Multi‐layer energy filter for realizing conformal irradiation in charged particle therapy

Abstract

A new type of filter for charged particle radiotherapy is developed to reduce unwanted dose transfer to the normal tissues around a tumor. The new filter can make a static irradiation field where the width of the spread-out Bragg peak (SOBP) is two-dimensionally adjusted. That makes the field conformal to the tumor three-dimensionally. The filter is made of many layers produced by using stereolithography. The layer has a miniaturized structure that has geometrical similarity to the conventional ridge filter. Shapes of cone and pyramid are also usable for the unit-cell constructing the layer. The spread of the field in the depth direction is decided by the thickness of the filter, or by the number of layers. The experimental result of the irradiation using the ridge-type construction shows a good agreement with an estimate by the Monte Carlo calculation. By combining this technique with intensity modulation that has lateral position dependence, the conformal irradiation can be achieved by a simple procedure.

Conformal irradiation by proton beam scanning and multilayer energy filter

“…The problem can be solved by using two regions designed for a narrow and a large spread of the SOBP. 9 Second, condensation of the energy deposition in the narrow SOBP should be compensated by lateral intensity modulation. The compensation is performed by modulation of the irradiation time in this work.…”

“…It was confirmed that the flat top of the peak in the dose distribution could be obtained by this structure. 9 The outer shape of the block is shaved to get a desirable shape for a step-shaped virtual target with 20, 40, and 60 mm thickness as shown in Fig. 2.…”

“…9 First, for a different spread of the SOBP, a different set of mixing factors of the beams is necessary. In other words, the design of the cell pattern should be changed for different thicknesses of the filter.…”

Application of dose kernel calculation using a simplified Monte Carlo method to treatment plan for scanned proton beams

“…However, since it takes much time for the FMC method to complete the dose calculation in clinical cases, it is difficult at present to apply the FMC calculation to routine clinical treatment planning. To reduce the calculation time, a number of faster Monte Carlo methods dedicated to proton therapy have been developed 10 , 11 , 12 . One of such faster FMC codes for TPS in proton therapy, named VMCpro, was reported to be 35 times faster than the general purpose FMC code GEANT4 for simulations in a phantom with large inhomogeneities (10) .…”

“…A simplified Monte Carlo (SMC) method 12 , 13 , 14 , 15 has been developed to obtain dose calculation results more accurately than the PBA in target heterogeneities in a shorter calculation time. It tracks individual proton paths scattered in material and uses a measured depth‐dose distribution in water for dose calculation.…”