On-line exit dose profile measurements by a diode linear array

Abstract

The possibility of using a commercial diode linear array (BMS Schuster Inc.) in on-line portal dosimetry has been investigated. The system is composed of 88 equispaced diodes (distance between diodes, 0.5 cm; detection area, ). The ability to measure exit dose profiles has been demonstrated by comparing relative exit dose profiles measured by an ionization chamber (NE 2571) with `portal' profiles measured by the diode array on a number of homogeneous and inhomogeneous phantoms in 6 MV x-ray beams (from a Clinac Varian 6/100). The influence of the variation of the scattered radiation at the exit level with respect to the detector level has been discussed and investigated by varying the air gap (from 0 to 80 cm for a homogeneous phantom and from 0 to 25 cm for inhomogeneous phantoms) as well as the field width. A good agreement (maximum difference 3.8%; 2.5% when the array was positioned in contact with the phantoms) between `portal' profiles and exit dose profiles in both homogeneous and inhomogeneous phantoms has been found if the array - phantom distance is kept below 7.5 cm. Results indicate that the system should be suitable for applications in transit dosimetry.