Assessment of a radiotherapy beam analyser

Abstract

A Protea Systems Corporation Radiation Beam Analyser (RBA-3) was acquired by this department with a view to providing non-Physics personnel with an easyto- use system to monitor both photon and electron output. The manufacturer describes the RBA-3 as not only fulfilling the above requirement reliably but also as providing a check on field flatness and symmetry. It is one of several commercially available systems for this type of application. Having acquired the RBA-3 and made appropriate Perspex build-up, its performance was assessed. The accuracy and precision of the RBA-3 was compared with the standard routine calibration facilities used in the department. Standard dosimetry calibrations and output checks are made using both NE 2570 and NE 2500/IONEX electrometers with NE 2571 graphite chambers. Machine calibrations are maintained in accordance with uncertainties specified in ICRU 24 and are done in the standard manner. A 10 cm × 10 cm field is set at a source-skin distance (SSD) equal to the source to axis distance (SAD) of high energy machines. The chamber is placed 5 cm deep in a closed water phantom which has been cross calibrated against an open water phantom. Field symmetry is monitored weekly using the closed water phantom and monthly using a full field inair scanner. The flatness of the accelerators at Coventry are in compliance with the IEC specification (1979) which concerns the uniformity variation across a field area defined geometrically relative to field edges determined at 50% of the central axis dose.