engleski

Dosimetric evaluation of Monte Carlo based treatment planning system in antorphomorpic phantom

1. Introduction Implementation of advanced radiotherapy techniques makes high demands on the treatment planning system (TPS). 2. Purpose Dosimetric evaluation of calculation algorithm built in Monte Carlo (MC) based TPS. 3. Materials and Methods Dose distributions of different complexity were calculated using Elekta Monaco TPS. Both calculation methods (Dose to Water-D2W and Dose to Medium-D2M) for 6MV photon beam were verified using 2D detector and ionization chambers of different volumes in inhomogenous phantom. 4. Results Comparison of measured and calculated data in point of ‘tissue’ and low density medium show good agreement regardless the calculation method. In high density medium deviation was much higher. Also, deviations of measured and values calculated as D2W and D2M, respectively, were larger and of opposite signs. To overcome this we propose to define minimum volume of water cylinder to be placed within high density medium. This volume should allow that D2W and D2M calculated doses differ less than 0.5% and consequently good agreement with measurements. Furthermore, 2D methodology used for patient specific dosimetry was used to evaluate TPS accuracy, but antrophomorphic thorax phantom was used instead of the homogeneous one. Anlayze shows that gamma criterion 3% or 3mm was not fulfilled when inhogeneities are inside treated volume. 5. Conclusion MC TPS evaluation using point dose measurements show large uncertainties within high density medium. It can be overcome by using proposed approach. Also, despite the manufacturer’s claim of 2D verification superiority over point measurements, our investigation shows otherwise when measurements in inhomogeneous phantom are considered.

radiotherapy ; dosimetry ; calculation algorithm

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