Radiosurgical stereotactic systems that use a linear particle accelerator (linac) as the source of x-ray radiation. These systems typically consist of a dedicated linac with a movable gantry and a single or multileaf collimator; standard linacs may be adapted for limited radiosurgery applications by the addition of stereotactic accessories. Linac-based stereotactic systems used for intracranial or neck procedures typically employ a metal head frame attached to the patient's skull for precise targeting of the radiation beam; other systems incorporate a miniature linac mounted on a flexible robotic arm and an image-guidance system that can track target location during treatment using fiducials as references without the need for the invasive head frame. Systems that can be used in any location in the body and employ computer-controlled beam-shaping (known as Intensity Modulated Radiation Therapy [IMRT]) to conform the radiation dose to the shape of the tumor or other lesion are also available, but those systems may not be as accurate in targeting and are used mainly for multiple treatment sessions (i.e., radiotherapy treatment systems) rather than as surgical systems. Most radiosurgical linac systems include computing capabilities and dedicated software appropriate for specific treatments. Radiosurgical linac stereotactic systems are used in neurosurgery (i.e., for brain, neck, and spinal tumors) and, less frequently, for radiosurgery of extracranial tumors (e.g., to ablate abdominal tumors).