1. Potential
- Current proton therapy facilities costing upwards
from £100,000,000 1
due to great size, weight,
complexity and low production volumes.
- Currently ~50 installations worldwide. Access is
limited or impossible for many patients who would
stand to benefit 2
. No full power systems in UK,
though two are planned.
- Dielectric Wall Accelerator (DWA) in commercial
development which promises to drastically reduce
size & cost of accelerator, democratising availability.
Possible to retrofit into existing linac bunker 3
.
Advantages of proton therapy vs x-ray
- Protons (p+
) and heavy ions interact with matter in
a different way to photons & electrons (e-
), which is
more favourable in general for radiotherapy 1
. See
figure 1.
- Characteristic 'Bragg peak' energy distribution with
zero dose beyond a given depth (no exit dose).
Dielectric Wall Accelerator – the future of proton radiotherapy ?
William Newman, MSc Radiotherapy and Oncology, University West England, October 2015.
- Modulation of p+
energy creates spread out Bragg
peak and confines dose to tumour 1
. Entry dose is
lower and exit dose zero. Thus non-target integral
dose is much reduced, 50% for prostate 4
.
Disdvantages of proton therapy
- Predominantly cost, currently.
- Higher stopping power of p+
means energy must be
much higher for sufficient penetration. 250 MeV
protons will transit 37 cm of water 5
.
- This and and greater mass of p+
(2,000 times mass
of e-
) means accelerating apparatus must be much
larger. It is the main reason for the expense of a p+
therapy facility.
- Difficulty of in-vivo dosimetry, PET may provide an
answer 6
.
- Difficulty modelling dosimetry in software 5
.
History of DWA
- DWA originally developed at Lawrence Livermore
National Laboratory (LLNL) as part of a nuclear
weapons testing program 3
.
- DWA team learned that a compact p+
accelerator
was needed by Oncology and adapted design.
- Compact Particle Acceleration Corporation (CPAC)
set-up by Accuray in 2008 to develop the technology
under licence from LLNL 7
.
- 35 MeV prototype working 8
.
- 150 MeV machine estimated to be commercially
available in 2016 (2013 prediction) 9
.
Technology
- Uses patented novel technology, proprietary to
LLNL / CPAC. See schematic in figure 2.
- Uses a virtual travelling wave electric field concept,
analogous to a Mexican wave passing through a
stadium crowd. No radio frequency carrier wave as
per conventional radiotherapy electron linacs.
- High electrical gradient insulator forming beam
tube permits extreme field strength.
- Stacked Blumlein pulse generators fire in order to
produce a local electrical field that travels the length
of the beam tube, accelerating a p+
bunch.
- Optically activated switches which allow precise
temporal firing of Blumlein pulse generators 3
.
Concerns
- No news output from CPAC since 2012. Company
is still recruiting however 11
.
- Only one independent scientific review 12
of the
project can be found. It verified claims and
considered objectives to be achievable.
- Rival technologies may catch up. Bad news for
DWA but good news for radiotherapy 8
.
Figure 1 dose depth distribution protons and x-rays 1
.
Figure 2 DWA schematic 10
.
