TH‐C‐141‐12: Evaluation of Potential Internal Target Volume for Liver Tumor with Consideration of Respiratory Variation Using Cine MRI

Abstract

Purpose: Although four‐dimensional CT (4DCT) is valuable for evaluating the respiratory motion and internal target volume (ITV) for liver stereotactic body radiotherapy (SBRT), 4DCT cannot consider the respiratory variation. Here, we developed a novel technique to evaluate the potential ITV with consideration of respiratory variation using cine‐magnetic resonance imaging (cine‐MRI). Methods: We retrospectively evaluated 6 patients who received liver SBRT in 2012. The planning CT (pCT) was acquired with free breathing, and then 4D‐CT and cine‐MRI images were also acquired with the same set‐up condition. The respiratory signal was derived from the peak of the diaphragm on cine‐MRI frames. The motion vectors of 500 feature points in consecutive two frames were calculated using pyramidal implementation of the Lucas‐Kanade optical flow algorithm. The contour of tumor, blood vessel near tumor, or liver was delineated on one frame and the corner points of the contour were shifted onto the other frames using the motion vectors and then the tumor trajectory was calculated. On pCT images, gross tumor volumes delineated on the inspiration and expiration phases of 4D‐CT were three‐dimensionally translated using the tumor trajectory calculated with sagittal and coronal cine‐MRI. The distribution of the existence probability of the tumor was evaluated. Results: The concordance between ITV determined with 4D‐CT and potential ITV were evaluated with Dice similarity coefficient (DSC). When the voxels with the existence probability higher than cut‐off values were considered as potential ITV, the maximum DSC for each patient was 0.88 (range 0.84% to 0.93) and the cut‐off for maximum DSC was 24.5% (range 17% to 32%). Conclusion: We developed a novel technique to evaluate the potential ITV of liver tumor with consideration of the respiratory variation. Potential ITV may be approximately 20% larger than ITV determined with 4DCT. The volume will be larger for patients with large variation in respiration.