Particle accelerators are complex and fascinating machines
and it might be surprising that there's about 35,000
of them in the world
and around half of those are used for one specific
practical application.
And that is treating cancer.
The earliest mention of cancer dates back to 1600 BC,
where the Edwin Smith papyrus describes the ailment.
And for as long as humanity has suffered under it and known of it,
we've also try to cure it.
The same papyrus outlines instructions on how to
surgically remove a breast tumour.
Over the following centuries our knowledge and understanding grew,
but it wasn't until 1895 when Wilhelm R�ntgen discovered x-rays that we got our
first real modern breakthrough in cancer treatment.
R�ntgen used a cathode ray tube like this one which has a vacuum inside
the glass sphere and a cathode at one end,
which when heated with high-voltage
gives out electrons,
which would have been pulled towards the anode at the
other end, and when it hits the anode it generates x rays.
Within weeks of R�ntgen's discovery the damaging properties of
x-rays were already being applied positively to treat cancer.
X-rays kill cancer cells by damaging their DNA
through the mechanism of ionization.
The x-ray photons carry enough energy to remove the electrons from water molecules.
When molecules lose one or more of their electrons they become charged and
highly chemically reactive.
They bind to available DNA molecules nearby and
attack both the bases and the backbone of DNA in the cancer cells.
However the x-ray beam doesn't differentiate between cancerous cells and healthy cells
so it's possible in a course of treatment to actually kill some of the healthy
cells.
These machines are incredibly precise but getting the right dose distribution
is really challenging because of the fundamental property of photons
that once x-ray photons go into the body they'll keep on travelling and go out the
other side.
One way to address this challenge is to use a different type of particle,
say a heavy charged particle like a proton.
And particle accelerators can take protons from inside the
hydrogen atom, form them into high energy beams,
and that can then be directed at a tumor inside the body.
In a similar way to radiotherapy the particle enters the
body and hits the tumor.
However instead of passing through the body it stops at its target.
this happens due to a phenomenon called the Bragg
peak.
So if i I was to plot the depth inside the human body
when a fast-moving charged particle comes in,
it comes in at high speed and loses energy gradually
through ionization
until it stops at the end of its range.
The dose deposited when it does that starts quite low when
it's at high speed and as the particle decreases speed, it increases the dose it
deposits,
until it stops at the end of its range.
The higher the initial energy of the beam, the deeper that Bragg peak is inside
the body
and that means that we can tune the energy of the beam to reach the exact
depth of a tumour.
The main benefit for proton therapy over conventional radiotherapy is that the
dose to the surrounding healthy tissue is lower, and that's particularly useful
for cancers which a deep inside the body or close to sensitive organs,
and there are researchers around the world looking at
ways to make this treatment even more effective
by using heavier particles like carbon ions, or even increasing the
dose delivered during a single treatment to the cancer cells, while keeping the
low-dose in the surrounding tissue.
From early x-rays in the 19th century to cutting-edge proton beam treatments in
the 21st century,
particle accelerators have played a key role in our treatment
of cancer.
And while they're just one treatment modality,
we're constantly working to make better cheaper and more efficient accelerators
to make sure that doctors have the best tools available.