4 5 Gantry-free radiation therapy systems may be simpler and more cost effective, particularly for MRI-6 guided photon or hadron therapy. This study aims to understand and quantify anatomical 7 deformations caused by horizontal rotation with scan sequences sufficiently short to facilitate 8 integration into an MRI-guided workflow. 9

Four methodologies were evaluated for quantifying kilovoltage cone‐beam computed tomography (CBCT) dose: the Cone‐Beam Dose Index (CBDI), IAEA Report 5 recommended methodology (IAEA), the AAPM Task Group 111 methodology (TG111), and the current dose metric; the Computed Tomography Dose Index (CTDI) on two commercial Varian cone‐beam CT imaging systems; the Clinac iX On‐Board Imager (OBI); and the TrueBeam X‐ray Imaging system (XI). The TG111 methodology measured the highest overall dose (21.199 ± 0.035 mGy OBI and 22.420 ± 0.002 XI for pelvis imaging) due to the full scatter of the TG111 phantom and was within 5% of CTDI measurements taken using a full scatter TG111 phantom and 30‐cm film strips. CBDI measured the second highest overall dose, within 10% of the TG111, with IAEA measuring the third highest dose. For head CBCT protocols, CBDI measured the highest dose, followed by IAEA. The CTDI method measured lowest across all scan modes highlighting its limitations for CBCT dosimetry. The XI imaging system delivered lower doses for head and thorax scan modes and similar doses to the OBI system for pelvis scan modes due to additional beam hardening filtration in the XI system. The TG111 method measured the highest dose in the center of a CBCT scan during image guidance procedures; however, CBDI provided a good approximation to TG111 with existing CTDI equipment and may be more applicable clinically.

Introduction Recent advances in image guidance and adaptive radiotherapy could enable gantry‐free radiotherapy using patient rotation. Gantry‐free radiotherapy could substantially reduce the cost of radiotherapy systems and facilities. MRI guidance complements a gantry‐free approach because of its ability to visualise soft tissue deformation during rotation. A potential barrier to gantry‐free radiotherapy is patient acceptability, especially when combined with MRI. This study investigates human experiences of horizontal rotation within an MRI scanner. Methods Ten healthy human participants and nine participants previously treated with radiotherapy were rotated within an MRI scanner. Participants' anxiety and motion sickness was assessed before being rotated in 45‐degree increments and paused, representing a multi‐field intensity‐modulated radiotherapy treatment. An MR image was acquired at each 45‐degree angle. Following imaging, anxiety and motion sickness were re‐assessed, followed by a comfort questionnaire and exit interview. The significance of the differences in anxiety and motion sickness pre‐ versus post‐imaging was assessed using Wilcoxon signed‐rank tests. Content analysis was performed on exit interview transcripts. Results Eight of ten healthy and eight of nine patient participants completed the imaging session. Mean anxiety scores before and after imaging were 7.9/100 and 11.8/100, respectively (P = 0.26), and mean motion sickness scores were 5.3/100 and 13.7/100, respectively (P = 0.02). Most participants indicated likely acceptance of rotation if MRI were to be used in a hypothetical treatment. Physical discomfort was reported to be the biggest concern. Conclusions Horizontal rotation within an MRI scanner was acceptable for most (17/19) participants.