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CyberKnife Glossary of Terms

Last Updated 6/28/2010 3:44:39 PM

These terms are often discussed in association with the CyberKnife Robotic Radiosurgery program.

AVM (Arteriovenous Malformation)
An abnormal, congenital cluster of tangled blood vessels within the brain or spine that is at risk of bleeding. Depending on the AVM location, bleeding can cause symptoms ranging from headaches and seizures to paralysis and even death.

Benign Tumor
Benign tumors are not cancerous--i.e., they do not 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.

Cancer treatment that is administered through the use of drugs that are injected into the body or taken orally over a period of time. This is a form of systemic therapy, meaning that, as the drugs circulate in the bloodstream, the entire body is affected.

CNS (Central Nervous System) The combination of the brain and spinal cord.

Critical Structures
Refers to healthy tissues near the tumor or other target tissue; radiation to critical structures must be limited to low, non-damaging levels. For example, the spinal cord is the primary critical structure of concern when treating spinal lesions.

CT (Computerized Tomography)
A diagnostic imaging technique wherein an x-ray machine and computer are used to create detailed images of tissues and structures in the body. A dye, or contrast agent, may be injected into the patient to highlight abnormalities.

Robotic Radiosurgery
Non-invasive procedure in which a computer-controlled robot is used to deliver high-dose radiation to tumors throughout the body without stereotactic frames. It treats solid tumors anywhere in the body in one to five visits.

Refers to any location of the body “outside of the skull.” Examples of extracranial sites include the spine, lung, pancreas and other areas of the body.

Fiducials are markers found on either a stereotactic head frame or bite block. Additionally, x-ray-visible fiducials may be surgically implanted for treatment of spinal or other tumors. The fiducials act as markers to precisely identify the location of a tumor or other target.

Fractionated Radiosurgery
Delivers higher doses of focused radiation to a lesion over a series of two to five treatment sessions, thereby enabling a biologically more effective total dose to be administered. This is particularly beneficial for radiosurgical treatment of larger tumors and tumors located near critical structures. Each individual treatment is called a fraction.

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

Tumors that arise from the supportive tissue of the brain. These are the most common primary brain tumors. Examples include astrocytoma, ependymoma, oligodendroglioma and glioblastoma.

Head Frame (Stereotactic Head Frame)
An external metal ring that is affixed to the patient’s skull with four screws. It contains markers (fiducials) that are visualized on the CT and/or MRI scan. Local anesthesia is used during the frame attachment procedure. Nearly all radiosurgical techniques for brain tumors (excluding the CyberKnife system) use such head frames.

Inaccessible/Inoperable Tumor
A tumor that often cannot be removed surgically because it is located in an area that is difficult to access by open surgery. Because of location, surgical resection of these tumors has a high probability of damaging vital areas of the brain or spinal cord.

Refers to a location inside the skull or brain.

Isocentric Treatment Planning
All stereotactic radiosurgery system devices (with the exception of the CyberKnife system) are restricted to using a fixed isocenter as the standard for treatment.

Isocentric treatment--or multi-isocentric treatment--involves packing the lesion with a single or multiple overlapping, spherically shaped dose distributions.

Hot spots are areas where treatment volumes overlap, causing some tissue to be overdosed with radiation. Excessive radiation exposure of normal tissue increases the risk of complications, especially with critical structures such as the highly radiosensitive optic chiasm and acoustic nerves.

Cold spots are under-dosed areas within the target that receive a less than optimal amount of radiation dose. In situations of under-dosing, there is a risk that all tumor cells will not be destroyed.

Linac (Linear Accelerator)
A Linac, or linear accelerator, is a large x-ray machine that delivers high energy x-rays for therapeutic irradiation of benign and malignant lesions throughout the body.

Lumbar Puncture (aka Spinal Tap)
A procedure used to withdraw a small sample of cerebrospinal fluid from around the lower lumbar spinal cord. This fluid can be subsequently analyzed for abnormal cells and proteins.

Malignant tumors are cancerous and are capable of spreading from one site in the body to another, usually via the bloodstream in a process called metastasis.

Generally benign tumors that develop from the meninges, which are 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 anatomic site through the bloodstream.

MRI (Magnetic Resonance Imaging)
An imaging technique that uses magnetic fields rather than x-rays to delineate structures in the body. MRI generally provides more detailed images of soft tissue anatomy (as opposed to bone) compared to CT. A dye may be injected prior to the scan to improve visualization of many tumors. MRI scans are painless.

Non-Isocentric Treatment
The CyberKnife system's multi-jointed robotic arm enables the delivery of radiation for more complex-shaped lesions. The radiation beams are delivered from arbitrary points in the workspace to the lesion without intersecting a common point or isocenter. Non-isocentric treatment allows the CyberKnife system to “paint” the lesion volume with a nearly uniform dose while simultaneously helping to contour radiation away from nearby healthy tissue.

PET (Positron Emission Tomography)
An imaging technique that provides a picture of cellular activity by measuring positrons emitted from injected substances "labeled" with a radioactive marker.

Primary Brain Tumor
A tumor arising from unhealthy/mutant cells in the brain or surrounding tissue (in contrast to a metastatic tumor).

Rigid Immobilization
Typically refers to the use of an external metal frame that is attached to the skeleton and minimizes patient movement during radiosurgery. The CyberKnife system does not require rigid immobilization.

Spinal Tap
See Lumbar Puncture

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

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

Target Localization
Identifying the location of the target precisely in 3D space.

Treatment Planning
Customizing the radiosurgery treatment parameters (such as the radiation dose and the shape of the field) to the individual patient using specialized software. The process is typically computer-based and involves integrating information from CT/MRI scans to delineate and contour the target. The treating physician must define a specific dose and other key treatment parameters depending on pathology and the location of nearby critical structures. Typically the treating surgeon, radiation oncologist and medical physicist are all involved in this process.