Room: Karl Dean Ballroom B1
Purpose: We demonstrate here a novel approach that simultaneously enables real-time motion monitoring and truth-in-delivery analysis for integrated MR-guided radiation therapy (MR-gRT) systems.
Methods: Recently, we introduced an MRI sequence capable of acquiring real-time cine imaging in two orthogonal planes (SOPI). We extend SOPI here to permit dynamic updating of slice positions in one plane while keeping the other plane position fixed. In this implementation, cine images from the static plane are used for motion monitoring and as image navigators to sort stepped images in the other plane, producing dynamic 4D image volumes for use in dose accumulation. 4D-SOPI was acquired on a Siemens 3T in a healthy volunteer and in a dynamic motion phantom driven by an actual patient respiratory waveform displaying irregular breathing pattern and drifting. A custom 3D-printed target, designed to mimic the pancreas and duodenum and filled with radiochromic FXG gel, was interfaced to the dynamic motion phantom. Unique 4D-MRI epochs were reconstructed from a time series of phantom motion. Dose from a static 4cmx15cm field was calculated on each 4D respiratory phase bin and epoch image in a research Monaco TPS (incorporating a transverse 1.5T field), scaled by the time spent in each bin, and then rigidly accumulated in MIM. The phantom was then positioned on an Elekta MR-linac and treated while moving. Following irradiation, actual dose deposited to FXG was determined by applying an R1 versus dose calibration curve to R1 maps of the phantom.
Results: The 4D-SOPI cine images produced a respiratory motion navigator that was highly correlated with the actual phantom motion (CC=0.9981). The mean difference between the accumulated and measured dose inside the target was 4.4% of the maximum prescribed dose.
Conclusion: A novel, simultaneous cine and 4D imaging framework was developed for combined motion monitoring and truth-in-delivery analysis in MR-gRT.