Author: John Ford, Yu Fujiwara, Isao Murata, Zavier Ndum Ndum 👨🔬
Affiliation: Osaka University, Texas A&M University 🌍
Purpose: This study investigates how particle energy affects radiation quality and biological effectiveness with the inclusion of medical implants. Our research group used microdosimetric analysis to reveal that medical implants affect the biological effects via the production of secondary particles for two energies of proton or carbon ion sources. However, it is important to examine the energy dependency on the biological effectiveness over wider energy ranges in order to account for the effects of medical implants in detail. This is because the secondary particle production depends on the nuclear reaction cross-section of materials which vary with incident particle energy.
Methods: We employed the Particle and Heavy Ion Transport code System (PHITS) to calculate microdosimetric quantities (lineal energy and its frequency and dose distributions) for two more proton energies of 150 MeV and 250 MeV, and carbon-ion energies of 290 MeV/u and 400 MeV/u. PMMA was used to reproduce the depth curve in the human body, and Titanium and Gold were used as implant materials. Calculations were conducted for three regions: (a) before, (b) at, and (c) beyond the Bragg Peak for PMMA and equivalent thicknesses for each region derived from the ratio of stopping power for implants. The stopping power was calculated in SRIM. From these microdosimetric quantities, RBE and quality factor were evaluated using the MKM model.
Results: Microdosimetry spectra revealed energy-dependent discrepancies in peak positions and magnitudes, with higher peaks observed for implants due to secondary particle production. Titanium and Gold implants showed distinct effects on RBE and quality factor, particularly at the Bragg Peak.
Conclusion: We have demonstrated that medical implants' influence on beam quality depends on the particle’s energy. This highlights the importance of detailed energy-dependent assessments for clinical applications. Future work includes experimental validation to substantiate these findings further.