Volumetric modulated arc therapy treatment planning based on virtual monochromatic images for head and neck cancer: effect of the contrast‐enhanced agent on dose distribution

摘要

Virtual monochromatic images (VMIs) at a lower energy level can improve image quality but the computed tomography (CT) number of iodine contained in the contrast‐enhanced agent is dramatically increased. We assessed the effect of the use of contrast‐enhanced agent on the dose distributions in volumetric modulated arc therapy (VMAT) planning for head and neck cancer (HNC). Based on the VMIs at 40?keV (VMIsub40keV/sub), 60?keV(VMIsub60keV/sub), and 77?keV (VMIsub77keV/sub) of a tissue characterization phantom, lookup tables (LUTs) were created. VMAT plans were generated for 15 HNC patients based on contrast‐enhanced‐ (CE‐) VMIs at 40‐, 60‐, and 77?keV using the corresponding LUTs, and the doses were recalculated based on the noncontrast‐enhanced‐ (nCE‐) VMIs. For all structures, the difference in CT numbers owing to the contrast‐enhanced agent was prominent as the energy level of the VMI decreased, and the mean differences in CT number between CE‐ and nCE‐VMI was the largest for the clinical target volume (CTV) (125.3, 55.9, and 33.1?HU for VMIsub40keV/sub, VMIsub60keV/sub, and VMIsub77keV,/sub respectively). The mean difference of the dosimetric parameters (Dsub99%/sub, Dsub50%/sub, Dsub1%/sub, Dsubmean/sub, and Dsub0.1cc/sub) for CTV and OARs was 40keV/sub (2.4%), VMIsub60keV/sub (1.9%), and VMIsub77keV/sub (1.5%) plans. The effect of the contrast‐enhanced agent was larger in the VMAT plans based on the VMI at a lower energy level for HNC patients. This effect is not desirable in a treatment planning procedure.