gms | German Medical Science

Background: Based on the experience of Gantry 1 with a scanned proton beam a new gantry is under construction at PSI. The commissioning of the scanning system for this new Gantry 2 has started and first measurements are accomplished. Gantry 2 will also be the framework to develop new scanning techniques with the aim to treat moving tumours. To achieve this, repainting of the target volume is one of the promising strategies. In order to avoid the accumulation of unwanted dead time especially in the case of repainting, a fast scanning system is required. Besides the discrete spot scanning, modes with continuous scanning of lines or even contours are also considered.

Materials and methods: For the lateral beam displacement two fast sweeper magnets are used. Additionally a vertical deflector is installed close to the proton source inside the accelerator which allows a fast modulation of the beam intensity. Together with the mechanical degrader the beam line acts as energy selection system. To enable fast energy changes the complete beam line is built up with laminated magnets in order to avoid eddy currents. The accurate control of the scanning sequence requires a powerful control system. Precise synchronisation of the fast actuators is achieved using specific hardware based on field programmable gate arrays (FPGAs).

Results: Due to dedicated power supplies the two sweeper magnets have a setup time for typical spot steps below 3 ms and allow scanning the proton beam continuously up to 2 cm/ms in the target volume. In the line scanning mode the minimal time to paint a square of 10 cm² is 200 ms. Additionally the beam intensity of these lines can be modulated on a time scale of 50 μs using the deflector plate. With the help of FPGAs the control system allows controlling the fast actuators with a time resolution of 10 μs. Because of the laminated beam line the proton energy can be changed within 80 ms for the typical steps in depth.

Conclusions: Dedicated hardware enables a fast scanning on Gantry 2. It is supported by the control system which has the flexibility to enable different modes from discrete spot scanning to intensity modulated lines. With help of the deflector plate not only the beam intensity can be modulated but also the short reaction time enables the implementation of a control loop to stabilize the beam current at a desired value, which is a critical issue in the case of continuous scanning.

Only a fast scanning system with low dead time is able to apply repainting efficiently. The Gantry 2 has even the potential for volumetric repainting due to the fast energy changes. Repainting will be an important step in treating moving targets and will pave the way for new indications using scanning.