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Beam delivery system:

The beam delivery system consists of a 74 MeV beam taken from the TRIUMF 500 MeV cyclotron, a specialized chair that the patient sits in to receive the treatment, and X-ray equipment to alight the tumour to the beam.

Beam line 2C is capable of extracting protons with energies from 65 to 120 MeV. The extraction beam line has a common section immediately after the combination magnet and then a switching dipole magnet which can be set to transport protons down through the vault wall into the treatment area. This beam line has two dipole magnets, five quadrupole magnets for beam focusing and three steering magnets for beam centering. There are also many safety interlocks in this section. If any one of them detects that something is wrong, that beam is stopped instantaneously.


Proton Therapy

Alignment methods:

A laser system is used initially as a rough guide to align the beam directly on the tumour. This is done by a chair which is controlled electronically by servo motors. The chair's position can be changed in six ways. Precise patient alignment with the beam is accomplished using a series of X-ray polaroid films.

The treatment depth can be varied by a rotating wedge degrader or range shifter which is located between the first and second collimators. It is operated by a stepping motor with encoder readout. The range modulator which is just downstream of the range shifter rotates a plastic wheel which has been N/C machined to produce a flat spread out Bragg peak. The modulator wheel rotates at 240 RPM and has four modulations per rotation. A number of modulator wheels have been machined to vary the width of the spread-out Bragg peak over the required treatment depth of 5 to 25 mm in steps of 1 mm.


Patient treatment:

The patient is immobilized by way of a mask and a bite block, both custom made. Since the eye is not immobilized itself, it is very important that the patient not move the eye during treatment. However, to help the patient, the room lights are turned off and the patient is asked to stare at a small flashing light. The position of the eye is monitored during treatment by closed circuit television. If for any reason the patient moves the eye, the beam is stopped to avoid damage to other parts of the eye. If the eye being treated is blind, then the other eye can be used to fixate on the small flashing light instead.