Keywords – Proton Beam Therapy, VacBag, Setup Introduction: Effective, reproducible patient positioning is paramount for radiotherapy1,2. With the introduction of a new proton beam therapy (PBT) service, evacuated cushions (vacbags) have been implemented for patients receiving treatment to the limbs, abdomen, thorax and pelvis. This work reports on an audit reviewing set up times, image review times and positional reproducibility to confirm the suitability of the vacbag as an immobilisation tool in PBT. Method and Materials: The first ten patients treated on vacbags were included in this study (n=10). Patients were initially imaged with kilovoltage (kV) 2-dimensional (2D) image pairs as a gross error check (GEC), followed by a conebeam CT (CBCT) from which positional errors were corrected and verified with a final 2DkV image before proceeding to treatment. Images were compared to the planning CT to determine translational and rotational displacements. Treatment and image review times were also reported to audit the new service. Results: Compared to the planning CT, translational displacements were satisfactory with 95% <5mm and 100% <10mm. Greater variation was seen in rotational displacements, however, on average only 3% of all exceeded the 2⁰ tolerance. Agreement between 2D and 3D imaging were within the institutions defined GEC tolerance of <1cm and <2⁰. This agreement was reached in 99% of translational and 91% of rotational displacements. Treatment delivery times in minutes (mins) were reduced from a mean of 44 mins for the first patient to <30 mins for the most recent patient. Average Image review time for the GEC 2D orthogonal images also reduced from 6 mins to 3 mins, however the trend for review time of CBCT remained an average of <10 mins (range 7 mins – 12mins). Conclusion and Discussion: Initial results show that vacbags are suitable for accurate patient positioning and minimising translational displacements. Rotational displacements appeared more random. As such it has been determined, the use of a 6⁰ of freedom bed, combined with daily imaging, is paramount for accurate set up. Further investigation is ongoing to validate these results including the dosimetric impacts of changes in patient contours on the accuracy of intended treatment delivery. Numerical References (1) Aran Kim, BSc, MRT(T)a*, Valerie Kelly, MSc, MRT(T)ab, Colleen Dickie, MSc, MRT(T) (MR)ab, Charles Catton, MD, FRCPCab and Winnie Li, MSc, MRT(T)ab. Impact of Immobilization on Interfractional Errors for Upper Extremity SoftTissue Sarcoma Radiation Therapy, Journal of Medical Imaging and Radiation Sciences, 50, 308-316 (2) Winnie Li, B.Sc., R.T.T.,Arjun Sahgal, M.D.,Matthew Foote, M.D.,Barbara-Ann Millar, M.D.,David A. Jaffray, Ph.D.,and Daniel Letourneau, Ph.D. Impact of Immobilization on Intrafraction Motion for Spine Stereotactic Body Radiotherapy Using Cone Beam Computed Tomography, Int J Radiat Oncol Biol Phys, 2011, 84, 520-525