UV-Induced DNA Damage Per Phototherapy Session
Each session creates measurable DNA photoproducts in the epidermis.
- ~10³–10⁴: Dimers per session (per cell, placeholder estimate)
- ~24–48h: Repair window (nucleotide excision repair)
- 0.5–3 J/cm²: Typical session dose (depends on modality)
- 296–313nm / 320–400nm: Wavelength band (NB-UVB / PUVA)
How a single session damages keratinocyte DNA
Placeholder: UV photons form cyclobutane pyrimidine dimers in exposed skin cells.
Placeholder: most damage is repaired before the next scheduled session.
Tracking Cumulative Lifetime UV Dose
Dose accumulates arithmetically across hundreds of sessions over years.
- 20–40 sessions: Typical course length (per treatment cycle)
- multiple: Lifetime course count (for chronic psoriasis/vitiligo)
- J/cm² total: Cumulative dose metric (summed across sessions)
- per-patient log: Dose record keeping (required for safety)
Why cumulative dose matters more than per-session dose
Placeholder: long-term risk correlates with total lifetime dose, not single exposures.
Placeholder: clinics track running dose totals per patient chart.
Non-Melanoma Skin Cancer Risk Rises With Dose
Squamous and basal cell carcinoma risk climbs nonlinearly with sessions.
- cumulative dose: Risk driver (not single sessions)
- SCC > BCC: Cancer type (non-melanoma skin cancer)
- years: Latency period (after high cumulative exposure)
- nonlinear rise: Threshold behavior (placeholder risk curve)
Why risk accelerates past a cumulative threshold
Placeholder: repair capacity saturates as unresolved mutations accumulate.
Placeholder: risk rises steeply once lifetime sessions pass a few hundred.
PUVA Carries Higher Long-Term Risk Than NB-UVB
Psoralen photosensitization increases mutagenicity relative to narrowband UVB.
- higher: PUVA SCC risk (vs NB-UVB, dose-matched)
- psoralen + UVA: Mechanism (crosslinks DNA strands)
- lower risk: NB-UVB profile (placeholder comparative estimate)
- NB-UVB first-line: Clinical preference (when comparably effective)
Comparative mutagenicity of PUVA and NB-UVB
Placeholder: psoralen crosslinking adds mutagenic burden beyond UVB alone.
Placeholder: PUVA courses need stricter lifetime session limits.
Long-Term Dermatologic Monitoring Recommendations
High cumulative dose patients require scheduled skin surveillance.
- annual exam: Low-dose interval (placeholder guideline)
- every 3 months: High-dose interval (placeholder guideline)
- lifelong follow-up: PUVA history (placeholder recommendation)
- full skin exam: Key action (placeholder recommendation)
Setting a monitoring schedule from cumulative history
Placeholder: monitoring frequency scales with total dose and modality history.
Placeholder: PUVA patients typically need indefinite dermatology follow-up.