Analysis of Scanning Techniques for Hadron Therapy

The use of accelerators in cancer therapy has a long and successful history. Present-day developments are directed towards high-precision irradiation of tumours with hadron beams. Firstly, hadron beams localise the irradiation in a low-dose, entry corridor with a high-dose volume at the precise depth of the Bragg peak that is set by the beam energy. Secondly, the heavier the ion the lower the defocusing effect of the multiple scattering in the patient's body, but other considerations such as nuclear fragmentation and the physics of the energy transfer lead to an optimum in the mass range around carbon. The ability to deliver the irradiation to small volumes with millimetre precision opens the way to treating complex-shaped tumours that are in close proximity to vital organs. The creation of the well-focused ion beams at variable, but precise, energies over spill times of a few seconds is best done with a synchrotron using slow extraction. However, slow extraction is notoriously difficult to stabilise and it is essential that the beam scanning system just before the patient can be designed to be tolerant of beam fluctuations. This report analyses and then compares the performance of three scanning techniques: voxel scanning, mini-voxel scanning and true raster scanning. The analyses are made with a view to achieving a dose uniformity of ±2.5%. Voxel scanning combined with a fast chopper (Rapid Voxel Scanning) is proposed as a possible practical solution.