Cyberknife: Robotic Radiosurgery
What is robotic radiosurgery and how is it different from radiosurgery?
Radiosurgery is an ultra conformal, ultra precise, delivery of a single “shot” of a high dose of radiation, to a precisely demarcated pathological area, like a benign tumour or an AVM (Arterio Venous Malformation – a congenital abnormality where arteries are directly connected to veins without intervening capillaries).
The source of radiation could be X-rays from a Linear Accelerator (LINAC) or Gamma Rays from a Cobalt source (Gamma Knife). The pathological area to be irradiated is precisely demarcated, with an MRI/ CT or angiogram. Using advanced computer technology and mathematical calculations, “a treatment plan” is developed. A “prescription” is then agreed upon, by a team consisting of the neurosurgeon, radiation oncologist and medical physicist. After ensuring real time quality control, maximum safety, highenergy radiation is delivered, to bombard the abnormal area, without injuring surrounding normal tissue. In radiosurgery a fixed stereotactic frame is fixed on to the immovable skull, through a semi invasive procedure. The frame fixed on the skull, is used as a constant reference point or landmark.
In robotic radiosurgery, using sophisticated digitally reconstructed radiographs, the patient’s own unique skeletal signature is used as an internal landmark, for cranial and spinal lesions, thus dispensing with the invasive external rigid frame. With a lightweight LINAC, on a mobile robotic manipulator, the position of the radiation source can also be changed, ensuring greater conformality and total coverage of the most irregular volumes which need to be irradiated. The flexibility and versatility of the robot ensures total coverage of the most irregular shapes and volumes.
What are the specific advantages of the cyberknife?
- Dividing the total dose of radiation to four or five sessions is more advantageous in certain situations than a ‘single shot treatment’ as in conventional radiosurgery.
- The major inconvenience of treatment for 4-5 weeks required in conventional radiotherapy can be avoided.
- Larger lesions can be treated.
- A more effective dose can be given to lesions adjunct to critical structures.
- The distribution of radiation dose can be more uniform within the target.
- The CK robotic manipulator can aim the targeted, concentrated X-ray beams, of high-dose radiation, originating from the miniature LINAC, from multiple positions and angles, at hundreds of points within the target.
What is a cyberknife exactly?
The CyberKnife® is an extremely complex instrument, incorporating a combination of several advanced technologies. It must be operated only by a highly trained and skilled team of clinicians, physicists and technologists. The robotic manipulator moves and directs the LA (linear accelerator) with an extremely high level of precision and repeatability, allowing radiation from nearly any direction. Image detectors capture high-resolution anatomical images throughout treatment. Live images are continually compared with previously captured digital reconstruction radiographs to determine real-time patient positioning. The robotic manipulator instantly detects and corrects any changes in movement.
What are the functions of the robot?
The robot is a complex machine that carries out multiple complex tasks repeatedly, accurately and tirelessly. Inbuilt sensors enable sensory perception in real time. The processors process and react to information from sensors. Realtime tracking and correction for tumour movement (critical for lung tumours, pancreatic tumours, etc.) ensures that radiation is delivered only to the target
even though the target is moving. The path of the X-ray beams is corrected by the robot, on a real-time basis, without human intervention. Thus radiation strikes precisely at the intended area. This process is repeated several hundred times. Because of the high degree of mobility of the robotic arm – as the range of movement is not restricted like in a linear accelerator – highly conformal treatment is possible of even large, irregular targets. Every surface and area of the tumour is literally “painted” with radiation.
How does one decide that a patient is suitable for Cyberknife treatment?
Choosing the right patient at the right time in the right way is critical. Every patient is discussed at the Cyberknife Board. This consists of different sub-specialists, competent to assess and evaluate different treatment modalities for a particular pathology. The Board reviews the entire clinical data and the investigations. There should be a consensus of opinion that CyberKnife® treatment is indicated. Generally speaking, all patients suitable for frame-based stereotactic radiosurgery are suitable for CyberKnife® treatment.
CyberKnife® treatment is specifically indicated in complex cases in which more ultra-conformal high-precision radiation is required and when dosage to adjacent critical structures is a matter of deep concern. Lesions outside the brain, particularly those that are normally mobile, can only be treated with robotic radiosurgery. The target volume to be treated must be clearly visible on the MRI and/or angiogram.
- CT and MRI images are acquired as per an established protocol (occasionally other studies like PET-CT or digital subtraction angiography, may also be required). Prior to this, a mask is prepared, customized for the individual patient. The CT images are acquired with the face mask. The mask acts as an immobilization device restricting patient movement during treatment. Positioning on the treatment couch should match precisely the position in which the treatment planning CT images were initially acquired.
- All the images are transferred to the treatment planning computer. Using the mouse or an electronic pen, the neurosurgeon draws the tumour to be treated and other normal “critical structures” in the brain, eg. optic nerves, brain stem etc.
- A specially trained medical physicist then develops a “treatment plan” to ensure that the entire tumour is covered with the radiation beams in such a way that dosage to adjacent structures is negligible. Depending on the complexity of the tumour, it would take about 1 -2 hours to develop the plan.
- The radiation oncologist, the physicist and the neurosurgeon then fine tune the plan. After a thorough review and discussion, the team decides on the dose and jointly approves the plan. The number of sessions would depend on the size of the tumour, the biopsy report, location, presence of surrounding radiation dose-sensitive structures, prior treatment, expected duration of treatment, patient comorbidities, and radiobiological principles.
- Comfortable patient positioning for 45–60 minutes is crucial. For head and neck tumours, the face mask is used for immobilization. For the spine, appropriate immobilization is carried out. Following a very stringent quality control check of the robot, the LINAC and the entire system, the radiation commences. The CK robotic manipulator aims the beams emanating from the miniature LINAC, at thousands of points within the target volume. The dose is distributed equally throughout the volume of the tumour. The robot moves the LINAC along a specific path delivering radiation to the tumour.
- The LINAC then stops and a new pair of X-ray images are acquired.
- The target position is then redetermined and communicated to the robotic arm, which makes changes if necessary before delivering the next set of X-rays.
- The process is repeated until all the planned beams are delivered and the treatment is completed. This could take 30–60 minutes for intracranial lesions/ tumours. Theoretically, 1200 trajectories are available for a treatment, although in practice it is likely to be 10%–15% of this.
Do you think the cyberknife will replace other methods of treatment?
No, but for the properly chosen indication, it can make a huge difference, particularly because it is a non-invasive OP procedure with negligible procedure-related morbidity.