Water Equivalent Calorimeter for Quality Assurance in Proton Beam 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, whilst minimising the dose to the surrounding area to spare healthy tissue.

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. The advantages of proton therapy, coupled to the reduced cost of the equipment, has led to a surge in interest in proton therapy treatment worldwide: there are now over 20 centres, with this number set to double every 3 years over the next decade. The UK is currently constructing 2 full-sized proton therapy centres, to be based at University College Hospital in London and The Christie in Manchester and funded by the NHS. These will provide treatment for a much wider range of cancers, allowing more patients to be treated closer to home.

Treating these cancers requires machinery that is significantly more complex than a conventional radiotherapy system. Protons are accelerated to the right energy for treatment by a particle accelerator: once the beam leaves the accelerator, it then has to be transported to the treatment rooms many metres away by a series of steering and focussing magnets. When the proton beam reaches the treatment room, it has to be delivered through a gantry to the correct place. Proton therapy gantries are enormous - more than 3 storeys tall and weighing more than a hundred tonnes - and have to rotate around the patient to deliver the beam from any angle with millimetre precision. In order to ensure that treatment with such complex machinery is carried out safely, a range of quality assurance (QA) procedures are carried out each day before treatment starts. The majority of this time is spent verifying that the proton beam travels the correct depth and is carried out for several different energies: protons are counted at different depths in a plastic block that resembles human tissue. These QA measurements of the proton range take significant time to set up and adjust for different energies: the full procedure can take over an hour.

The focus of this project is to develop a detector that can make faster and more accurate measurements of the proton range than existing systems. The detector is built from layers of plastic scintillator - that have been developed for the SuperNEMO high energy physics experiment - and resembles a sliced loaf of broad. Protons passing through this scintillator stack deposit energy in each layer which is converted into light: by recording the light from each layer, the amount of energy the protons deposit along their path can be measured. Such a system provides a direct measurement of the range of protons in tissue, since the absorption of the plastic is virtually identical to human tissue. As such, a measurement of the proton range for multiple energies would allow the complete morning energy QA procedure to be carried out in a few minutes, with an accuracy of less than a millimetre. At the two UK centres, this would translate into being able to treat an extra 12 patients every single day.

The detector technology has already been tested successfully at the Clatterbridge proton therapy centre: the next step is to build a prototype of the layered detector and show that it can make measurements with the required resolution.