PUVA Dosing Calculator

Psoralen plus ultraviolet A (PUVA) photochemotherapy combines an orally administered photosensitising psoralen with long-wave ultraviolet A radiation (320 to 400 nm). Once activated by UVA, the psoralen forms cross-links with DNA and suppresses keratinocyte proliferation and cutaneous immune responses. PUVA remains an effective treatment for plaque psoriasis that has not responded to narrowband UVB, for early-stage mycosis fungoides, for palmoplantar dermatoses, and for selected patients with atopic eczema, vitiligo, and other photoresponsive conditions. The British Association of Dermatologists and British Photodermatology Group guideline (Ling et al. 2016) and the joint AAD-NPF phototherapy guideline (Elmets et al. 2019) are the principal sources for the regimens implemented here.

The psoralen dose is conventionally calculated from body weight: 0.6 mg/kg for 8-methoxypsoralen (8-MOP, methoxsalen) and 1.2 mg/kg for 5-methoxypsoralen (5-MOP, bergapten), rounded to the nearest 10 mg tablet. Some units dose by body surface area instead (25 mg/m2 of 8-MOP), and the AAD-NPF guideline offers a simplified weight-band table (10 mg under 30 kg, 20 mg from 30 to 65 kg, 30 mg from 66 to 91 kg, and 40 mg above 91 kg). Because the phototoxic peak of 8-MOP occurs at about 2 hours and that of 5-MOP at about 3 hours, most centres irradiate 2 hours after any oral psoralen for consistency. 5-MOP is used when nausea limits 8-MOP. UVA-protective eyewear is worn from ingestion until the end of the day.

Two approaches are used to select the starting UVA dose. In the minimal phototoxic dose (MPD) approach favoured in the United Kingdom, a series of test doses is applied after psoralen ingestion, the MPD is read at 72 to 96 hours, and treatment starts at 70% of the MPD. In the skin-type approach used in North America, the starting dose is read from the Fitzpatrick phototype: 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 J/cm2 for types I to VI respectively, with fixed increments of 0.5, 1.0, or 1.5 J/cm2 per treatment and maxima of 8, 12, or 20 J/cm2 for types I and II, III and IV, and V and VI. A randomised comparison by Collins et al. (1996) found that an MPD-based regimen cleared psoriasis with fewer treatments and a lower cumulative dose than a skin-type regimen, and MPD testing also confirms that sufficient psoralen has reached the skin.

Treatments are given twice weekly with at least 72 hours between exposures, because PUVA erythema peaks late. In the percentage-based regimens used by UK phototherapy networks, the dose is increased by 40% of the previous dose at each visit when no erythema is present (20% when the patient attends once weekly), and a low-increment regimen of 20% is used for atopic eczema, urticaria, polymorphic light eruption, and patients taking oral retinoids. Barely perceptible erythema that settles within 72 hours prompts a repeat of the previous dose and smaller increments; well-defined erythema prompts postponement of one treatment; painful erythema or blistering requires treatment to be withheld and reviewed, with resumption at half the previous dose. Practical cabinet maxima of about 15 J/cm2 apply to stand-up whole-body treatment. After one or two missed treatments the previous dose is repeated, after three the penultimate dose is given, after four to six the dose is halved, and after more than three weeks the MPD is repeated or treatment restarts at the initial dose.

Cumulative exposure should be recorded for every patient. The 30-year prospective PUVA Follow-Up Study (Stern 2012) showed that the risk of cutaneous squamous cell carcinoma rises steeply with the number of treatments and cumulative dose, and the BAD guideline advises limiting lifetime exposure, with figures of approximately 1000 J/cm2 or 150 to 200 treatments commonly cited as thresholds for heightened surveillance and for reconsidering further courses. This calculator applies published dosing rules and does not replace local protocols, dosimetry, or clinical judgement.