What is this equipment used for?
How does it work?
Who operates this
equipment?
How is safety
ensured?
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).
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.
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.
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.