The effect of prosthesis implant materials on doses in external beam treatment comparison with different dose calculation algorithms and dosimetry in phantom model

Purpose: This comparative study was to investigate the effect of high-Z pelvic prostheses on 6 MV-X external beam radiotherapy doses. Study was based on comparison of measurements of the radiation doses and calculation of these doses in two different dose calculation algorithms. Material and Methods: Paraffin materials and PMMA slabs have been used to form phantoms where we have implemented dose calculations and measurements. Prosthesis sample materials are Ti6Al4V and CoCrMo. These samples have been fixed in paraffin materials separately. In Nucletron Oncentra MasterPlan treatment planning system (TPS) and GAMOS code, phantom simulations have been carried out. Collapsed Cone (CC) algorithm have been used in TPS and Monte Carlo (MC) algorithm have been used in GAMOS code. Dose calculation results have been achieved in phantom as percentage depth doses along the beam central axis and dose profiles at 2.9 and 10 cm depths of the phantom. Afterwards doses have been measured with ion chamber and EDR 2 film. For percentage depth doses, ion chamber and for dose profiles EDR 2 film have been used. Results: Measurements and calculations of dose profiles shows that, EDR 2 film and Monte Carlo results are well matched. In comparison with EDR 2 films, Monte Carlo results have relative difference of minimum 0.11% and maximum 1.15%. However, Collapsed Cone results have deviations in the prostheses sample zone from EDR 2 and Monte Carlo results. Relative differences of Collapsed Cone results are minimum 7.87% and maximum 21.6%. Ion chamber measurements showed consistance only with Monte Carlo results below placement of prostheses sample. Above prostheses, ion chamber adaptor caused difficulty in fine measurements. Below 3.5 cm of prostheses sample, relative differences of Monte Carlo in comparison with ion chamber by minimum and maximum, are 0.24% and 0.86% respectively. In Collapsed Cone calculations, relative differences are minimum 10.4% and maximum 16.2%. Conclusion: Pelvic prostheses caused significant effects on the radiation doses. Above the prostheses samples, doses have been increased by aprroximately 10% and below the prostheses samples, decreasement of doses approaching to 45% have been observed. Our results show an agreement within 2% between Monte Carlo calculations and experiments, yet Collapsed Cone calculations didn't achieve the acceptable consistency. Keywords: High-Z prostheses, Collapsed Cone algorithm, GAMOS Monte Carlo code, EDR 2 film.