G. Qu1, and J. Zhu2,3; 1Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Jinan, P. R. China, Jinan, Shandong, China, 2Shandong Provincial Key Medical and Health Laboratory of Pediatric Cancer Precision Radiotherapy (Shandong Cancer Hospital), Jinan, Shandong, China, 3Shandong Cancer Hospital and Institute, Shandong First Medical University and Shandong Academy of Medical Sciences, Shandong Provincial Key Medical and Health Laboratory of Pediatric Cancer Precision Radiotherapy (Shandong Cancer Hospital), Jinan, China
Purpose/Objective(s): To study the differences and correlations between true non-contrast (TNC) and virtual non-contrast(VNC) which is reconstructed from dual-energy CT (DECT), and to investigate whether cranial VNC can replace TNC in proton radiotherapy dose calculation. Materials/
Methods: TNC and VNC images of 40 patients were enrolled, including 20 brain, 10 chest and 10 abdomen, to investigate the differences between VNC and TNC. Firstly, home-made software was used to implement layer-by-layer subtraction of the two sets of images according to the Hounsfield Unit (HU) values, which was used to comprehensively observe the differences in HU values between VNC and TNC in 40 patients. The correlation of HU values at sites with significant differences were studied and the correction model for predicting TNC HU values from VNC HU values were established. Proton radiotherapy plans for 20 cranial tumors patients were designed to studied the effect of VNC instead of TNC in proton dose calculations. Results: The differences between VNC and TNC were mainly concentrated in the areas containing bones, especially on skull and vertebrae. The mean HU values difference of the skull and vertebrae were 409.07±53.38 HU, 118.66±20.90HU, respectively. There was a significant association between the HU values of the VNC and TNC in skull and vertebrae in 40 patients (cranial: r=0.982, vertebral: r=0.991). The model established in this regard for predicting site-specific TNC HU values based on the VNC HU values was as follow: TNC=1.859×VNC+33.896 for cranial and TNC=1.827×VNC+5.491 for vertebral. There was a significant difference in the proton dose distribution on VNC and TNC images. The CTV, the 0.4cm region in front of the CTV and the 0.4cm region behind the CTV mean doses differed by 0.236%?1.803% and 3.561% between the two plans, respectively. Conclusion: The differences between VNC and TNC are mainly concentrated in the areas containing bones, the skull and vertebrae are the most prominent. There is a strong connection between the HU values of the VNC and TNC in skull and vertebrae, and the TNC HU values can be predicted based on VNC HU values in these areas. For cranial patients, VNC is not appropriate to replace TNC for proton radiotherapy dose calculation.
