ONLINE 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 i s composed of 88 equispaced diodes (distance between diodes, 0.5 cm; d etection area, 2.5 x 2.5 mm(2)). The ability to measure exit dose prof iles has been demonstrated by comparing relative exit dose profiles me asured by an ionization chamber (NE 2571) with 'portal' profiles measu red by the diode array on a number of homogeneous and inhomogeneous ph antoms 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 res pect to the detector level has been discussed and investigated by vary ing 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 goo d agreement (maximum difference 3.8%; 2.5% when the array was position ed in contact with the phantoms) between 'portal' profiles and exit do se profiles in both homogeneous and inhomogeneous phantoms has been fo und if the array-phantom distance is kept below 7.5 cm. Results indica te that the system should be suitable for applications in transit dosi metry.