The accelerated protons originating from the COMET cyclotron are focussed into a small beam called a spot, a pencil-thin beam of 5 to 7 mm width. The protons are then injected into the radiotherapy machine, the Gantry, which serves to steer the protons into the precise position in the patient to be treated. The high dose spots are scanned one after the other over the whole tumour volume (spot-scanning technique). In Gantry 1, the depth in the patient at which the proton spots stop is controlled by sliding plastic plates into the beam. These movements are very fast, lasting only a few milliseconds. The tumour is irradiated layer by layer in scan lines whilst the patient couch is moved slowly in steps of 5 mm. In this way all three dimensions of the tumour are scanned through with the beam.

In the new Gantry 2 facility, an advanced scanning technique is in place. The beam steering is facilitated in 2 dimensions. The third dimension, controlled by the energy of the beam, is performed at the exit of the cyclotron itself by a degrader on a timescale of fractions of a second.

With the treatment technique at PSI the placement of the proton pencil beam is computer-controlled, so that the high dose spot dwells for a precisely planned amount of time, precisely in the planned position in the tumour. Through the superposition of many single spots – around 10 000 for a volume of 1 litre – the tumour is irradiated with the desired radiation dose simultaneously monitoring the dose given by each individual spot. This allows for an extremely precise, homogeneous radiation treatment, that can be tailored optimally to even the most irregular tumour shapes. This method of producing a dynamic, three-dimensional conformal radiation treatment is termed spot-scanning. At PSI these precise cancer treatments have been in place since 1996, and have made it possible to deliver a high-precision radiation treatment to patients. These techniques limit the dose to the surrounding healthy tissues to a higher degree when compared to conventional photon therapy.

The principle of the PSI-developed Spot-scanning technique:

Through the scanning and superposition of dose-spots of a proton pencil beam, the desired dose distribution can be built up, and the dose can be precisely tailored to the shape of the tumour in three-dimensions.