Arthur Vandenhoeke¶
Magnetic field-map tracking methods for clinical beam modeling in proton therapy
Abstract¶
Recent improvements in radiotherapy treatments such as Volumetric Modulated Arc Therapy (VMAT) combine precise imaging techniques with modulated beam irradiations to enable the treatment of complexly shaped target volumes while maintaining the dose to healthy surrounding tissues at a low level. Proton therapy benefit from different dose giving processes allowing to deliver high doses to the target while keeping the dose to overall surrounding tissues at comparable levels. However, the comparison between the treatment planning systems of proton therapy and standard radiotherapy suggests that the only advantage of proton treatments is the decrease of the integral dose. The beam deliverance methods of proton therapy need to progress in response to this technological rise. In addition, the recent proton therapy facilities are designed to offer compact solutions such as the Ion Beam Application (IBA) Proteus one center. The compactness of the related Compact Gantry (CGTR) has shown evidence for cross-talk effects rising from the tight placement of its magnets. These effects are caused by the relatively large attenuation distance of the lateral magnetic fields referred as the fringe fields, compared to the length of the individual magnets. High order magnetic field components in those regions are responsible for non-linear effects on the beam optics, resulting in a deformed scanning patch at the isocenter. Results of magnet optimization studies have already been implemented in the actual design of the gantry to correct for optical imprecisions. The standard accelerator codes used for the design of large accelerator rings cannot be used to describe the non-linear effects occurring in the fringe field regions of the magnets. The Zgoubi ray-tracing code provides a solution to track charged particles in magnetic field-maps accessible via the magnetic model of the gantry defined at IBA. The Python interface to Zgoubi, called Zgoubidoo, is developed to load the individual field-maps and to translate the latter according to Zgoubi’s analytical field model. This translation is done by means of a parameterization that follows an Enge distribution accounting for the individual fringe fields. Zgoubidoo is also equipped with a decoupled plotting module enabling an independent verification of the geometry. Individual particles are tracked through the beamline in order to reproduce an experimental scanning patch. The simulations take the extensions of the fringe field of the CGTR’s last bending dipole as variable parameters and confirm an adequate scanning patch for extension values taken from the fringe field parameterization formalism of Zgoubidoo. Results show that the fringe field models of the scanning magnets strongly influence the scanning patch and allow to reduce the error between the experimental results and the numerical simulations. However they do not explain the distortion of the numerical scanning patch compared to the experimental patch. The latter is therefore attributed to the numerical design of the last bending dipole, which opens the way towards further optimizations of the model. The tracking of proton bunches also contributes to the optimization procedures of the entire beamline model through calculations of physically relevant parameters such as the beam transmission and the beam size.