  Proton therapy systems without a gantry can be more compact and less
expensive in terms of capital cost, and therefore more available to a larger
patient population. Would the advances in pencil beam scanning and robotics
make gantry-less treatment possible? In this study, we explore if high-quality
treatment plans can be obtained without a gantry.
  We recently showed that proton treatments with the patient in an upright
position may be feasible with a new soft robotic immobilization device and
imaging which enables multiple possible patient orientations during a
treatment. In this study, we evaluate if this new treatment geometry could
enable high quality treatment plans without a gantry. We created pencil beam
scanning (PBS) treatment plans for seven patients with head-and-neck or brain
tumors. Each patient was planned with two scenarios: one with a gantry with the
patient in supine position and the other with a gantry-less fixed horizontal
beam-line with the patient sitting upright. For the treatment plans,
dose-volume-histograms (DVHs), target homogeneity index (HI), mean dose, D_2
and D_98 are reported. A robustness analysis of one plan was performed with
+/-2.5 mm setup errors and +/-3.5% range uncertainties with nine scenarios.
  Most of the PBS-gantry-less plans had similar target HI and OAR mean dose as
compared to PBS-gantry plans, and similar robustness with respect to range
uncertainties and setup errors.
  Pencil beam scanning provides sufficient power to deliver high quality
treatment plans without requiring a gantry for head-and-neck or brain tumors.
In combination with the development of the new positioning and immobilization
methods required to support this treatment geometry, this work suggests the
feasibility of further development of a compact proton therapy system with a
fixed horizontal beam-line to treat patients in sitting and reclined positions.

Medical Physics