Using Megavoltage Computed Tomography to Estimate Radiotherapy Dose for High-Density Metallic Implants

Streak artifacts in kilovoltage computed tomography (kVCT), originated from metallic implants, are reduced by megavoltage CT (MVCT). This study aimed to establish the image value-to-density table (IVDT) of MVCT and validate dose accuracy. The IVDT of MVCT was established with different physical densities (0.290-19.427 g/cm
³
) and compared with kVCT for doses. The Acuros external beam (AXB) algorithm was compared with the analytical anisotropic algorithm (AAA) for heterogeneity correction in MVCT. Based on the experimental results, the IVDT of MVCT showed good linearity. Dose overestimations for the metallic hip prosthesis in a water phantom were reduced from 2.6%/2.8% in kVCT_AAA, 1.8%/-0.7% in MVCT_AAA and to -0.7%/-0.3% in MVCT_AXB with 6-MV/10-MV photons, respectively. The predictive formulas were validated and highly correlated with the measured doses (R
²
= 0.9933$ /0.9862 for 6-/10-MV photons, respectively). On average, the dose differences with a metallic eye shield on the Alderson radiation therapy phantom using 6-MV photon were reduced from 3.6% in kVCT_AAA to -1% in MVCT_AAA. In patient studies, the reductions of dose overestimations were from 28.6% and 7.4% to -3.9% with dental fillings, from 13.6% and -9% to -9.1% with the unilateral hip prosthesis, and from -25.3% and -5.9% to -4.3% with bilateral hip prostheses in kVCT_AAA, MVCT_AAA, and MVCT_AXB, respectively. In conclusion, MVCT with the AXB algorithm reduces metal artifacts on kVCT and plans accurate doses.