Program Information
Sensitivity of Electron Dose Calculations to CT-To-Density Curve Variations
P Alaei1*, (1) University of Minnesota, Minneapolis, MN
Presentations
TU-RPM-GePD-T-5 (Tuesday, August 1, 2017) 3:45 PM - 4:15 PM Room: Therapy ePoster Lounge
Purpose: To assess the sensitivity of electron dose calculations in a treatment planning system to small variations in CT-to-density curve.
Methods: The effect of changes in CT-to-density curves on electron dose calculations in Pinnacle treatment planning system (TPS) has been assessed. The CT-to-density curve is generally created at the time of TPS commissioning for each CT scanner and its accuracy is verified annually. In general, there are small variations in the curves which could be due to utilization of different imaging protocols or other factors. These small changes are generally believed to have little or no impact on dose calculations. In this study, the small variations in the curve and their effect on dose calculations were evaluated and compared to lager errors, i.e. using electron density instead of physical density.Two slab phantoms with lung and bone inhomogeneities were constructed and the dose to them was calculated using single 6, 9, 12, 16, and 20 MeV electron beams and 6 MV photon beam. The dose was calculated using the curve before and after its annual re-measurement/re-creation, as well as two artificially altered curves and one generated by using electron density values.
Results: The variation of the curve does not affect the photon beam dose calculations but affects that of electron beams to varying degrees. These subtle effects may not be clearly visible but there are dose calculation variations of up to 9% at the volume between 60 and 80 isodose surfaces and up to 15% in lower dose volumes between 30 and 40% isodose surfaces for low energy electrons. Much larger variations are observed when electron density is used instead of mass density.
Conclusion: Small CT-to-density curve variations have almost no effect on photon dose calculations but could have a substantial one in electron dose calculations, depending on energy.
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