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Irregularly shaped lesions with awkward configurations near
critical structures are well suited for proton beam therapy.
Protons have a physical advantage over gamma rays and
x-rays when it comes to sparing normal tissues. Protons
deposit most of their radiation energy in what is known
as the Bragg peak, which occurs at the point of greatest
penetration of the protons in tissue. The exact depth
to which protons penetrate, and at which the Bragg peak
occurs, is dependent on the energy of the proton beam.
This energy can be very precisely controlled to place the
Bragg peak within a tumor or other tissues that are targeted
to receive the radiation dose. Because the protons are
absorbed at this point, normal tissues beyond the target
receive very little or no radiation.
Radiation levels from a conventional x-ray therapy unit (Linac)
and protons of various energies as they penetrate in tissue
or water. X-rays have a maximum dose near the surface followed
by a continuously reducing dose with depth. Proton energy
can be adjusted to match the depth of the target with a sharp
drop in dose beyond the Bragg peak.
The figure on the right shows a widened proton beam as well
as an x-ray beam adjusted to treat an 8 cm thick target with
a maximum depth of 23 cm. The x-ray beam “spills”
unnecessary dose beyond the target compared to protons.
The proton Bragg peak is generally narrower than most lesions
therefore special equipment is used to combine protons of
various energies to broaden the Bragg peak to match the thickness
of individual targets. Properly selecting the thickness of
the broadened Bragg peak ensures uniform dose coverage of
a target with optimal reduction of dose at the entry surface
of the beam.
Tumors can have very irregular shapes and can be located
close to critical organs. Every patient’s tumor shape,
size and location are unique. Patient specific hardware, which
helps sculpt the proton beam, is customized to maximize the
dose to the tumor while minimizing the dose to normal structures.
Aiming proton beams, each with customized hardware, from various
directions further ensures that the dose to normal tissues
is reduced as much as possible therefore reducing the risk
of treatment related complications.
Pictured Above: Customized dose-shaping
devices used for proton beam therapy. A brass aperture
shaped to the outline of a target blocks the proton
beam outside a specified safety margin. The penetration
depth of the protons that pass through the aperture
opening is adjusted to match the shape of the target
with a Lucite range compensator. A target is depicted
in red on the rightmost figure with the proton radiation
dose conforming to its shape and avoiding a critical
structure shown in green.
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