Keywords: proton beam therapy, patient positioning, neck profile. Introduction and Background: For the treatment of paediatric patients undergoing proton beam therapy (PBT) under general anaesthetic (GA), two different headrests were evaluated with a view to enhancing the potential of a more prominent neck flexion in order to obtain a more stable airway. Method and Materials: Imaging data from 10 patients undergoing GA assisted PBT was used to assess consistency of treatment positioning using the standard (n=5) and neck profile (n=5) headrests. Cone-beam computed tomography (CBCT) data was collected throughout treatment; CBCTs were taken for fractions 1-5 then weekly unless required more frequently. Pearson correlations and student paired t-tests were used to assess translational and rotational set up errors in each of the vertical, longitudinal and lateral directions, as well as the pitch, yaw and roll. Notes on patients’ daily setups were also recorded to ensure we considered any non-data related positioning issues (e.g., complex patient setups). Results: On average 12 CBCTs were available to analyse for each patient (9-17) on the standard headrest, and 18 (9-33) on the neck profile headrest. 100% of patients on standard headrests required a re-setup at some point throughout treatment as opposed to 60% of neck profile headrest patients requiring a re-setup at some point. Pearson test for CBCT translations showed R-values 0.21, 0.54 and 0.45 in the right-left, superior-inferior and anterior-posterior directions respectively. Rotationally R-values were 0.18, 0.28 and 0.40 for pitch, roll and yaw. However, paired t-tests indicated no statistically significant differences between the two types of headrest resulting in p-values 0.40, 0.61 and 0.78 in the right-left, superior-inferior and anterior-posterior directions and 0.25, 0.17 and 0.29 for pitch, roll and yaw. Conclusion and Discussion: Overall, no statistically significant result was found between the two datasets, however practise was changed as a result of this project. The prominent reasons for re-setup in standard headrest patients were airway complications or cervical spine deformation impacting on target volume coverage. As a result of these findings, the neck profile headrests are being used for all GA patients. Further analysis is ongoing to validate these results. Numerical