Design and construction of a ripple filter for a smoothed depth dose distribution in conformal particle therapy.

The ripple filter was designed to broaden the Bragg maximum of carbon beams for the raster-scan technique, a special type of tumour-conformal ion beam treatment. In this technique the target volume is divided into individual layers that are treated sequentially by varying the energy from the accelerator stepwise. Because the unmodified Bragg maximum has a small half-width, below 1 mm for small energies (< 160 MeV u(-1)), homogeneous irradiation at small penetration depths of 2-6 cm can only be obtained by using a large number of energy steps. If the energy step is too large, ripples are produced in the superimposed depth dose distribution. The ripple filter widens a Bragg peak to a Gaussian peak with a half-width of more than 2 mm. This helps to smooth the extended Bragg peak and to reduce the number of energy steps required by a factor of two to three, leading to significantly shorter overall irradiation times and a higher particle fluence per layer. The ripple filter consists of a 2 mm thick Plexiglass (PMMA) plate with a periodic structure of fine grooves. It can be mounted 60 cm upstream of the patient as a stationary device, because the fine structure of the grooves is completely washed out by the lateral scattering of the beam.