Real‐time motion monitoring using orthogonal cine MRI during MR‐guided adaptive radiation therapy for abdominal tumors on 1.5T MR‐Linac

Background Real‐time motion monitoring (RTMM) is necessary for accurate motion management of intrafraction motions during radiation therapy (RT). Purpose Building upon a previous study, this work develops and tests an improved RTMM technique based on real‐time orthogonal cine magnetic resonance imaging (MRI) acquired during magnetic resonance‐guided adaptive RT (MRgART) for abdominal tumors on MR‐Linac. Methods A motion monitoring research package (MMRP) was developed and tested for RTMM based on template rigid registration between beam‐on real‐time orthogonal cine MRI and pre‐beam daily reference 3D‐MRI (baseline). The MRI data acquired under free‐breathing during the routine MRgART on a 1.5T MR‐Linac for 18 patients with abdominal malignancies of 8 liver, 4 adrenal glands (renal fossa), and 6 pancreas cases were used to evaluate the MMRP package. For each patient, a 3D mid‐position image derived from an in‐house daily 4D‐MRI was used to define a target mask or a surrogate sub‐region encompassing the target. Additionally, an exploratory case reviewed for an MRI dataset of a healthy volunteer acquired under both free‐breathing and deep inspiration breath‐hold (DIBH) was used to test how effectively the RTMM using the MMRP can address through‐plane motion (TPM). For all cases, the 2D T2/T1‐weighted cine MRIs were captured with a temporal resolution of 200 ms interleaved between coronal and sagittal orientations. Manually delineated contours on the cine frames were used as the ground‐truth motion. Common visible vessels and segments of target boundaries in proximity to the target were used as anatomical landmarks for reproducible delineations on both the 3D and the cine MRI images. Standard deviation of the error (SDE) between the ground‐truth and the measured target motion from the MMRP package were analyzed to evaluate the RTMM accuracy. The maximum target motion (MTM) was measured on the 4D‐MRI for all cases during free‐breathing. Results The mean (range) centroid motions for the 13 abdominal tumor cases were 7.69 (4.71–11.15), 1.73 (0.81–3.05), and 2.71 (1.45–3.93) mm with an overall accuracy of