Cinematic MRI imaging for hepatic malignancies: Implications for improving accuracy of radiation therapy.

Abstract

300 Background: Accurate radiation therapy (RT) targeting of hepatic tumors is critically dependent on organ and tumor motion assessment, most commonly performed with 4DCT. Since 4DCT captures only a snapshot of tumor motion in time and is prone to artifacts and irregular breathing, it is unclear how reliable 4DCT is in determining intra-fraction tumor motion. CineMRI allows for prolonged real-time organ and tumor motion assessment with no radiation exposure. We compared the extent of tumor motion between 4DCT and cineMRI in patients with hepatic tumors treated with RT. Methods: Sixteen patients with hepatocellular carcinoma (n=11), cholangiocarcinoma (n=3), and liver metastasis (n=2) underwent 4DCT and 2D biplanar cineMRI scans during simulation to determine the extent of target motion in the superior/inferior (S/I), anterior/posterior (A/P) and lateral directions. Biplanar cineMRI was performed over 5 minutes. Tumor motion from MRI was determined by tracking the centroid position of the gross tumor volume (GTV) using deformable image registration. Margins were determined by using histograms of 90% confidence interval of GTV motion in each direction. Motion estimates from 4DCT were performed by evaluation of the marker positions in each phase of the CT. We compared the extent of GTV motion and resulting margin size between cineMRI and 4DCT. Results: 4DCT underestimated the S/I margin in 50% of patients by a mean of 3.4 mm (range: 1.5-7 mm), the A/P margin in 44% of patients by a mean of 2.8 mm (range: 1-5.5 mm), and the lateral margin in 63% of patients by a mean of 1.6 mm (range: 0.2-4.5 mm). The average inferior motion (median to 10th percentile) was of greater magnitude (5.0 mm vs. 3.5 mm) compared to the average superior motion (median to 90th percentile). Superior, inferior, lateral, anterior, and posterior margins of 6, 8, 5, 6, and 4 mm respectively would be needed to cover 90% of the intra-fraction GTV motion as defined by cineMRI. Conclusions: CineMRI frequently detects larger differences in hepatic intra-fraction tumor motion when compared to 4DCT, most notably in the S/I direction, and may be useful when treating without respiratory management, particularly in patients with unreliable 4DCT imaging.