a vector linking a direction point, found usually within the target volume, and a source point, which correlates with the position of the linear accelerator’s focal point. Each source point forms a ‘node’ with a set of ‘nodes’ forming a path set. Path sets provide non- coplanar beam directions and can be achieved without repositioning the patient.
Treatment delivery Prior to and during treatment delivery, the digitally reconstructed radiographs (DRRs) generated from the 3D CT are automatically registered to the live images acquired using the X-ray imaging system, which determines the beam alignment. Internal radiographic reference points based on bony anatomy or implanted fiducial markers, in or around the tumour, are required for target localisation.
Before each fraction is delivered, the patient can be positioned using the adjustable treatment couch; this reduces the corrections required from the robotic manipulator. Any additional translational or rotational adjustments during treatment, identified from the real-time imaging, are relayed to the robotic manipulator and corrected, ie the alignment adjustments required are applied by correcting the beam position and orientation relative to the patient.
The unique tracking system featured on CyberKnife enables compensation for all small translations and rotations obtained from the latest image acquired. With the real time image guidance system and alignment correction, a very high dose conformality and steep dose gradient can be achieved. The ability to deliver non-isocentric and non- coplanar beams without repositioning the patient provides additional benefit.
Respiratory motion tracking A respiratory tracking system (Synchrony, Accuray, Sunnyvale, California, USA) is used for treatment delivery to tumours that move with respiration. The beam moves during treatment ensuring continuous alignment of the beam with the moving target volume. The concept of this system is based on a correlation model between tumour position and external marker position. Prior to treatment delivery, the tumour position is determined by acquiring x-ray images at multiple time points and a correlation model is generated by relating the tumour position at different phases of the respiratory cycle to the simultaneous external marker position.
Throughout treatment delivery the position of the tumour is determined by the position of the external markers using the correlation model. Optical markers are used to provide the external signal. Separate correlation models are built for each marker with each model providing an estimate of target position. An average of all three estimates is then calculated resulting in a final position estimate. The model is continually updated throughout treatment, adapting to any changes in target position or motion accordingly. There is however no fixed or constant relationship between external contour and tumour position for extra cranial sites between fractions and
aChieved through sub-millimetre positioning
a high degree of preCision is
during treatment delivery. Therefore the correlation model is regularly adjusted using the x-ray imaging data14
.
Fiducial Insertion Fiducial markers are required for most extra cranial lesions when using the CyberKnife system. This enables the system to track the tumour in six degrees of freedom (ie all translations and rotations) throughout treatment delivery. A minimum of three fiducials are required for the system to accurately track the lesion, however it is recommended that four to six fiducials are inserted due to the uncertainties in localisation of the individual fiducials15
.
Fiducials should be 0.7 to 1.2mm in diameter and 3mm to 6mm in length. Several requirements need to be met when undertaking fiducial insertion. These include a minimum distance of 2.0cm between fiducials, a 15 degree angle between three fiducials, non-collinear placement and maximum distance of 5-6cm from the lesion. If these requirements are not met, some fiducials may not be suitable for tracking purposes and consequently tracking accuracy may be compromised. An additional tracking algorithm does enable lung tumours to be tracked without markers, provided the tumour diameter is greater than 15mm in all directions and positioned in the peripheral region of the lung.
The fiducial placement procedure is very similar to a CT guided biopsy with comparable complications and challenges16
. Potential complications include the risk of
pneumothorax, haemoptysis, haemorrhage and perforation of bowel, depending on the location and position of the tumour.
25 2011
IMAGING & ONCOLOGY
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