External Beam Radiation Therapy

Linear Accelerator

We commonly perform external beam radiation therapy treatments with a linear accelerator.

A linear accelerator (LINAC) customizes high energy x-rays or electrons to conform to a tumor’s shape and destroy cancer cells while sparing surrounding normal tissue. It features several built-in safety measures.  These measures ensure that it will not deliver a higher dose than prescribed.  A medical physicist also routinely checks to ensure it is working properly.

If you’re scheduled for radiation therapy using a LINAC, your radiation oncologist collaborates with a radiation dosimetrist and a medical physicist. They develop a treatment plan for you. Your doctor then double-checks this plan before treatment begins. Your doctor also implements quality assurance procedures to ensure that we deliver each treatment in the exact same manner.

What is this equipment used for?

A linear accelerator (LINAC) is the device we commonly use for external beam radiation treatments for patients with cancer. The linear accelerator treats all parts/organs of the body. It delivers high-energy x-rays or electrons to the region of the patient’s tumor. We design these treatments to destroy the cancer cells while sparing the surrounding normal tissue. The LINAC treats all body sites, uses conventional techniques, Intensity-Modulated Radiation Therapy (IMRT), Volumetric Modulated Arc Therapy (VMAT), Image Guided Radiation Therapy (IGRT), Stereotactic Radiosurgery (SRS) and Stereotactic Body Radio Therapy (SBRT).

How does the equipment 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, it allows these electrons to collide with a heavy metal target to produce high-energy x-rays. These high energy x-rays conform to the shape of the patient’s tumor as they exit the machine. The customized beam is also directed to the patient’s tumor. The beam is usually shaped by a multileaf collimator that is incorporated into the head of the machine. The patient lies on a movable treatment couch. We use and lasers to make sure the patient is in the proper position. The treatment couch can move in many directions including up, down, right, left, in and out. The beam comes out of a part of the accelerator we refer to as the gantry. We can rotate the gantry around the patient. We are able to deliver radiation 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 physicist and the dosimetrist determine how to deliver the prescribed dose. They also 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 and is assured in several ways.

Before we deliver treatment to a patient, we develop a treatment plan. The radiation oncologist in collaboration with the radiation dosimetrist and medical physicist approve this plan. We also double-check and perform quality-assurance procedures to ensure that we can deliver the treatment as planned. 

Quality assurance of the linear accelerator is very important. There are several systems built into the accelerator so that it will not deliver a higher dose than the radiation oncologist has prescribed. Each morning before any patients are treated, the radiation therapist performs checks on the machine.  He or she ensures that the radiation intensity is uniform across the beam and is working properly. In addition, the medical physicist conducts more detailed monthly and annual checks of the linear accelerator.

Modern linear accelerators also have internal checking systems. These systems do not allow the machine to be turned on unless all the prescribed treatment requirements are met.

During treatment, the radiation therapist continuously observes the patient using 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) or other imaging tools such as cone beam CT are checked regularly to ensure the beam position doesn’t vary from the original plan.

Safety of the staff operating the linear accelerator is also important. The linear accelerator sits in a room with lead and concrete walls so that the high-energy x-rays are shielded and no one outside of the room is exposed to the x-rays. The radiation therapist must turn on the accelerator from outside the treatment room. Because the accelerator only emits radiation when it is actually turned on, the risk of accidental exposure is extremely low.