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

Cashmore, Jason (2014). Operation, characterisation & physical modelling of unflattened medical linear accelerator beams and their application to radiotherapy treatment planning. University of Birmingham. Ph.D.

Preview

Cashmore13PhD.pdf
PDF - Accepted Version
Download (12MB)

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:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):