UAB Radiation Oncology, Rays of Hope Volume 5 issue 1 | Page 10

CLINICAL CARE UPDATE
USING PROTONS TO FIGHT CANCER
The Science Behind the UAB Proton Center
     At every half circle inside the
cyclotron, the protons experience a brief
electrical field that accelerates them,
much like a child’s pumping a swing to
make it go higher. As the protons travel
faster and faster in the magnetic field,
their circular path widens.
     The result is a spiral that expands with
each swing the protons make around the
cyclotron. After 650 revolutions, the
protons will reach 60 percent of the speed
of light while moving in a 5-foot-3-inch-
diameter path. At that point, they exit the
cyclotron and speed through an energy
selection system and beam transport
system, headed toward a cancer patient.
Proton International at UAB will include a Varian ProBeam gantry and treatment room.
By: Jeff Hansen  | UAB News
The ability to make protons a cancer-fighting tool starts with speed. The protons must first
accelerate to about 60 percent of the speed of light, or around 112,000 miles per second.
Only then can the stream of protons be tuned and focused on its target — a human tumor. 
The science behind this proton therapy
has two elements. What happens when
bursts of those high-speed protons enter
the body of a patient? And how can you
make them go so fast?  Protons are tiny,
subatomic particles found in the nucleus
of every atom. They have a positive
charge, which is key to both their impact
in a patient and how they can be
accelerated.
     Inside cells of the body, the proton
wants to strip electrons — the negatively
charged subatomic particles — from
molecules. When this happens,
particularly to DNA in a tumor cell, it
damages the genetic material enough to
cause cell death. Kill enough cells in a
tumor, and the tumor is destroyed.
     The moving protons have an odd
characteristic with regard to this damage
— they do most of their electron stripping
just before they stop. Thus, as protons
speed through healthy tissue toward a
tumor, they cause less damage in that
healthy tissue than cancer radiotherapy
using traditional X-rays.
9 | RAYS OF HOPE
     By varying the speed of the protons,
radiation oncologists can vary how deeply
the protons will travel into the body before
they stop. Through fine control of speed
and aiming, they can target bursts of
proton ionizing energy to every point
inside the volume of a tumor. As the
protons reach the depth of the tumor and
begin to stop, they cause the most damage.
Unlike X-rays, there is no damage to
healthy tissue beyond the tumor.
Inside the Cyclotron
     The machine that will accelerate
protons for the UAB Proton Center is
called a cyclotron. The planned cyclotron
will weigh 90 tons, with a superconducting
magnet cooled by liquid helium to a
temperature of about minus 452 degrees F.
     Protons from hydrogen gas are injected
at the center of the cyclotron. Because
these moving protons have a positive
charge, the magnetic field from the
superconducting magnet will bend their
travel into a circular path.
Painting A Tumor
     These two systems are a crucial step
between the cyclotron and the patient. In
this long tube, the protons are focused
and slowed to the exact speeds needed to
penetrate to the proper depths for each
spot of the tumor. The fastest protons will
be able to penetrate nearly 12 inches into
the body. At the far end of the beam
transport, the patient lies on a robotic
patient-positioning table in the 16-foot-
wide treatment room.
     The cyclotron, energy selection system,
beam transport system and treatment
room will stand on a footprint the size of a
tennis court, part of a new three-story-tall
building to be built near 20th Street and
Fifth Avenu