Performance requirements of the MedAustron beam delivery system

The Austrian hadron therapy center MedAustron is currently under construction with patient treatment planned to commence in 2015. Tumors will be irradiated using proton and carbon ions, for which the steeply rising Bragg curve and finite range offer a better conformity of the dose to the geometrical shape of the tumor compared to conventional photon irradiation. The current trend is to move from passive scattering toward active scanning using a narrow pencil beam in order to reach an even better dose conformation and limit the need of patient specific hardware. The quality of the deposited dose will ultimately depend on the performance of the beam delivery chain: beam profile and extraction stability of the extracted beam, accuracy and ramp rate of the scanning magnet power supplies, and precision of the beam monitors used for verifying the delivered dose. With a sharp lateral penumbra, the transverse dose fall-off can be minimized. This is of particular importance in situations where the lesion is adjacent to critical and radiosensitive structures. Multiple Coulomb scattering upstream of the patient in e.g. air gap, monitors and vacuum windows will limit the sharpness of the beam, in particular for protons in the lower therapeutic energy range. In this work, clinical requirements on target dose distribution are translated into specifications along the beam delivery chain. By using an analytical model for third integer resonant extraction in combination with particle tracking, the effect of power supply ripple in the main ring magnets on extraction stability has been analyzed. A model for calculating beam growth caused by scattering in a heterogeneous slab geometry along a transfer line (taking focusing and defocussing of the beam into account) has been implemented and used for optimizing the MedAustron nozzles and proton gantry with respect to beam growth at the isocenter. By combining the scattering model with parameterized proton and carbon ion Bragg curves, a three dimensional dose calculator for spot scanning has been implemented to perform a dose distribution error analysis with respect to the combined effect of various beam delivery imperfections and limitations. This approach enables for identification of ”bottlenecks” at an early stage in the design procedure while avoiding unnecessarily strict specifications. Different scanning modes have been compared and preventive means of suppressing the impact of beam delivery imperfections already on the treatment planning stage are discussed.