SU‐EE‐A3‐01: 4D Locally Adaptive Radiation Delivery Using Integrated In‐Room MRI and DMLC Tracking

Purpose: The ideal IGRT‐based delivery to manage intrafraction motion has two requirements: complete spatio‐temporal knowledge of the anatomy, and real‐time beam adaptation corresponding to motion‐induced anatomical changes. Toward this goal, we investigate an integrated strategy combining two powerful techniques — online image‐guidance using fast cine‐MR imaging (4D‐MRI) and real‐time, locally adaptive delivery using dynamic multileaf collimator (DMLC)‐based tracking. Method and Materials: Image SNR and, therefore, field strength (B_o) requirements for an integrated MRI+Linac for the task of radiotherapy guidance were investigated. Using multisection, multiphase steady‐state free precession (SSFP) and spoiled gradient recall (SPGR) sequences, 3D volumes (1.4s/volume) and 2D coronal slices (0.5s/slice) of a volunteer's thoracic region were acquired with a 1.5T MRI. For each set, a region of interest encompassing the diaphragm was segmented and trajectories of superior‐inferior and left‐right motion were computed for voxels within. In order to simulate real‐time imaging with lower field strength MRI+Linacs, the SNR for each set was progressively degraded and corresponding motion trajectories recalculated. 4D locally‐adaptive IMRT was implemented using a DMLC tracking algorithm which adapts the beam aperture(s) in real‐time using 3D position information from an independent monitoring system. The aforementioned trajectories were programmed into a high‐resolution 3D‐programmable motion platform and geometric accuracy of DMLC tracking was measured. Results: The SSFP‐acquired 2D and 3D images yielded adequate SNR for registration, while the SPGR sequence yielded faster acquisition but poorer SNR. For SSFP images, even with a factor‐of‐six SNR reduction (corresponding to B_o ∼0.2T), no significant changes were observed in estimated motion trajectories. Finally, sub‐millimeter tracking accuracy was observed for these traces for simultaneous target motion in the S‐I and left‐right directions. Conclusion: These initial studies provide valuable insights into design requirements of integrated MRI+Linac systems and indicate that 4D‐MRI combined with DMLC tracking represents a highly promising approach for intrafraction motion management.