agents
- Patient inability or unwillingness to use systemic immunosuppressants or biologics
- Pregnancy, where many systemic options are contraindicated
- Reimbursement approval through payer pathway (phototherapy is a covered benefit under most major insurance plans when medical necessity criteria are met)

Contraindications and relative exclusions:
- Personal or family history of melanoma or extensive non-melanoma skin cancer — cumulative UV exposure may elevate risk further; skin cancer types and warning signs documents the baseline risk landscape
- Photosensitizing medications (fluoroquinolones, thiazides, tetracyclines, phenothiazines) that lower the MED and increase erythema risk
- Genetic photosensitivity disorders including xeroderma pigmentosum, lupus erythematosus with photosensitive phenotype, and Bloom syndrome
- Active skin infections or open wounds in treatment areas

Dosing and safety monitoring protocol:

  1. Establish minimum erythema dose (MED) or skin phototype (Fitzpatrick I–VI) to set initial dose
  2. Begin at 50–70% of MED or phototype-based starting dose per AAD protocol
  3. Increase by 10–15% per session if no erythema is observed, or hold dose if erythema develops
  4. Document cumulative joules/cm² across all sessions in patient record
  5. Monitor for acute side effects: erythema, pruritus, blistering
  6. At cumulative PUVA exposure exceeding 150 treatments, ophthalmologic surveillance is indicated due to cataractogenic risk — a requirement reflected in FDA device labeling for PUVA units

The FDA's Center for Devices and Radiological Health (CDRH) maintains oversight of phototherapy device performance standards, and facilities operating these devices are subject to state radiation control program requirements in most U.S. jurisdictions. For a broader view of how regulatory frameworks shape dermatological practice, the skin conditions overview provides foundational context alongside information available at the site index.

Phototherapy is not a permanent cure for chronic conditions like psoriasis or vitiligo; relapse rates after cessation of NB-UVB typically reach 40–50% within 6 months, based on data summarized in Cochrane systematic reviews of phototherapy maintenance trials. Maintenance protocols — reduced to once weekly sessions — can extend remission but add to cumulative UV burden, requiring ongoing risk-benefit evaluation.

References

The law belongs to the people. Georgia v. Public.Resource.Org, 590 U.S. (2020)