Probe: Proton Beam Extension for Imaging and Therapy

The UK is presently contracting to supply two centres where the NHS will carry out treatment of many cancers with protons. Whilst the current generation of X-ray linacs can deliver excellent treatment using the technique known as intensity-modulated radiotherapy (IMRT), there is nevertheless a small dose inherently deposited in tissues outside of the intended tumour treatment site. For certain hard-to-treat tumours near to critical organs, and in particular when treating some childhood cancers, it would be beneficial to avoid this so-called out-of-field dose. Proton therapy can do this because protons interact in tissue quite differently to X-rays; there is a much more pronounced peak in the delivered dose which can be varied in depth to target the tumour accurately. The two new centres at Christie Hospital and UCLH in London can treat at any depth as they will use high-energy accelerators, augmenting the present low-energy proton centre at Clatterbridge which is restricted to shallow eye treatments.

But there's a problem. Whilst the proton dose is deposited at a specific depth, it can be hard to set the proper energy to reach that depth. This is because current-generation imaging such as X-ray computed tomography doesn't do a good enough job at allowing clinicians to estimate the amount the protons will slow down on their way to the tumour. One promising way around this is to do imaging with protons as well: no conversion from X-ray measurements is needed, but proton imaging needs more energetic protons that can pass right through the patient where their residual energy is measured to work out how much was lost on the way. Suitable high-resolution detectors for this are under development in the UK, but as yet there is no suitable source of the protons themselves.

This is where our linac comes into the story. Recent research at our institute and elsewhere means that we think we can build a linear accelerator (linac) that can boost the protons from the energy available at present adult treatment centres such as Christie up to the energies required for imaging. To retrofit to existing treatment centres requires such a linac to be small, and hence that energy gain must occur in a very short distance; this is the hard part, and requires the use of high-frequency ('X band') accelerating cavities previously developed for use in particle physics experiments. We hope in this project to demonstrate the first truly high-gradient proton linac for imaging, taking the knowledge previously developed for physics research.

Our aim in developing a high-gradient linac is that it can be used to provide improved imaging for patients in the UK and abroad. It is thought that improved imaging using protons could reduce the required margins during tumour treatment by as much as 5mm, sparing surrounding sensitive tissues and thereby reducing side-effects and improving long-term outcomes for patients. Later on, we think the same linac technology could also be used to provide protons directly for treatment, where the use of a linac can allow finer control of the treatment depth. Also, if the accelerating structures can be made small enough, they could even themselves be fitted onto the gantry that rotates the proton beam around the patient, meaning that smaller, cheaper treatment facilities become possible. These single-room centres are seen as one way to increase the access to proton therapy for patients.

We hope our project and the advantages it brings will in the future widen the range of cancers for which proton therapy is beneficial.