| Linear Accelerator:
 |
A
linear accelerator (LINAC)
is the device most commonly used
for external beam radiation treatments
for patients with cancer.
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What
is this equipment used for?
A linear accelerator (LINAC) is the device
most commonly used for external beam radiation
treatments for patients with cancer. The
linear accelerator can also be used in stereotactic
radiosurgery similar to that achieved using
the gamma knife on targets within the brain.
The linear accelerator can also be used
to treat areas outside of the brain. It
delivers a uniform dose of high-energy x-ray
to the region of the patient's tumor. These
x-rays can destroy the cancer cells while
sparing the surrounding normal tissue. A
linear accelerator is also used for Intensity-Modulated
Radiation Therapy (IMRT).
How
does it work?
The
linear accelerator uses microwave technology
(similar to that used for radar) to accelerate
electrons in a part of the accelerator called
the "wave guide", then allows
these electrons to collide with a heavy
metal target. As a result of the collisions,
high-energy x-rays are scattered from the
target. A portion of these x-rays is collected
and then shaped to form a beam that matches
the patient's tumor. The beam comes out
of a part of the accelerator called a gantry,
which rotates around the patient. The patient
lies on a moveable treatment couch and lasers
are used to make sure the patient is in
the proper position. Radiation can be delivered
to the tumor from any angle by rotating
the gantry and moving the treatment couch.
Who
operates this equipment?
The
patient's radiation oncologist prescribes
the appropriate treatment volume and dosage.
The medical radiation physicist and the
dosimetrist determine how to deliver the
prescribed dose and calculate the amount
of time it will take the accelerator to
deliver that dose. Radiation therapists
operate the linear accelerator and give
patients their daily radiation treatments.
How
is safety ensured?
Patient safety is very important.
During treatment the radiation therapist
continuously watches the patient through
a closed-circuit television monitor. There
is also a microphone in the treatment room
so that the patient can speak to the therapist
if needed. Port films (x-rays taken with
the treatment beam) are checked regularly
to make sure that the beam position doesn't
vary from the original plan.
The
linear accelerator sits in a room with lead
and concrete walls so that the high-energy
x-rays do not escape. The radiation therapist
must turn on the accelerator from outside
the treatment room. Because the accelerator
only gives off radiation when it is actually
turned on, the risk of accidental exposure
is extremely low. Indeed, pregnant women
are allowed to operate linear accelerators.
Modern
radiation machines have internal checking
systems to provide further safety so that
the machine will not turn on until all the
treatment requirements prescribed by your
physician are perfect. When all the checks
match and are perfect, the machine will
turn on to give your treatment.
Quality
control of the linear accelerator is also
very important. There are several systems
built into the accelerator so that it won't
deliver a higher dose than the radiation
oncologist prescribed. Each morning before
any patients are treated, the radiation
therapist uses a piece of equipment called
a "tracker" to make sure that
the radiation intensity is uniform across
the beam. In addition, the radiation physicist
makes more detailed weekly and monthly checks
of the accelerator beam.
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