CyberKnife Glossary of Terms

Gain a better understanding of CyberKnife® and its applications by learning these commonly used terms related to radiosurgery.

If you’re considering CyberKnife treatment at El Camino Health’s Radiation Treatment Center, many of these terms may come up in conversations with your doctor. If you're unsure about what something means, please ask us — we're here to answer your questions.

Arteriovenous malformation (AVM) – An abnormal cluster of tangled blood vessels that begins at birth within the brain or spine. An AVM may cause bleeding, and symptoms may range from headaches and seizures to paralysis and even death.

Benign tumor – Growths that aren’t cancerous — they don’t spread to other areas of the body. However, their growth and enlargement can cause symptoms by compressing nearby tissue or structures such as nerves. Over time, some benign tumors may become malignant (cancerous).

Chemotherapy – Cancer treatment that puts anti-cancer drugs into the bloodstream and circulates them throughout the body. Chemotherapy can be taken intravenously (IV) or as pills.

Central nervous system (CNS) – A term describing the combination of the brain and spinal cord.

Critical structures – Healthy tissues near a tumor or other tissue targeted for removal. If you’re receiving radiation, doctors must limit the exposure of these structures to low, non-damaging levels. For example, the spinal cord is the primary critical structure doctors must avoid when treating spinal tumors.

Computerized tomography (CT)Computerized tomography is a diagnostic imaging technique that uses X-rays and a computer to create detailed images of tissues and structures in the body. A dye may be injected into a particular area of the body to highlight abnormalities.

Extracranial – Any location of the body outside of the skull, such as the spine, lung, pancreas and other areas of the body.

Fiducials – Markers made of metal that precisely identify the location of a tumor or other target for radiation treatment. Your doctor places them on a stereotactic head frame (see head frame below) or bite block (mouthpiece to hold your mouth still during radiation treatment). A doctor also can surgically implant a fiducial that’s visible with X-ray to help locate spinal or other tumors.

Fractionated radiosurgery – Delivers higher doses of focused radiation to a tumor over a series of two to five treatment sessions to achieve maximum effect. This is particularly beneficial for radiosurgical treatment of larger tumors and tumors located near critical structures. Each individual treatment is called a fraction.

Fractionation – Dividing the total dose of radiation into multiple smaller doses (usually administered daily) to permit surrounding, exposed healthy tissue time to repair.

Glioma – Tumor that arises from the supportive tissue of the brain. Gliomas are the most common primary brain tumors. Examples include astrocytoma, ependymoma, oligodendroglioma and glioblastoma.

Head frame (stereotactic head frame) – An external metal ring that’s affixed to a person’s skull with four screws. It contains markers (fiducials) that are visualized on a CT or MRI scan. Local anesthesia is used during the frame attachment procedure. Nearly all radiosurgical techniques for brain tumors (except CyberKnife treatment and radiosurgery) use such head frames.

Inaccessible/inoperable tumor – A tumor that often can’t be removed surgically because it’s located in an area that is difficult to access by open surgery. Because of the location, surgical removal of these tumors has a high probability of damaging vital areas of the brain or spinal cord.

Intracranial – A location inside the skull or brain.

Linear accelerator (Linac) – A large X-ray machine that delivers high-energy X-rays to destroy benign and malignant tumors throughout the body.

Lumbar puncture (spinal tap) – A procedure used to withdraw a small sample of cerebrospinal fluid from around the lower lumbar (low back) spinal cord. Doctors analyze this fluid to look for abnormal cells and proteins.

Malignant – Cancerous tumors that are capable of spreading from one site in the body to another, usually via the bloodstream, in a process called metastasizing.

Meningioma – Generally, a benign tumor that develops from the meninges — the thick, strong membranes that cover the brain.

Metastatic tumor – A tumor arising from cancer cells that originate elsewhere in the body and travel to a new area of the body through the bloodstream.

Magnetic resonance imaging (MRI) – A noninvasive, painless imaging technique that uses magnetic fields rather than X-rays to visualize structures in the body. MRI generally provides more detailed images of soft tissue anatomy (as opposed to bone) compared with CT. A dye may be injected before the scan to improve visualization of many tumors.

Non-isocentric treatment – The CyberKnife System's multi-jointed robotic arm enables the delivery of radiation for more complex-shaped tumors. The CyberKnife System delivers radiation through unlimited angles, thus producing the most conformal, targeted treatment available. Non-isocentric treatment allows the CyberKnife System to “paint” the entire tumor with a nearly uniform dose, while simultaneously directing radiation away from nearby healthy tissue.

Positron emission tomography (PET)PET scans provide images of cellular activity by measuring positrons (subatomic particle) emitted from injected substances that have a radioactive marker that can be tracked.

Primary brain tumor – A tumor arising from unhealthy/mutated cells in the brain or surrounding tissue (in contrast to a metastatic tumor, which has migrated from another site).

Rigid immobilization – Typically refers to the use of an external metal frame that minimizes a person’s movement during radiosurgery. CyberKnife doesn’t require rigid immobilization.

Robotic radiosurgery – Noninvasive procedure in which a computer-controlled robot is used to deliver high-dose radiation to tumors throughout the body without a head frame — a frame attached to the head to keep it as still as possible during radiation treatment. Radiosurgery makes adjustments for slight movements so the frame isn’t needed. It treats solid tumors anywhere in the body in one to five visits.

Spinal tap – See lumbar puncture above.

Stereotactic (stereotaxis or stereotaxy) – “Stereo” refers to one’s position within 3D space. Stereotaxy or stereotaxis is the science and practice of locating a tumor precisely within 3D space.

Stereotactic head frame – See head frame above.

Surgical resection – Conventional open surgery (with a scalpel) to remove a tumor or other lesion.

Target localization – Identifying the precise location of the target tumor in 3D space.

Treatment planning – Customizing radiosurgery treatment parameters (such as the radiation dose and shape of the field) to the individual, using specialized software. The process is typically computer-based and involves integrating information from CT or MRI scans to map out the target. The doctor must define a specific dose and other key treatment parameters depending on the diagnosis and the location of nearby critical structures. Typically, the surgeon, radiation oncologist and medical physicist are involved in the process.

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