GHOST: Geant Human Oncology Simulation Tool

With funding from the STFC Global Challenge Concepts Fund, topped up by the Department of Physics to make a total of 12 months full time, we will model direct and scattered radiation doses associated with different approaches to image guidance, specifically in the context of radiotherapy for curing cancer. Image guidance scans can be done to include different lengths of the body; longer lengths facilitate the image guidance workflow, but at the expense of higher imaging dose. Imaging frequency can be varied, for example imaging on the first 3 or 5 days of treatment, or daily. The schedule alters the safety margin required, and in turn the amount of normal tissue receiving a high treatment dose, and affects the probability of accurate treatment delivery.

A key concept here is the current perception of risk, which is quite different between professionals and regulators, and patients. Patients readily accept the notion that image guidance improves accuracy of treatment, and in general are keen to maximise image guidance for their own case. Amongst some professionals and regulators there is a fear about radiation exposure which begins to approach a "dread risk". This has been partly responsible for inhibiting the uptake of image guidance in the UK.

Within the UK the use of image guidance has not been as widespread as fundamental principles would suggest. Part of this is due to concerns over the risk of radiation dose for image guidance, especially where resources are limited. Image guidance is especially important where intensity modulated radiotherapy is used, because this introduces steep dose gradients which need to be accurately delivered. At the time of the last UK survey of RT practice, less than 70% of head and neck cases were managed with on-line image guidance, despite IMRT having been shown to provide a major clinical value to patients.

Since image guidance takes a finite time on the treatment machine, where resources are limited, this often leads to an argument to reduce the use or the frequency of the image guidance imaging. A more sophisticated estimate of risks will add to the argument in favour of expanding the use of image guidance, and the need for adequate provision of NHS resources. The same arguments apply around the world, especially in the developing world, where similar resource limitations exist.