Passive ion beam modulation techniques for particle therapy facilities utilizing active pencil beam scanning delivery systems
Particle therapy (PT) cancer treatment is an alternative to conventional radiotherapy with the possibility for more conformal and tissue sparing treatments. PT is realized using either passive or active beam delivery methods. With the latter, also coined the scanned beam technique, magnets are used...
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|Summary:||Particle therapy (PT) cancer treatment is an alternative to conventional radiotherapy with the possibility for more conformal and tissue sparing treatments. PT is realized using either passive or active beam delivery methods. With the latter, also coined the scanned beam technique, magnets are used to cover the target laterally and for in-depth variation energy modulation is deployed, making the dose delivery even more conformal. However some of the current limitations of scanned beams are the longer irradiation times and a sensitivity to intra-fractionally moving targets. In PT centers with synchrotrons the irradiation time is directly related to the spot scan size and the width of the Bragg peak (BP), with the time needed from the accelerator to change energy as the bottleneck. The number of energy shifts required to cover a typical tumour in a homogeneous manner can in particular for heavy ions be as large as many hundreds. By broadening the BPs through the use of passive energy modulators, the number of energy shifts can be lowered, which would not only reduce the irradiation time but also results in a higher particle fluence per energy step, leading to higher precision in the beam monitoring systems.
This work addresses the implementation of such passive energy modulators, in particular the ripple filter (RiFi). A “first generation” RiFi is currently used in carbon ion treatments in Germany, Italy, China and Japan. This first generation RiFi has 1D groove shapes, which requires a non-modulating base layer of material leading to unnecessary scattering. It is furthermore restricted to a maximum thickness of 3 mm. A new second generation RiFi with two-dimensional cone structures has been designed. Compared to the old design the resolution and the mass distribution are significantly improved, reducing the overall lateral beam width. Using 3D printing for manufacturing, the obtainable RiFi thickness is higher, with further BP widening and shorter irradiation times as a results. The new 2D design is thought to be usable in treatments with protons as well, where RiFis as of now are commonly not used in proton treatments.
In this thesis, we show a methodological presentation of planning with the second generation RiFi design. It was found that treatment plans with 2D RiFis with 4 and 6 mm thicknesses yielded for the studied cases comparable dosimetric results to the standard 3 mm thick RiFi in terms of plan homogeneity and conformity but with significantly reduced irradiation times: Compared to the 3 mm RiFi, the 4 and 6 mm RiFis lower the irradiation time by 25-30% and 45-49% respectively. Plan homogeneity and conformity were slightly improved for thinner RiFis but satisfactory results are obtained for all cases with RiFi performances in general increasing with penetration depth due to straggling and scattering effects. Certain plans for 6 mm RiFis indicate that there might be an upper limit on the RiFi thickness in treatments of small and very superficial tumours.
The work of this thesis also continues the investigations of the RiFi-induced fluence inhomogeneities and dose range inhomogeneities begun in the author's master thesis and covers new findings in this topic related to the beam spot sizes and the ion optical focusing of the beam.
Lastly, during the thesis, plates of porous materials such as foams or lung substitutes will be shown to be usable as passive energy modulators in a manner similar to RiFis and to furthermore function as a range shifter, which placed close to the patients leads to reduced beam penumbras for low penetration depths.
This work furthermore contains a short outlook with a perspective on other methods reducing the energy shifts as well as comments on new future designs of energy modulators.|
|Physical Description:||119 Pages|