Room: AAPM ePoster Library
The use of high Z material to improve local biological effectiveness has been discussed for many years. Both in vitro and in vivo test results showed that iodine can improve relative biological effectness(RBE). RBE can also be calculated using models such as the local effect model (LEM) and the microdosimetric kinetic model (MKM). In this study, a Geant4 simulation is performed to simulate the microdosimetry indices of an iodine-coated subcell target and to calculate the RBE enhancement owing to iodine using LEM and MKM.
The subcell target in this study is an 1 mm diameter sphere made from the tissue composition recommended by the ICRU with a density equal to 1 mg/cm3 to bring the equivalent diameter of this target to 1 µm. This subcell target is coated with a 100 nm thick iodine layer and a 5 mm thick shell made of tissue. X-rays with 80 kVp and 250 kVp are simulated to compare the iodine enhancement on different photon energies. Finally, the RBEs were calculated using the MKM model based on Okamoto’s process and the LEM model which convolute GEANT4 simulated secondary electron spectra with DNA damage cross-sections.
For 250 kVp X-ray, the RBE enhancement ratio of iodine is 2.3 (MKM) and 9.27 (LEM). LEM gave a superior ratio because the secondary electron spectra peaks at about 100 eV and the Geant4 DNA demonstrates that electrons have the largest DSB cross-section in the range from 100 eV to 1 keV and reach the maximum at 400 eV. For 80 kVp X-ray, the RBE enhancement is even more significant.
Iodine has strong potential to improve local RBE for kV X-rays.