Mechanistic modelling of relative biological effectiveness of carbon ion beams and comparison withexperiments.

Relative biological effectiveness (RBE) plays a vital role
in carbon ion radiotherapy, which is a promising treatment method
for reducing toxic effects on normal tissues and improving treatment
efficacy. It is important to have an effective and precise way of obtaining
RBE values to support clinical decisions. A method of calculating RBE
from a mechanistic perspective is reported.
Approach: Ratio of dose to obtain the same number of double strand
breaks (DSBs) between different radiation types was used to evaluate
RBE. Package gMicroMC was used to simulate DSB yields. The DSB
inductions were then analyzed to calculate RBE. The RBE values were
compared with experimental results.
Main results: Furusawa's experiment yielded RBE values of 1.27,
2.22, 3.00 and 3.37 for carbon ion beam with dose-averaged LET of 30.3
keV/µm, 54.5 keV/µm, 88 keV/µm and 137 keV/µm, respectively. RBE
values computed from gMicroMC simulations were 1.75, 2.22, 2.87 and
2.97. When it came to a more sophisticated carbon ion beam with 6
cm spread-out Bragg peak, RBE values were 1.61, 1.63, 2.19 and 2.36
for proximal, middle, distal and distal end part, respectively. Values
simulated by gMicroMC were 1.50, 1.87, 2.19 and 2.34. The simulated
results were in reasonable agreement with the experimental data.
Significance: As a mechanistic way for the evaluation of RBE for
carbon ion radiotherapy by combining the macroscopic simulation of
energy spectrum and microscopic simulation of DNA damages, this
work provides a promising tool for RBE calculation supporting clinical
applications such as treatment planning.