# Phototherapy and PUVA

> Phototherapy and PUVA in NEET-PG Dermatology: NB-UVB 311 nm, psoralen dosing, MED-based protocols, indications and long-term cancer risk.

- Canonical URL: https://prepelephant.com/topics/neet-pg/dermatology/phototherapy-puva
- Exam / course: NEET-PG · Subject: Dermatology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Phototherapy and PUVA", PrepElephant, https://prepelephant.com/topics/neet-pg/dermatology/phototherapy-puva

## Direct answer

Phototherapy is the controlled delivery of ultraviolet radiation as a drug: broadband UVB (290-320 nm), narrowband UVB centred at 311 nm (the modern workhorse), or PUVA — psoralen plus UVA (320-400 nm), where 8-methoxypsoralen 0.6 mg/kg taken 1-2 hours before irradiation intercalates into DNA and, on UVA photoactivation, forms adducts that suppress pathologically rapid epidermal turnover and inflammation. Treatment begins at a percentage of the measured minimal erythema dose (MED) for UVB or minimal phototoxic dose for PUVA and climbs by increments of roughly 10-20%, two or three sessions weekly. The flagship indications are extensive plaque and guttate psoriasis, vitiligo, atopic dermatitis, cutaneous T-cell lymphoma and pityriasis lichenoides; acute harms are erythema and blistering, and the chronic, dose-dependent one is squamous cell carcinoma after cumulative PUVA — the reason NB-UVB is preferred for long courses and children.

## What you must remember

- **Wavelength logic:** UVB acts superficially (psoriasis, vitamin D synthesis, guttate lesions); UVA penetrates deeper and needs a photosensitiser (psoralen) — PUVA clears thicker, more refractory plaques but carries the greater long-term cancer debt.
- **Dose protocol:** measure the MED, start at about 50-70% of it (UVB) or of the minimal phototoxic dose (PUVA), increase by 10-20% per session as tolerated; missed weeks demand dose reduction, not resumption — the arithmetic the nurse runs and the exam tests.
- **Psoralen conventions:** oral 8-methoxypsoralen 0.6 mg/kg, 1-2 hours pre-irradiation (take with food to blunt nausea); bath PUVA (0.5-1 mg/L dilute solution, 15 minutes soak) for lower cumulative dose; UVA-opaque goggles for 24 hours after oral psoralen and male genital shielding — SCC of the penis is a documented PUVA excess.
- **Indication map:** guttate psoriasis (NB-UVB excellent), plaque psoriasis (NB-UVB first for widespread; PUVA for thick, refractory), vitiligo (NB-UVB with or without tacrolimus), atopic dermatitis, cutaneous T-cell lymphoma (PUVA/NB-UVB for patch and plaque stage), pityriasis lichenoides, lichen planus, morphoea (UVA1).
- **Classic regimens named in exams:** Goeckerman (crude coal tar plus UVB) and Ingram (dithranol paste plus UVB) — the historical inpatient ancestors of modern protocols.
- **Acute adverse effects:** UVB erythema peaks within the first 24 hours; PUVA phototoxicity is delayed up to 48-72 hours, so treatments never on consecutive days in classical schemes; nausea from psoralen, pruritus, and photo-recall of previous sunburn.
- **Chronic harms:** photoaging, PUVA lentigines, cataracts (hence the 24-hour eye protection rule) and dose-dependent SCC risk — carcinogenic synergy with ciclosporin and prior arsenic exposure is the reason a careful treatment history precedes any first course.
- **Contraindications:** xeroderma pigmentosum and DNA-repair disorders (absolute), lupus with photosensitivity, prior arsenic or radiation exposure, and pregnancy for PUVA (UVB remains acceptable when necessary); photosensitising drugs (doxycycline, thiazides, amiodarone) force dose caution or postponement.

## Running a course from measurement to clearance

A 32-year-old with psoriasis over 30% body surface area, uncontrolled by topicals, is measured for MED — geometric doses on untanned lower back, read at 24 hours. His narrowband UVB course starts at 70% of MED, thrice weekly, with 10% increments as long as no persistent erythema follows; clearance typically takes 20-30 sessions, tapered to weekly maintenance before stopping. This is the textbook course and, in Indian teaching institutes, the most commonly phototreated disease after vitiligo.

Contrast the thick, palmoplantar plaques that shrug off UVB: PUVA takes over — 8-methoxypsoralen 0.6 mg/kg with food, UVA cabinet an hour later, genitals shielded and goggles worn through the day, with instruction that sun exposure on treatment days risks a phototoxic burn. The examinable discipline is rhythm: never on consecutive days, never escalating through unresolved erythema, never without a cumulative-dose record.

## Where the exam switches on the lamp

Question-writers test mechanism first — psoralen intercalates DNA and UVA cross-links it, whereas UVB acts directly on DNA and epidermal immunity. Dose arithmetic comes second: percentage-of-MED starts and 10-20% increments, and what to do after a missed fortnight (drop the dose). The safety list is third — 24-hour ocular protection and genital shielding in PUVA, the SCC risk of cumulative PUVA, and the absolute bar in xeroderma pigmentosum. Finally, the matching exercise: guttate psoriasis to UVB, CTCL to PUVA, morphoea to UVA1, and Goeckerman to tar-plus-UVB — four pairs that recur almost verbatim across question banks.

## Frequently asked questions

### Why is narrowband UVB preferred over PUVA for long-term therapy?

NB-UVB at 311 nm clears psoriasis comparably with a lower long-term squamous cell carcinoma risk and no systemic photosensitiser, making it safer for extended and maintenance courses.

### How is the starting dose of phototherapy determined?

By measuring the minimal erythema dose (or minimal phototoxic dose for PUVA) and starting at roughly 50-70% of it, with 10-20% increments per session as tolerated.

### What protective measures accompany oral PUVA?

UVA-opaque goggles for 24 hours, genital shielding during exposure, sun avoidance on treatment days, and contraception for women of childbearing age.

### Which phototherapy suits cutaneous T-cell lymphoma?

PUVA (or NB-UVB) for patch and thin plaque-stage mycosis fungoides, often combined with retinoids or interferon as stage-appropriate.

### Name the historical combination regimens of UVB.

Goeckerman regimen — crude coal tar plus UVB; Ingram regimen — dithranol (anthralin) paste plus UVB; both born in psoriasis wards decades before narrowband lamps.