High Resolution Fast Detector 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.

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.

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 energy QA measurements take significant time to set up and adjust for different energies: the full procedure normally takes an hour.

This project is looking to develop a detector that will make more accurate and more rapid measurements of the proton energy than existing systems. A calorimeter - used to measure a particle's energy - that was developed for the SuperNEMO high energy physics experiment has been modified to record the energy of a proton therapy treatment beam. This system can measure proton beam energies much more quickly than the existing energy QA technology primarily because it is much simpler. Protons are absorbed by a plastic scintillator that converts the particle energy into light: this light can then be detected to measure the particle energy. By making the scintillator the right size and shape, proton beams over the full range of treatment energies can be measured without having to change anything about the detector system. This would allow the complete morning energy QA procedure to be carried out in a few minutes. At the two UK centres, this would translate into being able to treat an extra 12 patients every single day.

In addition, because so much light is produced by protons stopping in the scintillator, the proton energy can be measured to better than 1%. This means that the accuracy of the energy measurement will also be better than the existing technology.