Improving the accuracy of converting dose to medium to dose to water algorithms in small megavoltage photon fields in
Xiaoyu Yang1, Ying Cao1, Qigang Shao1
1Oncology Department, Xiangya Hospital Central South University, Changsha, People's Republic of China.
Purpose:
To present a formalism to improve the accuracy of converting absorbed dose to medium in medium (Dm) to absorbed dose to water in medium (Dw) in small megavoltage photon fields for different human tissues in Dm-based treatment planning systems (TPS).
Methods:
Eight kinds of real human tissues were simulated to convert Dm to Dw. Four kinds of virtual water media were deliberately designed to analyze source of deviations from the conventional Bragg-Gray theory. Mass electronic stopping powers were calculated using the ESTAR code. The phase-space data was generated by the EGSnrc/BEAMnrc Monte Carlo code. The dose deposition was calculated with the EGSnrc/DOSRZnrc code. Electron fluence spectra calculated with EGSnrc/FLURZnrc code were utilized to analyze fluence perturbations and determine fluence intensity (Φw,mint) and fluence spectral shape (Φw,mS) correction factors.
Results:
Large conversion errors of Dw using Bragg-Gray theory were observed, such as 19.65% ± 9.58% (average value ± standard deviation, type A) for inflated lung (ICRU). Fluence perturbations could be exacerbated by severe charged particle disequilibrium conditions. These deviations were caused by the synergy between tissues' different mean excitation energies and smaller mass densities compared to those of water. Adding Φw,mint and Φw,mS correction factors to modify Bragg-Gray theory could greatly reduce Dw conversion errors, within 1.00% for all tissues studied.
Conclusions:
The current clinically used Dw conversion algorithm in commercial Dm-based TPS isn't appropriate for some human tissues in small field dosimetry. Correction factors should be exploited to improve the accuracy.
