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Operation, characterisation & physical modelling of unflattened medical linear accelerator beams and their application to radiotherapy treatment planning

Cashmore, Jason (2014)
Ph.D. thesis, University of Birmingham.

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Abstract

The flattening filter is a conical shaped piece of metal sitting within the treatment head of a linear accelerator, used to produce a flat, uniform beam of X-rays from the forward-peaked distribution exiting the target. Despite their routine use since the introduction of the linac in the 1950’s, however, there are still several unresolved issues surrounding their use. The photon scatter and electron contamination introduced by modifying the fluence are difficult to model, as is the variation in energy spectrum caused by differential absorption across the field. Leakage radiation also causes increased whole body doses to the patient, and the filter itself causes acts as an amplifier for beam bending and steering issues.

With advances in tumour imaging, dose optimisation and in-room image-guidance it is now possible to locate a tumour accurately in space and to design radiation fields to conform to its shape, avoiding adjacent normal and critical tissues. This active production of non-flat fields means that the prerequisite for flat fields no longer exists, and the filter is potentially no longer a necessary component.


This thesis reports on research to produce a filter-free linear accelerator, from basic operation and optimisation, dosimetric characterisation and beam modelling, through to treatment planning and dose delivery. FFF beams have been shown reduce many of the problems seen with the current generation of linear accelerators, producing beams that are inherently more stable, simple to model and with reduced patient leakage (leading to reduced secondary cancers). The increase in dose rate also translates into shorter treatment times for many treatments, aiding patient comfort and reducing problems associated with intra-fraction motion.

Type of Work:Ph.D. thesis.
Supervisor(s):Green, Stuart and Beddoe, Alun
School/Faculty:Colleges (2008 onwards) > College of Engineering & Physical Sciences
Department:School of Physics and Astronomy
Subjects:R Medicine (General)
RC0254 Neoplasms. Tumors. Oncology (including Cancer)
Institution:University of Birmingham
ID Code:4616
This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder.
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