Volumetric modulated arc therapy planning based on virtual monochromatic images: Effect of inaccurate CT numbers on dose distributions

Abstract Purpose Though virtual monochromatic images (VMIs) at low energy levels can improve image quality, the measured Hounsfield unit (HU) values can be inaccurate. We assessed the dosimetric error due to inaccurate HU estimation in volumetric modulated arc therapy (VMAT) planning. Methods Based on the VMIs at 50 keV (VMI 50keV ), 77 keV (VMI 77keV ) and single-energy CT (SECT) image for a phantom with different sizes, lookup tables (LUT L and LUT S ) were created. Using an anthropomorphic phantom (head and spine regions), VMAT plans were generated based on VMI 50keV , VMI 77keV and SECT using the corresponding LUT L , and then, the doses were re-calculated using LUT S . For clinical cases, 30 VMAT plans (prostate, brain, and spine cases) were generated based on VMI 50keV and VMI 77keV . Results In the anthropomorphic phantom study, the difference in the dosimetric parameters for planning target volume (PTV) in the VMAT plan based on the VMI 77keV was smallest (within 0.1 Gy) among three types of treatment planning approach. In clinical cases, in general, the differences of the 3-dimensional gamma passing rate and dosimetric parameters in the treatment plans based on the VMI 50keV were larger than those in the VMI 77keV . Especially for brain cases, the difference for PTV was more prominent when AXB was used (the maximum difference was 0.5 Gy) than AAA. Conclusions The dosimetric error due to the inaccurate HU estimation was larger in the VMIs at low energy levels. This may be clinically insignificant, but should be avoided in the VMAT treatment planning.