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The Physics of CyberKnife®System
CyberKnife® is an integrated image guided linear accelerator with computer controlled robotics to treat tumors anywhere in the body with sub millimeter accuracy. The CyberKnife® linear accelerator produces photon beams of 6MV nominal energy. The compact light-weight linac head is attached to a robotic arm that is free to rotate and translate with six degrees of freedom. The robot is capable of delivering radiation from 100 nodes, which are distributed on the surface of an imaginary sphere, with each node capable of giving 12 different beams. Treatment field sizes are determined by the use of secondary circular cones of diameters ranging from 5 mm to 60 mm. The treatment couch of the CyberKnife® also has movements in six degrees of freedom. It has got 3 translation movements (longitudinal, lateral and vertical) and 3 rotational movements (pitch, roll and yaw).

The treatment delivery to any body site, with the CyberKnife® system is based on image guidance with five tracking methods, namely, Skull tracking (used for intracranial lesions), Fiducials tracking (for lesions in the soft tissue), Synchrony tracking (for lesions which move in synchrony with respiration), Spine tracking (for spinal cord lesions) and Lung tracking (for compact lung lesions). Image guidance is provided by kV X-ray beams from two diagnostic X-ray tubes fixed at 45 degree oblique on the ceiling.

The images are captured by two solid state detectors that are fixed on to the floor and compared with the reference images for the alignment of the patient. The system doesn’t allow treatment till the alignment position off-set values, are well within limits. The robot corrects its position to compensate for the residual off-set, making the delivery to sub-millimeter accuracy. This is done before each and every beam is delivered. In other words, the imaging is continuous, which is a unique feature, which is not provided with any other system till date. In addition to X- ray image guidance, optical guidance called synchrony motion tracking is used especially for the treatment of tumours, which move with respiration. Unlike other systems, where the radiation is delivered only at certain phases of breathing cycle, CyberKnife® delivers radiation continuously, adjusting its position in synchrony with chest movement.

To make sure that the targeting accuracy is up to sub millimeter, rigorous periodic quality assurance checks are done daily and monthly, with phantoms, which involve static and moving targets. These tests include CT scanning of the phantom, outlining the target and planning on the CT scans, executing the treatment plan on the phantom and analysing the dose distribution.
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