Calorimetry for Proton Therapy

Modern cancer treatment is largely a combination of 3 techniques: surgery, chemotherapy and radiotherapy. Radiotherapy uses beams of X-rays to irradiate the tumour from many different directions. The effect is to kill the cancer by depositing as much radiation dose in the tumour as possible.

Proton therapy is a more precise form of radiotherapy that provides significant benefits over conventional X-ray radiotherapy. Protons lose energy - and therefore deposit their dose - in a much smaller region within the body, making the treatment much more precise: this leads to a more effective cancer treatment with a smaller chance of the cancer recurring. This is particularly important in the treatment of deep-lying tumours in the head, neck and central nervous system, particularly for children whose bodies are still developing and are particularly vulnerable to long-term radiation damage.

In 2011 the UK government announced funding for 2 full-sized proton therapy centres, to be based at University College Hospital in London and The Christie in Manchester. These will provide treatment for a much wider range of cancers, allowing more patients to be treated closer to home. Procurement for these centres began in 2013, with doors expected to open some time after 2018. Unlike the majority of proton therapy centres worldwide - particularly in the US - the 2 UK centres are publicly funded and will treat some of the most challenging cancers.

In order to treat such difficult cancers, extremely sophisticated imaging is needed to identify the cancer and spare the surrounding tissue. Traditional treatment planning with X-rays requires multiple CT scans to create a patient treatment plan and to monitor the size and position of the cancer during treatment. However, since protons are much more precise than X-rays, the quality of the imaging must also be that much better. In addition, X-ray CT images don't provide information on how protons lose energy, so a conversion factor has to be used to estimate how the dose will be delivered with protons rather than X-rays. Also, in existing proton therapy centres CT scans are not taken with the patient in position and ready for treatment, since it's very difficult to squeeze the imaging equipment around the proton delivery nozzle. When the patient moves the shape of their body changes, making the treatment plan less accurate.

An alternative is to use higher energy protons to image the patient while the patient is sitting in position ready for treatment. The energy of the protons is increased so that they penetrate right through the body: by tracking the protons before and after the patient and measuring their energy, it's possible to reconstruct an image of the tumour and the surrounding tissue with the patient in the right position, that also tells you how much dose the protons will deposit during treatment. This system is called proton CT.

In order to create a proton CT image, you need a very accurate measurement of the proton energy when it leaves the body. This project is looking at modifying a particular type of energy detector, called a calorimeter, to measure the energy of protons in a proton CT system. The calorimeter was developed for the SuperNEMO experiment to measure high energy electrons but it can also measure proton energies very accurately. In addition, it can also measure the energy of the proton beam that is used for treatment. This detector will also be used to check the energy of the proton beam at the Clatterbridge Cancer Centre that uses lower energy protons to treat eye tumours. Clatterbridge is the only hospital of its kind in the UK.

In 2011 the UK government announced funding for 2 full-sized proton therapy centres, to be based at University College Hospital in London and The Christie in Manchester. These will provide treatment for a much wider range of cancers, allowing more patients to be treated closer to home. Procurement for these centres began in 2013, with doors expected to open some time after 2018. Unlike the majority of proton therapy centres worldwide - particularly in the US - the 2 UK centres are publicly funded and the indications list prioritises some of the most challenging cancers, particularly central nervous system (CNS), head and neck and paediatric cases. In addition, the target is to treat 1,500 patients a year, split between both centres, which requires significant improvements in throughput over other full-scale proton therapy centres.

A significant research programme at UCL is already expected to run alongside the clinical facilities at UCLH, not only to improve the quality of treatment for such difficult cases but also to address the unique challenges present in such a large and complex indications list. Given the complexity of the UK PBT indications list, a number of research areas must be investigated in order to provide the level of service required by the NHS: these include more advanced in-room imaging techniques to provide the necessary resolution to match the more precise dose distributions available from protons. Traditional X-ray CT imaging provides neither the requisite resolution nor a proton-specific absorption map that is needed for optimum treatment with protons. As such, imaging techniques using protons above 300 MeV are highly desirable: particle tracking in front and behind the patient, coupled with a final measurement of the proton energy, allows a proton CT image to be reconstructed to optimise the treatment plan and dose delivery. While improvements in imaging techniques are particularly pressing for the UK centres as a result of the complex indications list, developing proton imaging is recognised by the international hadron therapy community as one of the two areas - alongside improved gantry design - that will have the greatest impact on improving proton therapy treatment.

The benefits of developing a high resolution, fast calorimeter for proton therapy are twofold:

1) To provide the calorimeter stage for a proton imaging system;
2) To provide fast measurements of the clinical proton beam energy as a part of the daily quality assurance process.

The development of a complete proton imaging system beyond the scope of this proposal. However, having a system available for making high resolution measurements of the proton beam energy in the treatment room before the UK centres commence clinical operation is highly desirable. A unique aspect of this proposal is that a close relationship already exists between UCL High Energy Physics, UCL Medical Physics, the UCL Centre for Medical Image Computing (CMIC) and UCLH, providing a unique opportunity to develop and implement technology for direct clinical implementation.

In addition, there are clear benefits for the Clatterbridge Cancer Centre. An accurate measurement of the proton energy and energy spread in the treatment room provides valuable diagnostic information. Early experimental measurements using a prototype calorimeter also indicated a number of unexpected operating modes of the Clatterbridge cyclotron ion source and RF system that provided several insights into the complexities of low current operation. It is expected that further measurements as set out in this proposal will provide important information not just into the energy characteristics of the beam, but also the bunch structure and time distribution of protons during clinical operating conditions.