Design and application of a high temporal resolution radiation beam analyser

• Main Author: Hunt, Stephen Graham
• Other Authors: Horsfield, Mark
• Published: University of Leicester 2012
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.561132

The introduction of dynamic radiotherapy has resulted in the development of medical linear accelerators capable of high dosimetric accuracy. A novel test instrument has been developed that is capable of simultaneously measuring the radiation profile and four associated accelerator control signals in a single pulse of radiation, fully synchronised to the beam generation system of the accelerator. The system was constructed, validated and tested to investigate its ability to provide a greater insight into and deeper understanding of the behaviour of accelerator beam transport and generation control systems during dynamic operating conditions pertinent to Intensity Modulated Radiotherapy (IMRT) beam delivery. The performance of the electron gun filament and its associated servo control system was investigated in relation to the lower limit of small Monitor Unit (MU) segments for ‘step and shoot’ IMRT. Accelerator electron beam current control, using the electron gun filament current as the controlling element, and its significance to small segment doses is presented. The behaviour and characteristics of the radial steering system during gantry rotation were also measured. First, the fundamental properties were characterised to explore the use of the instrument for engineering development, and then the accelerator performance during gantry rotation was investigated. Low frequency periodic oscillations found in the radiation beam during rotation were considered from an engineering standpoint, and the consequences for patient dose were evaluated. The instrument has the ability to observe the relationship between radiation beam profile and control system events concurrently with the radiation pulses, at high temporal resolution, which was crucial in identifying the source of and possible cure for unexpected system behaviour in the work presented in the thesis. This was demonstrated by identifying the processes involved that set the lower limit for small MU segments and by the characterisation of the radial control system during gantry rotation on the Elekta SLi series accelerator. The multi-input analogue capability proved useful in understanding the interactions between the different control system elements involved in beam production. The instrument should prove to be of particular benefit for improving the accuracy and reliability of patient treatment in the context of dynamic radiotherapy.