Image-guided Frameless Radiotherapy
Image-guided Frameless Radiotherapy
It is more than half a century since Lars Leksell developed stereotactic radiosurgery and the ability to deliver protons to discrete areas of the brain to irradiate tumours. However, while there have been many improvements made to the nature and method of radiation delivery, the stereotactic head frame used to co-ordinate the beams and keep the patient in position has remained at the core of the procedure – until recently. New devices are now available that use stereo imaging technology to accurately identify, target and destroy tumours with greater accuracy and in shorter time-frames, without the need for a painful and restrictive head frame.
Challenges Faced During Radiosurgery/Radiotherapy
Radiosurgery (a single high dose of radiation) and radiotherapy (where the effective dose is fractionated into a series of lower doses) require immobilisation of the patient and precise target localisation to allow the dose to be concentrated in the target via multiple crossing beams. A single dose is normally delivered in a session that takes about 20 minutes. With fractionated radiotherapy there may be one daily dose five times per week; if 10 fractions are being used, they could be delivered over two weeks, or with two fractions per day for five days with an interval between fractions of at least six hours.
The Frame-based System
In traditional techniques, a stereotactic head frame was attached to the patient’s head using four screws inserted into the skull under local anaesthesia. This ensured the patient was fastened to the linear particle accelerator (linac) so he or she could not move. Once the frame was fitted the patient was scanned using computed tomography (CT) to establish the targeting relationship. Therapy was initiated almost immediately afterwards. One assumption made with the linac was that the patient could not move at all between CT scanning and initiation of therapy and, furthermore, he or she had to remain still during the treatment. In order to protect healthy brain tissue during conventional radiosurgery/radiotherapy, the target was outlined and then a safety margin added to cope with both any unintentional movements made and the inherent inaccuracies of the equipment. This increased the radius of the irradiated sphere created by a millimetre or so, which effectively doubled its volume.
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