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Introduction
UV phototherapy is one of the most evidence-backed treatments in dermatology. Yet its clinical outcomes are profoundly sensitive to one thing most devices get wrong: the starting dose. Get it right and patients see consistent repigmentation or clearance. Get it wrong and you're managing burns, erythema, and patient dropout.
This guide walks through the complete process — from determining Minimal Erythema Dose (MED) to condition-specific escalation schedules — for dermatologists prescribing 308nm excimer and 311nm NB-UVB phototherapy. It applies to both clinic-based treatment and connected home therapy platforms like UVThera.
What Is MED and Why Does It Matter?
The Minimal Erythema Dose (MED) is defined as the lowest dose of UV radiation that produces just-perceptible erythema on unaffected skin 24 hours after irradiation. It is the gold standard reference point from which all phototherapy dosing begins.
MED is not a fixed number. It varies significantly between patients based on skin phototype, anatomical site, prior UV exposure, and immune status. A Fitzpatrick Type I patient may have a MED of 15–20 mJ/cm², while a Type VI patient may not erythema until 100 mJ/cm² or beyond.
Clinical principle: Starting phototherapy without MED testing is equivalent to prescribing a drug without knowing the patient's weight or renal function. You may get away with it — but you're flying blind, and the consequences of overdosing UV are both visible and distressing to patients.
For targeted 308nm excimer therapy, MED matters even more than in broadband UVB, because the irradiance is concentrated on a small area of affected skin. The differential between therapeutic dose and burn threshold is narrower, making calibrated dosing essential.
The Fitzpatrick Scale: Your Starting Map
Before conducting a formal MED test, classify your patient using the Fitzpatrick Skin Phototype scale. This provides an initial dosing estimate and helps set test site exposure levels.
Type | Skin characteristics | Sun response | MED range (NB-UVB) | Starting dose (% MED) |
Type I | Very fair, freckles, red/blonde hair | Always burns, never tans | 15–30 mJ/cm² | 50% |
Type II | Fair skin, blonde/light brown hair | Usually burns, sometimes tans | 25–40 mJ/cm² | 50–60% |
Type III | Medium skin, brown hair | Sometimes burns, usually tans | 40–60 mJ/cm² | 60–70% |
Type IV | Olive/light brown skin | Rarely burns, always tans | 60–90 mJ/cm² | 70% |
Type V | Brown skin (common in South Asia) | Very rarely burns | 80–120 mJ/cm² | 70–80% |
Type VI | Deeply pigmented skin | Never burns | 100–150+ mJ/cm² | 80% |
Important — Indian skin context: The majority of Indian patients fall into Fitzpatrick Types IV and V. MED ranges for these phototypes are considerably higher than the values typically cited in Western clinical guidelines, which are calibrated for Type II–III populations. Do not apply European dosing tables without adjustment.
Step-by-Step: Conducting the MED Test
The formal MED test should be performed on unaffected, non-lesional skin — typically the lower back, inner forearm, or buttock. Avoid test sites that have had recent UV exposure or topical treatment in the past 48 hours.
Step 1 — Select test site and protect surroundings Use a MED template with 5–6 apertures (typically 1 cm² each). Cover surrounding skin with opaque material. For 308nm devices, use the smallest reduction tip to isolate each test field precisely.
Step 2 — Set incremental dose series Expose each aperture to a different dose in an ascending series. A standard NB-UVB series: 20, 40, 80, 160, 320 mJ/cm². For 308nm excimer, use 50, 100, 200, 300, 400 mJ/cm² for Types IV–VI; start lower (25–200 mJ/cm²) for Types I–III.
Step 3 — Read at 24 hours Instruct the patient to return or send a photograph at exactly 24 hours post-exposure. The MED is the lowest dose producing a well-defined, just-perceptible redness with clear boundaries. Faint pinkness without clear demarcation does not qualify.
Step 4 — Document and record Record the MED value in mJ/cm² along with date, device used, test site, and Fitzpatrick type. In connected systems like UVThera, this becomes the anchor value for the patient's protocol, and all subsequent dose escalations are calculated from it automatically.
Step 5 — Set initial treatment dose Begin treatment at 50–80% of the confirmed MED, depending on phototype and condition. Never start at 100% MED — leave margin for tissue variability across the treatment area.
Contraindication check before MED testing: Confirm the patient has no active use of photosensitising medications (tetracyclines, thiazides, NSAIDs, St John's Wort, fluoroquinolones, amiodarone). If present, either substitute or document the increased photosensitivity risk and reduce starting doses by 25–30%.
Condition-Specific Starting Doses and Treatment Goals
Once MED is established, the treatment protocol diverges by indication. The therapeutic window, session frequency, and acceptable erythema response differ meaningfully between vitiligo, psoriasis, eczema, and atopic dermatitis.
Vitiligo Start at 50–70% MED on depigmented patches. Frequency: 3–5 sessions per week. Goal: mild, asymptomatic erythema within 24–48 hours. Response assessment at 12 weeks.
Psoriasis Start at 70% MED on plaques. Frequency: 3 sessions per week. Erythema in psoriatic plaques is expected and acceptable. Clearance typically achieved within 20–30 sessions.
Eczema (Chronic) Start at 50% MED. Lower frequency is tolerated: 2–3 sessions per week. Avoid irradiating skin that is actively weeping. Reassess barrier function monthly.
Atopic Dermatitis Start at 50% MED. Second-line use after topical therapy failure. Monitor for paradoxical flare in the first 2 weeks. Combine cautiously with low-dose topical steroids if clinically indicated.
Note on vitiligo dosing: Depigmented skin in vitiligo patches has significantly higher UV sensitivity than surrounding normal skin due to the absence of melanin. Always test MED on the lesional skin separately. The MED on vitiligo patches is often 30–50% lower than on perilesional skin. Applying perilesional MED values to lesional skin risks significant burns.
Dose Escalation Schedules
Phototherapy is not a static prescription. Dose must be escalated over time as the skin adapts. Under-escalation leads to tachyphylaxis — the skin acclimatises and therapeutic response plateaus. Over-escalation causes painful erythema and patient dropout.
Erythema response at last session | Action | Dose increment | Notes |
No erythema | Increase dose | +10–15% per session | Skin is under-dosed; escalate confidently |
Faint, transient erythema (<24h) | Continue escalating | +10% per session | Ideal therapeutic response zone |
Asymptomatic erythema lasting 24–48h | Maintain current dose | No change | Acceptable; hold and reassess next session |
Symptomatic erythema, mild discomfort | Reduce dose | −25% from last dose | Resume escalation only when erythema resolves |
Blistering or severe burn | Pause treatment | Hold until healed | Restart at 50% of dose that caused burn; review photosensitiser list |
"The therapeutic window in UV phototherapy is narrow but consistent. The clinician's job is to find the upper edge of that window and hold the patient there — not below it where nothing happens, and not above it where you lose their trust."
Anatomical Site Variation: What Most Protocols Miss
UV sensitivity is not uniform across the body. The same prescribed dose will produce different biological effects depending on treatment site. This matters especially in vitiligo, where lesions appear across vastly different anatomical locations.
As a general clinical rule, skin on the face and neck is significantly more sensitive than the trunk, while the palms and soles are the most resistant. A patient with vitiligo on both the face and hands will need site-specific dose adjustments, not a single protocol applied uniformly.
Site-specific dose adjustment guide:
Face and eyelids: reduce by 30–40% relative to trunk dose
Neck: reduce by 20%
Dorsal hands and feet: reduce by 10–15%
Palms and soles: may require doses 2–3× the trunk dose for adequate response
These are starting approximations — individual MED testing at each major site is the gold standard.
Managing Protocols in Connected Home Therapy
When prescribing home phototherapy, the dermatologist's protocol is only as reliable as the device enforcing it. Timer-based home devices introduce a compounding error: as the lamp ages, output drops — but the timer runs the same. A patient faithful to their 3-sessions-per-week schedule is actually receiving progressively less UV than prescribed, with no visible indication that anything has changed.
Connected platforms with real dosimetry — such as UVThera — address this by measuring actual UV output using a calibrated photodiode at every session. If tube output has degraded, the device automatically extends session time to deliver the prescribed mJ/cm². The dose the dermatologist sets is the dose the patient receives, reliably, across the full course of treatment.
From a protocol management standpoint, this means:
Set protocol once, trust it over time. Starting dose, escalation schedule, and maximum dose cap are set in the clinic dashboard. The device enforces the protocol — the patient cannot override the escalation ceiling.
Review remotely, not just at the next appointment. Session logs, cumulative dose, and repigmentation photos are visible in your dashboard. Adjust escalation pace between appointments without requiring the patient to travel.
ABHA-linked records travel with the patient. If a patient moves or changes dermatologist, their full phototherapy history — MED value, cumulative dose, session log — is accessible via their ABHA health ID. No treatment history is lost.
Monitoring, Review Points, and When to Stop
UV phototherapy is a long-course treatment. Vitiligo patients may need 6–12 months of consistent sessions before significant repigmentation is achieved. Psoriasis patients may achieve clearance faster, but require maintenance therapy to prevent relapse. Clear review criteria prevent both premature discontinuation and unnecessary prolongation.
Recommended review schedule: Formal assessment at weeks 4, 12, 24, and 48. At each review, photograph treatment sites under standardized conditions and document percentage repigmentation or clearance. If no response is apparent by week 12, reassess diagnosis, patient adherence, and whether dose escalation has been adequate.
When to discontinue: Treatment should be paused or stopped if the patient develops a photoallergic reaction, progressive erythema unresponsive to dose reduction, or if cumulative NB-UVB dose approaches 1,000 sessions or 1,000 J/cm² — at which point the long-term carcinogenesis risk warrants formal reassessment.
Cumulative dose tracking: Maintain a running cumulative dose record in mJ/cm² for every patient on long-term phototherapy. This is standard practice in most European dermatology centres and increasingly expected under NABH clinical documentation standards in India. Connected devices that log every session make this automatic.
Summary: The Prescribing Checklist
Classify Fitzpatrick phototype — use this to set initial MED test exposure series
Conduct formal MED test — on unaffected skin, and separately on lesional skin for vitiligo; read at 24 hours
Set starting dose at 50–80% MED — adjust by phototype, anatomical site, and condition
Prescribe escalation schedule — +10–15% per session with no erythema; hold or reduce on symptomatic erythema
Review photosensitiser drug list — document and reduce dose by 25–30% if photosensitising agents are present
Track cumulative dose — log every session in mJ/cm²; set a maximum threshold for long-term patients
Review formally at weeks 4, 12, and 24 — photograph under standardised conditions; adjust protocol based on response
This article is intended for qualified dermatologists and medical professionals. It is a clinical reference guide, not a substitute for individual clinical judgement. All dosing decisions should account for patient-specific factors, comorbidities, and local clinical guidelines.