Cyberknife Radiosurgery – A New Treatment Method for
Image-guided, Robotic, High-precision Radiosurgery
Alexander Muacevic European Cyberknife Center Munich , Berndt
Wowra European Cyberknife Center Munich
Cyberknife Radiosurgery – Overview
Cyberknife technology is based on radiosurgical
principles that have been in clinical practice for 30 years. Radiosurgery is the precise application of a high (tumour-destructing) dose of radiation in a precisely
defined target volume, while protecting the surrounding healthy tissue. During radiosurgery, many radiation beams from different
directions intersect in the tumour region, where they
accumulate. The surrounding healthy tissue receives only a small amount of the
total dose. Until recently, the Gamma Knife system was the standard instrument
for neurosurgical applications, and some centres use
linear accelerators (linacs) for clinical radiosurgical procedures. These systems, which are used
mostly in conventional radiation oncology, have to be readjusted for every radiosurgical treatment. They must also undergo physical
testing, because radiosurgical applications demand
significantly higher quality and precision requirements than conventional
radio-oncological applications. However, Gamma Knife
and conventional linacs share the same necessity of
applying an invasive stereotactic ring on the
patient’s head in order to achieve the desired accuracy of ±1mm. The
revolutionary development of the Cyberknife
technology, combining integrated image-guided and robotic technology, has led
to a paradigm shift in radiosurgery.1,2
Advantages of Cyberknife
Radiosurgery
With Cyberknife technology, an invasive stereotactic head frame is no longer required in order to
obtain the highest possible accuracy for brain treatments. Non-invasive and
pain-free radiosurgical treatment is now available to
patients. In addition, if feasible, the treatment can be divided into several
stages,2 which makes it even safer to treat
larger lesions or those in highly sensitive areas. Apart from standard neurosurgical
indications (i.e. acoustic neurinomas, meningeomas, brain metastases), radiosurgical
applications are currently evolving to include extracranial
indications. It is now possible to treat tumours of
the spine, pancreas, lung and liver safely and effectively with radiosurgical techniques in cases where the tumours are well delineated from the surrounding healthy
tissue. In selected cases, this could replace a surgical procedure. Because of
the physiological breathdependent organ movement,
until now it has not been possible to apply high-radiation doses to body
lesions. The newest development in Cyberknife radiosurgery is a breath-triggered realtime
movement-correction system, which makes it possible to apply radiosurgical doses to these lesions. The Cyberknife moves according to the tumour’s
movement. Infrared cameras follow the breath excursions and send information
online to a robot, which steers the linac into
position.6 Anaesthesia or dedicated body stereotactic frames, as used with conventional radiation
devices to suppress respiration, are no longer necessary. Cyberknife
treatment is designed for outpatient treatment, which significantly enhances
the quality of life of cancer patients. A hospital stay or rehabilitation is
not needed in most cases. Treatment time is dependent on tumour
location and size and the organs at risk. A full course of radiosurgery
lasts between 60 and 90 minutes.
Technology
The Cyberknife technology is
a composition of two main parts: the radiation source, which is a lightweight
and compact photon device (6 MeV linac,
dose rate 4 Gy/minute) coupled with a robotic arm
capable of moving in six degrees of freedom (Kuka
GmbH,
Dynamic Patient Correction
Moving target volumes can be monitored, and
irradiation dynamically adapts to the movement accordingly. After fiducial markers are applied percutaneously,
internal organ movement is defined by the X-ray imageguided
system. Simultaneously, external light-emitting diodes, which are fixed on the
breast of the patient, measure the breath excursions. The software calculates
the organ movement, taking into account the internal and external target
volume. Systemically performed X-ray image updates are the basis for iterative
corrections of the correlation model.5,6
Frameless Stereotaxy – Accuracy
Academic studies at the European Cyberknife Center Munich, Germany, and Stanford University,
US, have shown that frameless Cyberknife technology
is as precise as the conventional frame-based systems. Phantom studies achieved
total accuracy results (including imaging, planning and treatment) of
0.42±0.4mm.3,4
Treatment Schedule
An individual head mask is used for brain treatments. This helps to stabilise the patient’s head during treatment. For lesions
in moving organs such as the lung, liver and pancreas, small (5mm) metal
markers are implanted percutaneously in the vicinity
of the lesion. They are used as landmarks for image registration and are
detected automatically by the X-ray camera system. Image registration for brain
treatments is performed without external fiducials
and uses bony scull structures. All treatments are performed on the basis of CT
and magnetic resonance imaging. The frameless technology allows the planning of
images days before radiation application. Dose planning is performed using an
inverse dose planning algorithm whereby the dose prescription to the target
volume is defined by the tolerance dose to the organs at risk. During
treatment, the patient is awake and is monitored by a video camera system. During
treatment, the robot moves to 100 defined virtual points that are distributed
homogeneously above the target. From each of these points, the robot can be
directed to each point inside the treatment area (non-isocentric
radiation). Complex optimisation techniques weight
the individual beam in such a way that a high dose of radiation is directed to
the tumour, taking into account the specific
limitations of the organs at risk. The stereotactic
X-ray system records images during treatment and compares them with the
generated DRRs of the planning CT, providing the
highest possible precision during treatment.
Medical Indications
The European Cyberknife Center Munich is a
co-operative institution with the
Future Perspectives
New research activities are focusing on the full potential of robotic
techniques for radiosurgical applications. One of the
primary goals is the introduction of realtime image
guidance of tumours in soft tissue without the
implantation of fiducials.7 This would
require a fast, fully automated analysis of X-ray imaging during treatment. The
technology is currently undergoing phantom tests. In addition, research
activities are being carried out in order to optimise
the inverse planning algorithm and to take into account the relative organ
movement during respiration.