CyberKnife has been hailed by the media as the ‘must have’ radiotherapy technology. But what are its true capabilities?
INTRODUCTION Intracranial stereotactic ‘radiosurgery‘ was developed in 1951 by a Swedish neurosurgeon, Lars Leksell1
. The term ‘radiosurgery’ was initially defined as the delivery of an extremely
high radiation dose to an often critically located small intracranial lesion during a single session. In order to minimise dose to normal tissues, multiple non-coplanar beams entering the patient at different locations were used to deliver the high target dose.
Since 1951, radiosurgery has undergone significant technical and clinical developments and, in more recent years, stereotactic radiotherapy has been investigated, developed and extended to extra cranial treatment sites with the intent to deliver a very high dose over a small number of fractions in order to have an ablative effect on the tumour2
delivered over several sessions, it is commonly referred to as fractionated stereotactic radiotherapy (SRT) and is currently used to treat both benign and malignant tumours.
Most stereotactic platforms available currently consist of a dedicated stereotactic delivery system and an image guidance system - an essential requirement for SRT delivery. Existing radiosurgery systems include CyberKnife, Gammaknife and Tomotherapy. In addition, several linear accelerators are now equipped with stereotactic beam collimators and head frames.
One such system is the Novalis Tx radiosurgery platform. This system consists of a Varian Trilogy linear accelerator (Varian Medial Systems, Palo Alto, California, USA) with a micro multi leaf collimator in conjunction with the ExacTrac (Brainlab, Munich, Germany) image guidance system, offering real time imaging and corrections in six degrees of freedom. The Elekta Axesse (Elekta, Stockholm, Sweden) also offers a similar integrated stereotactic system.
The Gammaknife (Elekta, Stockholm, Sweden) is a well established radiosurgery system
CYBERKNIFE The CyberKnife system has been under technical development for almost 20 years, whilst the basic concept has remained unchanged. However, significant improvements and additions were implemented more recently. CyberKnife is routinely used to treat brain,3,4
head and neck,5,6 . When spine,7,8 lung,9,10 liver,11 addition to nodal or other tumour recurrences.
The treatment delivery system for the CyberKnife includes an X-band cavity magnetron, a standing wave and side-coupled waveguide, which produces 6MV x-ray beams at a dose rate of 1000cGy/min14
. Bending magnets and beam flattening filters are not required
and secondary collimation is provided by either fixed circular collimators ranging in size from 0.5mm-60mm, or a variable aperture collimator, which enables the same selection of collimator sizes without a physical change of collimator. Small collimator fields can be complex to calibrate due to steep dose gradients and electronic disequilibrium.
The robotic arm or manipulator, (Kuka Roboter, Augsburg, Germany) on which the linear accelerator is mounted, has the ability to move in six degrees of freedom and moves around the patient with a high degree of precision providing sub-millimetre position reproducibility. The increased geometric flexibility does, however, require more extensive primary barriers than those needed for conventional gantry mounted linear accelerators14
Treatment planning As with all current radiotherapy systems, a 3D CT dataset is required for planning purposes, from which a 3D patient model is generated. Treatment beams are defined by
it may beCome the treatment of ChoiCe for prostate CanCer
pancreas and prostate12,13
for intra-cranial work requiring a fixed head frame for treatment delivery purposes, whilst the Tomotherapy Hi-Art system (Tomotherapy, Madison, USA) utilises a ring gantry and delivers helical intensity modulated radiotherapy (IMRT) by means of thousands of small beamlets. This system utilises on-board image guidance with megavoltage computed tomography. The CyberKnife system (Accuray, Sunnyvale, California, USA) is an image guided robotic radiosurgery system featuring a robotic couch with six degrees of freedom and continual real-time kV image guidance (see figure 1). The robotic arm on which the linear accelerator is mounted enables the delivery of non-isocentric and non coplanar treatment beams with a high degree of precision. It is suitable for lesions anywhere in the body including structures that move with respiration.
23 2011
IMAGING & ONCOLOGY
tumours, in
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