| Summary: | 碩士 === 長庚大學 === 醫學影像暨放射科學系 === 103 === In radiotherapy, particle beams are superior to photon beams for the depth-dose distributions due to the Bragg peaks. In addition, particle beams provide narrower penumbras, in the lateral dose distributions and higher relative biological effectiveness (RBE) values due to the production of nuclear fragments.
The RBE enhancements of particle beams are especially significant near the particle ranges where the linear energy transfer (LET) increases more rapidly. Beyond the particle ranges, nuclear fragments continue to travel and deposit their energies in the dose tails region. All these contribute to increased RBE-weighted doses in the tumor and extend the biological doses to the normal tissue behind the tumor.
In the present work, the FLUKA (FLUktuierende KAskade) Monte Carlo code was used to simulate the dose penumbras, fragment tails, and RBE values for therapeutic proton, helium-ion, and carbon-ion beams in a water phantom. RBE values were determined from the individual fluence spectra of primary particles and secondary fragments and Substituting these spectra into the MCDS (Monte Carlo Damage Simulation) code, the fluence averaged DNA damage yields were computed. RBE values for the induction of DNA double-strand break were then calculated at different locations in the water phantom.
For proton, helium and carbon ions of similar ranges, RBE values of protons range from 1.1 at the entrance depth to 1.7 at the distal edge; RBE values of helium ions range from 1.2 at the entrance to 2.5 at the distal edge; RBE values of carbon ions range from 1.3 at the entrance to 2.8 at the distal edge.
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