Durham MED tester acceptance testing and calibration using EBT3 Gafchromic film

Abstract

The Hybec Durham Erythema MED tester is a hand held test device designed to measure the Minimal Erythema Dose for a patient receiving UVB phototherapy. It has a series of 10 apertures with a superimposed foil grid of varying attenuation so that 10 doses of UVB are delivered to a patient simultaneously in increments of 1.26. Several groups have researched this device and published results differ. A more optimum method of calibration is required. There are three challenges in the measurement of the output of the 9 mm diameter apertures: availability of a detector of appropriate size, use of appropriate input optic for the ‘in contact’ geometry and the variation in bulb output with time. Calibrated EBT3 Gafchromic film is ideal for this application as it allows simultaneous measurement of all apertures and quantitative investigation in the patient exposure geometry. The EBT3 and a Bentham spectral radiometer were used to quantify the output characteristics of two handheld testers. Lamp stability, warm up characteristics, relative output at apertures and uniformity of bulbs were evaluated in simulated patient exposure geometry. Previous research was carried out with the tester free in air. Measurements carried out in this research demonstrate that the geometry has an influence on warm up and relative output of apertures, due to heating effects. Results allowed a treatment protocol to be established with revised warm-up time and modified relative aperture outputs. EBT3 is suitable for the commissioning and routine quality control of the device.