Abstract
Protons and heavy ion particles are considered to be ideal particles for use in external beam radiotherapy due to superior properties of the dose distribution that results when these particles are incident externally and due to their relative biological effectiveness. While significant research has been performed into the properties and physical dose characteristics of heavy ions, the nuclear reactions (direct and fragmentation) undergone by He 4 , C 12 and Ne 20 n u c l e i u s e d i n r a d i o t h e r a py i n m a t e r i a l s o t h e r t h a n w a t e r i s s till l a r g e l y unexplored. In the current project, input code was developed for the Monte Carlo toolkit Geant 4 version 9.3 to simulate the transport of several mono-energetic heavy ions through water. The relative dose contributions from secondary particles and nuclear fragments originating from the primary particles were investigated for each ion in both water and dense bone (ICRU) media. The results indicated that the relative contribution to the total physical dose from nuclear fragments increased with both increasing particle mass and with increasing medium density. In the case of 150 MeV protons, secondary particles were shown to contribute less than 0.5% of the peak dose and as high as 25% when using 10570 MeV neon ions in bone. When water was substituted for a bone medium, the contributions from fragments increased by more than 6% for C 12 and Ne 20 .
Highlights
It has been demonstrated that the use of protons and heavy ions for the treatment of cancer results in relatively large doses being administered to the tumour region while minimising the damage of healthy tissue surrounding it
When 150 MeV protons were used, secondary particles produced in nuclear reactions contributed less than 0.5% of the maximum dose in the Bragg peak as illustrated in figure 1(a)
Of all the secondary particles tracked in the simulation, the most significant secondary particles were alpha particles which contributed a maximum of 0.19% of the dose at the Bragg peak followed by secondary protons with 0.12%
Summary
It has been demonstrated that the use of protons and heavy ions for the treatment of cancer results in relatively large doses being administered to the tumour region while minimising the damage of healthy tissue surrounding it. This effect is enhanced by the higher relative biological effectiveness of heavy ions at low energies (high linear energy transfer). Low mass fragments produced in nuclear reactions may gain a large fraction of the primary particles energy enabling them to travel beyond the Bragg peak depositing their energy in this region
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