Feasibility Of Mr-Guided Radiotherapy Using Beam-Eye-View 2d-Cine With Tumor-Volume Projection

Purpose. Current magnetic resonance imaging (MRI) guided radiotherapy (MRgRT) applies sagittal/ coronal 2D-cine to monitor major tumor motions, however, the beam eye's view (BEV) with volumetric tumor projection would be the best measure for radiation beam conformality, independent of tumor through-plane motion. The goal is to assess the feasibility, accuracy, and performance of the BEV approach. Methods. Beam-specific BEV 2D-cine with volume-projected tumor contours were simulated to establish a 2D/3D tumor match against a tumor-motion library based on multi-breath time-resolved (TR) 4DMRI images. Two BEV-library-matching methods were developed: (1) fast screening with tumor center-of-mass (Delta COM), in-plane area ratio, and DICE similarity, and finalizing with the highest DICE score and (2) DICE screening for top-3 candidates and finalizing with rigid registration. A 4D-XCAT digital phantom and 8 lung-cancer patients were used for assessment. For each patient, 3 sets of 40 s TR-4DMRI were acquired at 2Hz and 6 representative BEV were created with the isocenter set at tumorCOMin mid-respiration. One TR-4DMRI set (40x2 = 80-images) was used to simulate BEV 2D-cine and the other two (160-images) were used to create a library. The matching result was validated against the ground truth within the test set. Using a leave-one-out strategy, the success rate, accuracy, and speed of tumor matching were assessed for volume-projected tumors over 11520 time-points (=8patients.3sets.80images.6BEVs). Results. Volume-projected tumor contour area on the 6 BEVs varies by 60%.+/-.8% and %A(V) (I) (in-plane/ volume-projected) varies by 82%.+/-.9%. The %A(V) (I) changes with tumor shape, orientation, and through-plane motion. Method-1 produces 96% matching success (Delta COM = 0.7 +/- 0.2 mm, %A (V)(V)=1.01 +/- 0.02, Dice= 0.92 +/- 0.02) with the computational time of 15 +/- 1ms/match, while method-2 produces 94% +/- 1% success (Delta COM = 0.2 +/- 0.1mm,%A(V)(V)=1.00 +/- 0.01, Dice = 0.94 +/- 0.02) with 223 +/- 13 ms/match. Conclusion. This study has demonstrated the feasibility, accuracy, and benefits of BEV 2D-cine imaging with tumor-volume projection, allowing real-time tumor motion monitoring and beam conformality checking. Further clinical evaluation is necessary before MRgRT applications.