Ultraviolet phototherapy: review of options for cabin dosimetry and operation

Ultraviolet (UV) treatment dose is determined by the length of time that a patient spends in a phototherapy cabin. The output from UV fluorescent lamps declines with use and a method is needed to compensate for the change in irradiance and to identify and replace any lamps that fail. The decline in lamp output with age and the magnitudes of localized areas of low irradiance resulting from failed lamps have been measured and results used to assess different approaches to lamp replacement. In current cabin models, single failed lamps give cold spots with 7%-12% lower irradiances and replacements with new lamps give 3%-6% higher localized irradiances. However, cold spots with 30% lower irradiances may result from lamp failures in some older dual UVA/TL01 lamp cabins. The use of internal cabin detectors that can be employed to compensate for changes in irradiance level is beneficial. Cabins having pairs of internal detectors provide a reasonable reflection of the changes in irradiance that occur and the position of the patient in the cabin should not affect the treatment dose by more than +/-6%. There should be a robust system to identify and replace failed lamps to minimize the risk of erythema.