Water Equivalent Calorimeter for Quality Assurance in Proton Beam Therapy

Lead Research Organisation: University College London
Department Name: Physics and Astronomy

Abstract

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.

Publications

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Kelleter L (2020) A scintillator-based range telescope for particle therapy. in Physics in medicine and biology

 
Description The purpose of the project was to develop a system for measuring the range of protons used for proton beam therapy treatment. During the research project the design of the system changed radically to meet clinical needs, allowing the system to function at much higher intensities than was previously possible. A very rough working prototype was assembled but the grant ran out before further progress could be made.
Exploitation Route We would need further grant funding in order to develop the system into a fully fledged clinical prototype. We are still waiting to publish results and are considering a patent application that would assist in the commercialisation and medical certification of the technology.
Sectors Healthcare

 
Description Improved beam monitoring at MedAustron
Geographic Reach Local/Municipal/Regional 
Policy Influence Type Influenced training of practitioners or researchers
 
Description STFC RCUK Innovation Fellowship
Amount £304,453 (GBP)
Funding ID ST/R004870/1 
Organisation Science and Technologies Facilities Council (STFC) 
Sector Public
Country United Kingdom
Start 04/2018 
End 03/2021
 
Title Quenched Bragg curve reconstruction 
Description The Bragg curve is well-characterised for dose deposition in proton beam therapy. Bortfeld derived an analytical formula describing the shape of this curve as a function of a few simple parameters. Due to quenching of the scintillation light due to Birk's Law this formula does not describe the resulting light output curve for dose deposition in scintillators. An extension to Bortfeld's formula has been derived that includes Birk's Law to describe the shape of the scintillation light output. 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? No  
Impact Derivation of this formula was a prerequisite for the analysis and fitting of measurements from the water equivalent calorimeter and has enabled the successful fitting of the resulting light output curves. 
 
Title Water equivalent range calorimeter 
Description Construction of a range telescope assembled from sheets of plastic scintillator for measuring the range of clinical proton beams in proton therapy. Detector is currently a first generation prototype but is intended for use in clinical centres. 
Type Of Material Physiological assessment or outcome measure 
Year Produced 2018 
Provided To Others? No  
Impact Detector is in early stages of development. 
 
Title Clatterbridge beamline model 
Description Modelling of the Clatterbridge proton therapy beamline in Geant4. Includes all components that influence the scattering and absorption of the beam before and after the vacuum exit, including the patient specific collimator. 
Type Of Material Computer model/algorithm 
Year Produced 2017 
Provided To Others? Yes  
Impact Provides the capability for other users to build on this model to accurately simulate the beam properties at Clatterbridge for both research and clinical purposes. It is anticipated that other users at Clatterbridge will be able to make use of this model for their research and in turn improve upon it. Model will be described and published online to assist this process. 
 
Description NUVIA a.s. 
Organisation Nuvia Limited
Country United Kingdom 
Sector Private 
PI Contribution Construction and testing of the water equivalent calorimeter, based on plastic scintillator technology provided by Nuvia a.s. as part of the IPS partnership agreement. Testing of scintillator to determine optimum light output, effects of radiation damage and machining improvements necessary to ensure the thickness tolerances required for a clinically relevant device.
Collaborator Contribution Production of 2mm, 3mm and 10mm scintillator sheets for use in prototype water equivalent calorimeter. Improvements in scintillator composition and machining tolerances to improve light output, radiation damage and flatness.
Impact Research project is still in progress: first beam tests with the scintillator sheets in the correct detector configuration have only just been carried out. As such, preliminary results have been presented at group meetings and the 2017 PPRIG workshop but have not been published. The collaboration is strongly cross-disciplinary, drawing on expertise from high energy physics, medical physics and clinical proton beam therapy.
Start Year 2017
 
Title Proton range calorimeter 
Description The water equivalent proton range calorimeter will make measurements of the range of clinical proton beams as part of the daily QA process necessary to ensure safe treatment. The detector will make these measurements more quickly and more accurately, improving clinical throughput. 
Type Diagnostic Tool - Non-Imaging
Current Stage Of Development Initial development
Year Development Stage Completed 2018
Development Status Actively seeking support
Impact Detector is at an early stage of development: full analysis of results has not yet been possible. 
 
Title Proton range calorimeter 
Description Water equivalent range calorimeter is currently under development for the measurement of the range of proton beams for proton therapy. Detector is constructed of layers of water equivalent plastic scintillator with the resulting light output imaged by a pixel sensor to determine the proton range. 
Type Of Technology Detection Devices 
Year Produced 2018 
Impact Detector is in an early stage of development so impacts are yet to be realised.