Status of ultraviolet A dosimetry in methoxsalen plus ultraviolet A therapy.

The carcinogenic risk of methoxsalen plus ultraviolet A treatment (PUVA) is almost certainly related to the cumulative UVA exposure dose. A reliable estimate of UVA dose is a major component, therefore, in determining the long-term safety of this treatment modality. Thirty-one UVA meters from twenty-nine different PUVA treatment centers were compared with a calibrated UVA meter. The results inferred that 95% of UVA meters used in PUVA centers are reading between 57% and 159% of the true UVA irradiance, a factor of 2.8 in relative sensitivity. This wide variation in accuracy between the UVA meters gives cause for concern. These variations were attributed to differences in calibration procedure, optical properties of the sensors, and, most importantly, poor quality control by the manufacturers. The situation that appears to have existed since the introduction of PUVA therapy can be improved only if dermatologists agree that the present inaccurate and uncertain approach to UVA measurements is unsatisfactory. Manufacturers of UVA meters should be encouraged to provide instruments as reliable as the dosimeters used in radiotherapy, in which the prescribed dose is probably within 5% of the true value. Ideally, a standard type of UVA meter with appropriate physical properties and an agreed calibration procedure should be used by all centers engaged in PUVA treatment.