Journal List > Prog Med Phys > v.26(1) > 1098471

Bae, Cho, Kwak, Park, Lim, and Chung: Monte Carlo Simulation of the Carbon Beam Nozzle for the Biomedical Research Facility in RAON

Abstract

The purpose of the Monte Carlo simulation study was to provide the optimized nozzle design to satisfy the beam conditions for biomedical researches in the Korean heavy-ion accelerator, RAON. The nozzle design was required to produce C12 beam satisfying the three conditions; the maximum field size, the dose uniformity and the beam contamination. We employed the GEANT4 toolkit in Monte Carlo simulation to optimize the nozzle design. The beams for biomedical researches were required that the maximum field size should be more than 15×15 cm2, the dose uniformity was to be less than 3% and the level of beam contamination due to the scattered radiation from collimation systems was less than 5% of total dose. For the field size, we optimized the tilting angle of the circularly rotating beam controlled by a pair of dipole magnets at the most upstream of the user beam line unit and the thickness of the scatter plate located downstream of the dipole magnets. The values of beam scanning angle and the thickness of the scatter plate could be successfully optimized to be 0.5o and 0.05 cm via this Monte Carlo simulation analysis. For the dose uniformity and the beam contamination, we introduced the new beam configuration technique by the combination of scanning and static beams. With the combination of a central static beam and a circularly rotating beam with the tilting angle of 0.5o to beam axis, the dose uniformity could be established to be 1.1% in 15×15 cm2 sized maximum field. For the beam contamination, it was determined by the ratio of the absorbed doses delivered by C12 ion and other particles. The level of the beam contamination could be achieved to be less than 2.5% of total dose in the region from 5 cm to 17 cm water equivalent depth in the combined beam configuration. Based on the results, we could establish the optimized nozzle design satisfying the beam conditions which were required for biomedical researches.

References

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Fig. 1.
The schematic diagram of the experimental beam nozzle for GEANT4 simulation.
pmp-26-12f1.tif
Fig. 2.
The full width half maximums of field versus the thicknesses of scatter plate.
pmp-26-12f2.tif
Fig. 3.
The distances of hone shapes in each lateral profile according to the various tilting angle for beam wobbling.
pmp-26-12f3.tif
Fig. 4.
The normalized lateral dose profiles under conditions with/without the optimized beam tilting angle 0.5o and the diagram of the combined beam delivery: (a) the lateral profile in scatter thickness 0.05 cm and beam tilting angle 0o, (b) the lateral profile in scatter thickness 0.05 cm and beam tilting angle 0.5o, (c) the diagram of the beam combination with a central static beam and a peripheral wobbling beam, and (d) the lateral profile in the combined beam mode for 3500 million MC histories of C12.
pmp-26-12f4.tif
Fig. 5.
The Bragg peaks for C12 beams with energies ranged from 160 to 310 MeV/u.
pmp-26-12f5.tif
Fig. 6.
The C12 ion SOBP beam and the corresponding normalized weighting function for each Bragg peaks with C12 ion energies ranging from 160 to 310 MeV/u.
pmp-26-12f6.tif
Fig. 7.
The percentage of dose delivered by non C12 particles tagged at the water phantom surface was interpreted as the beam contamination level.
pmp-26-12f7.tif
Table 1.
Carbon beam characteristics & requirements for the biomedical research project in RAON.
  Previous design Final design
Beam energy 430 MeV/μ 310 MeV/μ
Beam diameter 3 mm 2.5 mm (1∼5 mm)
Beam size 15×15 cm2 (50%) 15×15 cm2 (90%)
Dose uncertainty <5% <3%
Energy spread <1% <1%
Nozzle length 3 m 11 m
Range modulator Required Not necessary
Wobbler Disabled Enabled
Beam purity <85% > 95%
TOOLS
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