Monte Carlo simulations of new 2D ripple filters for particle therapy facilities

At particle therapy facilities with pencil beam scanning, the implementation of a ripple filter (RiFi) broadens the Bragg peak (BP), which leads to fewer energy steps from the accelerator required to obtain an homogeneous dose coverage of the planned target volume (PTV). At the Universitätsklinikum Gießen und Marburg, Germany, a new second generation RiFi has been developed with two-dimensional groove structures. In this work we evaluate this new RiFi design.

The Monte Carlo (MC) code SHIELD-HIT12A is used to determine the RiFi-induced inhomogeneities in the dose distribution for various ion types, initial particle energies and distances from the RiFi to the phantom surface as well as in the depth of the phantom. The beam delivery and monitor system (BAMS) used at Marburg, the Heidelberg Ionentherapiezentrum (HIT), Universitätsklinikum Heidelberg, Germany and the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany is modeled and simulated. To evaluate the PTV dose coverage performance of the new RiFi design, the heavy ion treatment planning system TRiP98 is used for dose optimization. SHIELD-HIT12A is used to prepare the facility-specific physical dose kernels needed by TRiP, and for recalculating the physical dose distribution after TRiP optimization.

At short distances from the RiFi to the phantom surface fine structures in the dose distribution are observed. For various RiFis, ion types and initial particle energies the distance dmax at which maximum dose inhomogeneity occurs is found and an expression for dmax is deduced. The distance d0.01 at which the dose inhomogeneity is less than 1% is estimated and used as a threshold distance at which dose distributions are considered homogeneous. The MC data are found to agree with analytical expressions for dmax and d0.01; both are inversely related to the angular distribution. Increasing scatter from the beam delivery and monitoring system results in reduced dmax and d0.01. Furthermore, dmax and d0.01 are found to be proportional to the RiFi period λ.

Our findings clearly indicate that the dose inhomogeneity induced by RiFis does not add uncertainties to the dose distribution in the clinical setting. The new RiFi design can be used in treatments to obtain homogeneous PTV dose coverage with fewer energy steps while improving lateral penumbra, thereby reducing the required treatment time.