Room: Room 202
Purpose: The internal tumor volume (ITV) is normally created based on planning 4DCT to account for intrafraction motion. However, the intrafractional motion may vary between radiation treatment (RT) fractions and such variation has not been well understood due to the lack of robust imaging during RT delivery. The purpose of this study is to assess this variation based on longitudinal MRIs acquired on an MR-Linac.
Methods: The 2D cine and 3D T2 MRIs were acquired on an MR-Linac equipped with a 1.5T MRI scanner for patients with abdominal tumors on a weekly basis during the course of RT. The weekly cine images were acquired for a duration of 133 seconds on three orthogonal planes at a frequency of 5 Hz. Liver was delineated on weekly T2 images and was used to investigate the motion. An algorithm that rigidly registers streaming 2D images to a 3D reference volume within a region surrounding the 3D target volume was used to track the liver motion. The tracking algorithm was tested on both phantom and patient images. The weekly motion data were compared with those from the 4DCT during simulation.
Results: Liver respiratory motions assessed from MRIs varied between different fractions and were different from the baseline motions at RT simulation. Compared to the center of mass motion measured with 4DCT, the varied motion amplitude can cause the target to be outside the ITV by more than 5 mm and 10 mm in about 15% and 5% respiratory cycles, respectively.
Conclusion: Intrafractional respiration organ motion can vary between treatment fractions as assessed from weekly MRIs. This interfractional variation needs to be accounted for by either adding a margin to the ITV, or preferably by real-time motion tracking during RT delivery.
Funding Support, Disclosures, and Conflict of Interest: This work was partially supported by Elekta