Linear Accelerator

A medical linear accelerator (LINAC) customizes high energy x-rays or electrons to conform to a tumour’s shape and destroy cancer cells while sparing surrounding normal tissue. It features several built-in safety measures to ensure that it will deliver the dose as prescribed and is routinely checked by a medical physicist to ensure it is working properly.

If you’re scheduled for Radiation therapy using a LINAC, your radiation oncologist will collaborate with a radiation dosimetrist and a medical physicist to develop a treatment plan for you. They will double-check this plan before treatment begins and implement quality assurance procedures to ensure that each treatment is delivered in the exact same manner.

What is Linear Accelerator used for?

A medical linear accelerator (LINAC) is the device most commonly used for external beam radiation treatments for patients with cancer. It delivers high-energy x-rays or electrons to the region of the patient’s tumour. These treatments can be designed in such a way that they destroy the cancer cells while sparing the surrounding normal tissue. The LINAC is used to treat all body sites, using 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 allows these electrons to collide with a heavy metal target to produce high-energy x-rays. These high energy x-rays are shaped as they exit the machine to conform to the shape of the patient’s tumour and the customized beam is directed to the patient’s tumour. The beam is usually shaped by a multileaf collimator that is incorporated into the head of the machine. The patient lies on a moveable treatment couch and lasers are used 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 called a gantry, which can be rotated around the patient. Radiation can be delivered to the tumour from many angles 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 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 and is assured in several ways.

Before treatment is delivered to the patient, a treatment plan is developed and approved by the radiation oncologist in collaboration with the radiation dosimetrist and medical physicist. The plan is double-checked before treatment is given and quality-assurance procedures are performed to ensure that the treatment will be delivered 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 patient is treated, the radiation therapist performs checks on the machine to make sure that the radiation intensity is uniform across the beam and that it 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 that 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 make sure that 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.

Gamma Knife

Gamma Knife is a Radiation therapy that uses computerized treatment planning software to help physicians locate and irradiate small targets within the head and brain with very high precision. The treatment delivers intense radiation doses to the target area while sparing surrounding tissue.

If you’re scheduled for Radiation therapy using Gamma Knife, a treatment team consisting of a radiation oncologist, a medical physicist and a neurosurgeon will work together to provide your treatment. Safety is ensured by the medical physicist who tests the machine’s mechanical functions and verifies that the imaging and treatment planning computers and software are correct and acceptable.

What is this equipment used for?

The Gamma Knife and its associated computerized treatment planning software enable physicians to locate and irradiate relatively small targets in the head (mostly inside the brain) with extremely high precision. Intense doses of radiation can be given to the targeted area(s) while largely sparing the surrounding tissues. The Gamma Knife can be used for a wide variety of problems. For example, it can be used to treat selected malignant tumours that arise in or spread to the brain (primary brain tumours or metastatic tumours), benign brain tumours (meningiomas, pituitary adenomas, acoustic neuromas), blood vessel defects (arteriovenous malformations) and functional problems (trigeminal neuralgia). Possible future uses are being evaluated for epilepsy and Parkinson’s disease.

Single-session Gamma Knife treatment is usually unsuitable for targets larger than three or four centimetres in size.

How does the equipment work?

The Gamma Knife utilizes a technique called stereotactic radiosurgery, which uses multiple beams of radiation converging in three dimensions to focus precisely on a small volume, such as a tumour, permitting intense doses of radiation to be delivered to that volume safely. Currently, the available models use advanced robotic technology to move the patient in submillimetre increments during treatment to focus radiation successfully on all parts of the target.

Treatments using Perfexion

Most treatments are given in a single session. Under local anaesthesia, a special rigid head frame incorporating a three-dimensional coordinate system is attached to the patient’s skull with four screws. Imaging studies, such as magnetic resonance imaging (MRI), computed tomography (CT), or angiography, are then obtained and the results are sent to the Gamma Knife®’s planning computer system. Together, physicians (radiation oncologists and neurosurgeons) and medical physicists delineate targets and normal anatomical structures. They use a planning computer program to determine the exact spatial relationship between the target, normal structures and the head frame to calculate Gamma Knife treatment parameters. Targets often are best treated during the treatment session with combinations of several successive highly focused treatments, commonly known as “shots.” The physicians and physicists routinely consider numerous fine-tuning adjustments of treatment parameters until an optimal plan and dose are determined.

Using the three-dimensional coordinates determined in the planning process, the frame is then precisely attached to the Gamma Knife unit to guarantee that when the unit is activated, the target is placed exactly in the centre of approximately 200 precision-aimed, converging beams of (Cobalt-60 generated) gamma radiation. Treatment takes anywhere from several minutes to a few hours to complete depending on the shape and size of the target, the number of “shots” and the dose required. Patients do not feel the radiation treatment as it is delivered, although they may experience seeing flashes of light while the treatment is being delivered. Following treatment the head frame is removed and the patient may return to normal activity.

Treatments using Icon

Icon treatments differ from Perfexion treatments in that:

• Treatments are given in single or multiple sessions.
• An external frame is not required.
• Icon uses a combination of on board stereotactic Cone Beam CT imaging and real-time infrared motion detection and management to achieve 0.15 mm treatment accuracy even without a rigid frame.

Who operates this equipment?

A multidisciplinary team approach provides patients with the greatest safety. The team is most commonly comprised of a radiation oncologist, a medical physicist and a neurosurgeon—all specially trained in the use of the Gamma Knife®—with support from nursing staff, anaesthesiologists (for patients who are unable to cooperate, such as children) and radiation therapists, who work together to provide patients with the high-quality care they deserve. It is a requirement of the Nuclear Regulatory Commission that an “authorized user,” usually a radiation oncologist, be at the treatment console during the entire procedure.

How is safety ensured?

Because placement accuracy of the shots is critical to localization of the radiation (to a fraction of a millimetre), anything that would degrade this precision is unacceptable. Rigid attachment of the head frame, geographic targeting accuracy of the imaging studies, shaping of the volume of tissue to be treated (selection of the number, size and relative intensity of the shots) and accuracy of attachment of the frame to the Gamma Knife unit are all critical. As is true of all Radiation therapy procedures, correct selection and calculation of the amount of radiation to deliver are essential. A qualified medical physicist assures that the imaging and treatment planning computers and software are correct and acceptable. The mechanical functions of the machine are tested on a regular basis to ensure the safety of patients and medical staff.

Cyberknife.

The CyberKnife System is a Radiation therapy device manufactured by Accuray Incorporated. The system is used to deliver radiosurgery for the treatment of benign tumours, malignant tumours and other medical conditions.

The device combines a compact linear accelerator mounted on a robotic manipulator, and an integrated image guidance system. The image guidance system acquires stereoscopic kV images during treatment, tracks tumour motion, and guide the robotic manipulator to precisely and accurately align the treatment beam to the moving tumour.^[2] The system is designed for Stereotactic Radiosurgery (SRS) and stereotactic body Radiation therapy (SBRT). The system is also used for select 3D conformal Radiation therapy (3D-CRT) and intensity modulated Radiation therapy (IMRT).