Impact of accuracy of CT/MRI image fusion on dosimetric parameters of 3-dimensional conformal radiotherapy for hepatocellular carcinoma.

Objective:To study the accuracy of image fusion with computed tomography (CT) and magnetic resonance imaging (MRI) and its impact on dosimetric parameters of tumor target region and critical organs at risk in the three dimensional conformal radiotherapy (3D-CRT) for hepatocellular carcinoma (HCC) patients. Methods:Eight patients were enrolled in this study. The CT image was taken at peak inhalation phase, and MRIT2 was performed at the time of peak exhalation. Additional image was taken by fast scanning of MRIT2 (MRIT2F) at peak inhalation. Mutual information method was used to make CT/MRI image registration, and manual adjustment was made to optimize the accuracy of image fusion. The gross tumor volume (GTV) and liver in each patient were contoured on CT, MRIT2, and MRIT2F images, and critical organs were delineated on CT images. The accuracy of image fusion was assessed by the distance between bone markers of CT and MRI on the fused image (DCT-MRIT2, DCT-MRIT2F), and the percentage of liver overlap (P-LIVERCT-MRIT2, P-LIVERCT-MRIT2F). The composite volumes of GTV based on CT and MRIT2 or MRIT2F (V-GTVCT+MRIT2, V-GTVCT+MRIT2F) were calculated. We made the two radiotherapy plans according to GTVCT+MRIT2 and GTVCT+MRIT2F and compared the diffe-rences in the radiation dose of 95% PTV volumes (D95) and the dose to critical organs at risk between the two groups. Results:The means DCT-MRIT1 and DCT-MRIT2 were (2.3±0.9) mm and (2.0±1.0) mm, and the difference was not significant (P0.05). The means P-LIVERCT-MRIT2 and P-LIVERCT-MRIT2F were (85.4±2.3)% and (93.0±1.4)%, respectively. The difference was significant (P0.05). V-GTVCT+MRIT2 and V-GTVCT+MRIT2F were (642±561) cm3 and (547±474) cm3, respectively. There was a statistical difference between them (P0.05). There was no statistical difference in D95, maximal dose to duodenum, maximal dose to stomach and mean dose to left kidney between the two groups receiving different treatment plans(P0.05), but the statistical difference existed in maximal dose to spinal cord, mean dose to right kidney, mean dose to normal liver and volume percentage of normal liver receiving 5, 10, 15, 20, 25, 30, 35, and 40 Gy irradiation (V5, V10, V15, V20, V25, V30, V35, and V40, P0.05) . The values of CT+MRIT2 group were higher than those of CT+MRIT2F group. Conclusion:When application of image fusion with CT and MRI for HCC patients, the image of CT and MRI should be obtained at the same respiratory phase to reduce the dose to critical organs at risk.