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Evaluation of Different Physical Parameters to Interpret the Biological Effects of Particle Therapy

D Ma*, L Bronk, U Titt, D Grosshans, R Mohan, F Guan, MD Anderson Cancer Center, Houston, TX

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose:
To find the appropriate physical parameters that can be used to interpret the biological effects of therapeutic protons and ions.
Methods:
The clonogenic survival data of the non-small cell lung cancer (NSCLC) cell line H460 with the irradiations of protons, helium and carbon ions were obtained using the HIT facility in collaboration with the DKFZ in Heidelberg, Germany. The Geant4 Monte Carlo toolkit was used to perform the calculations for the physical parameters such as dose, linear energy transfer (LET), and lineal energy, y. The nominal energy of protons, helium and carbon ions are 80.04, 84.0, and 153.66 MeV/u. The dose to each cell layer in the wells, track- and dose-averaged LET (LETt and LETd), and the microdosimetric spectra (in 2-µm water sphere) were scored. The fluence- and dose-mean lineal energy (yF and yD) and saturation-corrected dose-mean lineal energy (y*) were derived from the microdosimetric spectra.
Results:
For protons, both of different LET quantities and lineal energy quantities increase along the Bragg curve. For helium and carbon ions, the variation trends of these quantities are quite different. Finally, yD was selected to correlate with the relative biological effectiveness (RBE) of different particles at the cell surviving fraction of 0.1. For protons, RBE increases from 1.2 to 3.2 in a non-linear trend with the yD from 3.6 to 20.6 keV/µm. For helium and carbon ions, the RBE increases from 1.7 to 4.7 with the yD from 10 to 87.9 keV/µm. After this, the RBE of carbon ions deceases to 1.7 until the yD of 270 keV/µm showing the overkill effect.
Conclusion:
Our in vitro experimental data show that the relations between RBE and physical parameters are complex, and the microdosimetric quantity yD is superior to other physical quantities. More experiments and physical calculations are needed to support this conclusion.

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