Ultraviolet phototherapy: review of options for cabin dosimetry and operation

Abstract

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.

Investigations of cabin design in UV phototherapy

“…Due to the highly variable nature of UV treatments, as well as the abundance of different cabin constructions avail-able, there has been much discussion over different factors and approaches for determining dose in practice. [9][10][11] Current methods of dose estimation in treatment cabins are ad hoc and involve a radiation protected clinician making a series of averaged measurements, which while practical can lead to large errors and rely on extrapolating data from the clinician's build and frame 12 which can lead to very large selfshielding errors and uncertainties. 13 A major benefit of having an accurate dose model is it would clarify such uncertainties, making it possible to quantify patient dose from first principles and in doing so allow better designed and tolerated patient doses and better control of patient irradiance.…”

Dose modeling in ultraviolet phototherapy

“…10 Due to the highly variable nature of UV treatments, as well as the abundance of different cabin constructions available, there has been discussion over different factors and approaches for determining dose in practice. 11,12 Unlike some other radiation therapies, UVR phototherapy is nonionizing and acts directly upon the epidermis, not penetrating deeply into biological tissue beyond that. For this reason, irradiance in phototherapy is synonymous with dose, and hence is the quantity a clinician wishes to measure in practice.…”

Guidelines on the measurement of ultraviolet radiation levels in ultraviolet phototherapy: report issued by the British Association of Dermatologists and British Photodermatology Group 2015

“…In treatment cabins, for example, patient positioning and the nonuniformity of irradiation of the skin due to its curvature may be > 15% . Similarly, differences in the output of the different lamps in the array can be 15% and fluctuations in the output of lamps during treatment can be as much as 10% …”

“…The user sets an exposure time corresponding to the prescribed treatment dose, and based on prior irradiance measurements. The cabin's inbuilt electronic timer then controls the exposure; there is no automatic allowance for differences in patient size or variations in the cabin irradiance . To maintain accuracy, a programme of regular irradiance calibration tests is necessary, typically after 50 h of use and repeated at least every 4 months.…”