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

The possibility of using a commercial diode linear array (EMS 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, 2.5 x 2.5 mm(2)). 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.