How it's done

Since 1968, when Leksell Gamma Knife started being used for regular treatment, a stereotactic frame has been used as an immobilization device. Prof. Lars Leksell used a Leksell frame, which was developed based on an arc-centred principle.

Arc Centre Principle

'A trajectory perpendicular to an arc will pass through its centre.'

Leksell Frame with orthovoltage x-ray tube mounter for radiosurgery experiment

Lars Leksell with his stereotactic frame

The Gamma Knife was also based on the same principle. The Leksell frame fixation provided accuracy and immobilization.

Leksell Frame G with Model U Gamma Knife

Vantage Frame for Radiosurgery to be used in the new ESPRIT Gamma Knife

Other Frames for Linac Radiosurgery

CRW Frame System, Arc-centered Principle

Linac Radiosurgery

When Betti O, et al. in Paris in 1982 and Loeffler J & Alexander E, et al. in Boston in 1992 demonstrated Linac-based radiosurgery, the race to use Linac for radiosurgery began. Different centres used different frames and fixation devices, like CRW frames by Radionics. The objective was to get a perfect frameless fixation device.

There were four goals in developing a fixation device to facilitate stereotactic axial imaging:

Reduce patient movement to lessen motion artifacts.
Allow reproducible positioning for long procedures.
Enable precise and transferable geometric target delineation across numerous diagnostic and therapeutic modalities.
Facilitate reproducible head positioning for scans performed on different days. Their solution was to use mouldable thermoplastics to create a rigid helmet based on bone anatomy instead of a metal invasive frame.

Frameless Immobilization

The thermoplastic mask provided immobilization, but the fear of error remained. Various devices were made to detect intra-fraction movement, including light emitting diodes, stereoscopic feature detecting cameras, etc.

Position Verification in Frameless Treatment

In 1994, at Stanford, a new device was developed called ‘Neurotron 1000’. It was a Linac mounted on a robotic manipulator giving 6 degrees of freedom and 3𝛑 steradians beam access to the patient’s head. Currently it is known as Cyberknife.

They used two special methods:

CT scan-based 3D modelling to detect bone features like Sella-turcica, perpendicular plates of ethmoid and the mastoid process, using computers.
Matching it with two oblique digital X-rays taken during treatment and matching them with digital versions created by a CT scan.

Bone feature matching using oblique X-rays gave high positional accuracy, even without a rigid invasive frame. This system guides through the fraction to maintain position and check errors.

This system led to successful development of Cyberknife which provided:

High precision targeting
Fractionated radiosurgery
Improved dosimetry for non-spherical tumour volumes
Body radiosurgery.

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