Development of MR-compatible head immobilization device and initial experience of MR-guided radiotherapy for central nervous system tumors

Purpose We aimed to develop and investigate positional reproducibility using a fixation device (Unity Brain Tumor Immobilization Device, UBID) in brain tumor patients undergoing magnetic resonance (MR)-guided radiotherapy (RT) with a 1.5 Tesla (T) MR-linear accelerator (LINAC) to evaluate its feasibility in clinical practice and report representative cases of central nervous system (CNS) tumor patients. Materials and Methods Quantitative analysis was performed by comparing images obtained by placing only the MR phantom on the couch with those obtained by placing UBID next to the MR phantom. Twenty patients who underwent RT for CNS tumors using 1.5T MR-LINAC between June 2022 and October 2022 were retrospectively analyzed. Among them, five did not use UBID, whereas 15 used UBID. The positional reproducibility of UBID was evaluated using the median interfractional and intrafractional errors in the first 10 fractions. Results Each MR quality factor of the MR phantom with UBID satisfied the criteria presented by Elekta. Median values of median shifts in the mediolateral, anteroposterior, and craniocaudal axes for interfractional errors were 2.98-mm, 2.35-mm, and 1.40-mm, respectively. For intrafractional errors, the median values were 0.05-mm, 0.03-mm, and 0.06-mm, respectively. The median values of the median rotations in pitch, roll, and yaw for both interfractional and intrafractional rotations were 0.00°. One patient diagnosed with an optic nerve sheath meningioma received RT with motion monitoring during irradiation. In two patients, changes in the tumor cavity and residual lesions were observed in the MR images obtained using 1.5T MR-LINAC on the day of the first treatment and immediately before the 21st fraction, respectively; therefore, offline/online adaptation was performed. Conclusion The reproducible and immobile UBID is clinically feasible in patients with CNS tumors receiving RT with 1.5T MR-LINAC. Based on our initial experience, we developed a workflow for 1.5T MR-LINAC treatment of CNS tumors.