Purpose: A more compact proton therapy system could be achieved if the gantry could be removed. However, moving patients into different positions may be needed to replicate different beam angles for optimal treatment planning. The shift of the internal organs between positions poses a potential problem for CT scanning/simulation and treatment planning. In this study, we imaged a volunteer using FONAR Upright MRI to gain insight into the practical treatment of patients without a gantry.
Method: Head-and-neck images were taken in an upright sitting position (6degrees from vertical plane) and a recumbent position. Abdominal images were acquired in an upright posture, at 30degrees from the vertical plane, and recumbent (Figure1). For head-and-neck imaging, we used a 2-point Dixon sequence. For abdominal imaging, we used a bSSPF sequence with breath holding at both end of inspiration and expiration. Rigid ROI-based registration was subsequently applied and verified manually.
Results: Head-and-neck image registration showed that brain is aligned well between two positions, while the neck region exhibits postural deformation (Figure2(a)). If one were to treat neck region, a separate registration could be done based on C-spine. Abdominal images showed a shift up to 1.3cm between the organs and body surface when comparing sitting to reclining at 30 degrees (Figure2(b)-(c)). 
Conclusion: The magnitude of the organ shifts observed between different positions can be included in treatment planning, measured with on-board imaging and corrected with adaptive planning. Together with the development of immobilization, this study shows that practically compact gantry-less proton therapy can be achieved.

PTCOG58