Radiosurgery
Radiotherapy depends primarily on tumour cells having greater sensitivity to radiation than normal tissue. To protect normal tissue as far as possible the treatment is fractionated over many sessions, usually over a period of several weeks.
In stereotactic
radiosurgery (SRS), a high power radiation beam is projected onto the target with much
greater accuracy. By cross-firing from many different angles the
exposure of adjacent healthy tissue is minimised and the number of
treatments can be greatly reduced.Radiosurgery does not remove the tumour but destroys tumour cells or stops growth of active tissue and the main forms of radiosurgery available today are Linac (linear accelerator), GammaKnife® and CyberKnife® , of which CyberKnife® is the most recent and the most flexible.
GammaKnife®
The first radiosurgical device was developed in
the
1950s, leading to the GammaKnife®. This is used for intra cranial
lesions, delivering precisely targeted gamma rays from a Cobalt-60
source through a helmet-shaped device with 201 separate holes and the
beams converge on the lesion to be treated. Although effective it is
necessary to screw an external metal frame to the patient's skull to
ensure accurate targeting. It can be difficult to treat targets on the
periphery of the brain and it cannot be used for fractionated
radiosurgery, which can be beneficial for larger tumours or lesions
located near nerves and other sensitive structures. Whilst GammaKnife® has a long and successful track record and is subject to continual
improvements it cannot be used on other parts of the body.
Modified Linear Accelerator Systems
Linear accelerators (linacs) were developed in the mid 1980s and do not require or generate any radioactive material. By modifying the conventional linear accelerators that are commonplace in many large hospitals and using specialised software it is possible to do many types of brain radiosurgery. Dedicated linac systems tend to be more carefully calibrated for spatial accuracy and optimised for radiosurgical efficiency. When treating brain tumours with linac radiosurgery, a metal head frame is still attached to the patient's skull and used to target the radiation beam.
Shaped Beam Systems
IMRT or Intensity Modulated Radiation
Therapy uses computer-controlled "beam-shaping" to give improved
accuracy and it can be used on most areas of the body. Using
sophisticated planning software a multi-leaf collimator, which is
attached to most modern
medical linear accelerators, dynamically reshapes the outline and
intensity
of the radiation field during treatment. This fits the radiation to a
target
much better than conventional radiation therapy can and reduces injury
to neighbouring healthy tissue. It is not as spatially precise as
radiosurgery however, so treatment is typically administered over 20-30
sessions. For brain tumours an intrusive head frame is required and it
does not offer the accuracy of GammaKnife® or CyberKnife®. Away from the
head accuracy is degraded as the patient moves, e.g. through breathing.
CyberKnife®
CyberKnife®
uses a miniature linear accelerator mounted upon a highly flexible,
robotically controlled arm to deliver precisely controlled beams of
radiation from many different angles to minimise impact on surrounding
tissue and focus on the tumour, AVM or other target. The position of
the tumour is tracked between doses, allowing CyberKnife®
to compensate for small body movements. For brain and spine tumours
the patient's own bone structure is used to provide markers, so
avoiding the need for intrusive frames. For other areas of the body
small metal markers (fiducials) are placed first. Complex shapes and
difficult, inoperable locations next to nerves or blood vessels can be
treated but the fine radiation beam employed means that CyberKnife®
is less suited to treating multiple or large tumours unless these are first reduced by surgery or chemotherapy.