No abstract

To improve accuracy of dose calculation on kilovoltage cone beam computed tomography (kV CBCT) images, a custom-made phantom was fabricated to acquire an accurate CT number to electron density curve by full scatter of cone beam x-ray. To evaluate the dosimetric accuracy, 9 volumetric modulated arc therapy (VMAT) plans for head and neck (HN) cancer and 9 VMAT plans for lung cancer were generated with an anthropomorphic phantom. Both CT and CBCT images of the anthropomorphic phantom were acquired and dose-volumetric parameters on the CT images with CT density curve (CT CT ), CBCT images with CT density curve (CBCT CT ) and CBCT images with CBCT density curve (CBCT CBCT ) were calculated for each VMAT plan. The differences between CT CT vs. CBCT CT were similar to those between CT CT vs. CBCT CBCT for HN VMAT plans. However, the differences between CT CT vs. CBCT CT were larger than those between CT CT vs. CBCT CBCT for lung VMAT plans. Especially, the differences in D 98% and D 95% of lung target volume were statistically significant (4.7% vs. 0.8% with p = 0.033 for D 98% and 4.8% vs. 0.5% with p = 0.030 for D 95% ). In order to calculate dose distributions accurately on the CBCT images, CBCT density curve generated with full scatter condition should be used especially for dose calculations in the region of large inhomogeneity.