What is Gamma Knife?

Gamma Knife is a machine designed to deliver high doses of therapeutic radiation to small abnormalities in the brain. Diseases treated with Gamma Knife include cancerous tumors, abnormal blood vessels (“arteriovenous malformations,” or AVMs), and malfunctioning nerves (trigeminal neuralgia, for example). This type of radiotherapy is called “stereotactic radiosurgery” because it is similar in spirit to traditional surgery, but instead of cutting out diseased tissues with a physical knife it uses radiation to treat the tissues without removing them. Radiosurgery can also be performed with medical linear accelerators.

Images courtesy of Elekta AB.

A Gamma Knife machine contains ~200 individual sources of cobalt-60 (⁶⁰Co) arranged in a hemispherical pattern inside a large helmet-like device. ⁶⁰Co is a radioactive isotope of cobalt, and as the sources decay they continuously emit gamma-ray (γ-ray) radiation, which consists of high-energy photons. These γ-rays are shaped by special cones into narrow beams which intersect at a central location known as the radiation isocenter. (The prefix “iso” means “same,” so isocenter means “the same center”).

For treatment, the patient lies on a flat table with their head inside a special frame or mask attached to the table. A head frame that is physically attached to the patient’s head by metal pins is usually used when only a single treatment session is needed to achieve the prescribed total dose, whereas an immobilizing head mask is usually used when multiple daily treatment sessions are needed to achieve the prescribed treatment total dose. The system then carries out a prepared plan which automatically moves the table with high precision (within 0.3 mm) to position diseased regions of the brain at isocenter. For each position, γ-ray beams of various direction, diameter, and duration are selected to irradiate the isocenter in a single delivery called a “shot.” Multiple shots may be used to sculpt the dose distribution to match the shape and location of the diseased regions. The power of Gamma Knife lies in its ability to deliver a very high dose of radiation to a small volume while minimizing the dose deposited in surrounding healthy tissue.

The Gamma Knife treatment process begins with a neurosurgeon placing a frame or mask on the patient. These rigid devices immobilize the patient’s head and contain reference points that define a so-called “stereotactic” coordinate system for accurately locating points inside the brain in three dimensions.

The patient is then imaged using magnetic resonance imaging (MRI), computed tomography (CT), angiography, or some combination thereof. The resulting images are used by the physician to locate diseased regions as well as nearby healthy tissues which should be avoided. After targets and normal volumes have been identified, a medical physicist uses specialized computer software to create a treatment plan from the images.

Once the treatment plan is ready, the patient is positioned on the treatment table with their head immobilized at the end by the frame or mask. When treatment is initiated, the Gamma Knife unit automatically follows the prepared treatment plan and delivers the radiation dose prescribed by the physician. When treatment is complete, the stereotactic frame or mask is removed from the patient’s head and the patient is monitored in a recovery area for a short period before being discharged, usually on the same day. A course of Gamma Knife treatment may consist of one or several such sessions, though usually not more than five.