64 AACR CANCER PROGRESS REPORT 2016
EXTERNAL BEAM RADIOTHERAPY directs radiation
at the tumor from outside the body; it is the most
common form of radiotherapy. Standard linear
accelerators use electromagnetic fields to accelerate
electrons, which can be used directly or collided
with a metal target to generate high-energy X-rays.
Electrons and photons (X-rays) are the most common
sources of radiation in external beam radiotherapy.
Conventional (2-D) external beam radiation therapy delivers a high-energy
X-ray beam from one or multiple directions. Imaging of the treatment area
is typically performed using low-energy diagnostic X-rays. It is chiefly used
in settings where high precision is not required, such as in the treatment of
3-D conformational radiotherapy (3DCRT) uses specialized imaging, usually
computed tomography (C T) and/or magnetic resonance imaging (MRI) and
planning software, to deliver high-energy X-rays via multiple beams that
more precisely fit the shape and size of the tumor.
Intensity-modulated radiotherapy (IMRT) is a further refinement of 3DCRT
that more precisely focuses and shapes the radiation by dividing each beam
into many “beamlets,” each of which can have a different intensity. IMRT is
particularly useful when a sharp dose gradient is required between the tumor
and sensitive tissues, for example, the optic nerves.
Intraoperative radiation therapy uses electron beam (superficial)
radiation directly on tumors that have been exposed during
Stereotactic radiotherapy is used in both stereotactic surgery (SRS) and
stereotactic body radiotherapy (SBRT). It uses many (typically more than
eight) beams with a highly sophisticated imaging system to direct radiation
to very well-defined smaller tumors. Typically, SRS is used to treat tumors of
the brain and central nervous system, whereas SBRT can be used on small
tumors within larger organs of the body.
There are two major
uses of ionizing
radiation in the
treatment of cancer.
radiation to control
and eliminate cancer, whereas radiology largely
uses lower-energy radiation to image tissues in
order to diagnose disease or treat disease via
the minimally invasive techniques used in
USING RADIATION IN CANCER CARE
Adapted from ( 24)
TYPES OF RADIOTHERAPY
Radiotherapy is the use of
high-energy rays (e.g., gamma
rays and X-rays) or particles
(e.g., electrons, protons, and
carbon nuclei) to control or
It works chiefly by damaging
DNA, leading to cell death.
uses protons or carbon
ions rather than X-rays
as the source of energy.
In contrast to X-rays
that pass through the
body, losing energy and
causing damage to the
through which they pass,
these heavier particles
deposit most of their
energy in the target. In
this manner, particle
therapy can deliver higher
doses with less damage
to surrounding tissue.
Although of great interest,
proton facilities are much
more expensive than
and the overall benefit to
the patient is still being