The Impact of Correcting for Translational and Rotational Errors Using the HexaPOD in Head and Neck Stereotactic Body Radiation Therapy Patients

The HexaPOD has been developed to correct for errors in six degrees of freedom (6DOF), which enables it with the potential to improve setup accuracy. This is critical in hypofractionated head and neck (H&N) stereotactic body radiation therapy (SBRT) plans, which have highly conformal dose distributions and close proximity of critical normal structures.

The purpose of the study was to investigate whether using the HexaPOD to correct for error in 6DOF will increase setup accuracy for H&N SBRT patients.

For each of the 20 SBRT H&N patients in this study, two sets of global computed tomography scans and cone beam computed tomography matches were performed for the first and last fraction of each patient's treatment. One of the global matches included corrections for rotations and the other did not. Next, sub-regions of interest, based on bony landmarks, were identified for each patient and local registrations were performed to match these sub-regions of interest to determine local sub-region errors in the global registration. The differences in local error, with and without rotations, were examined to determine whether the residual local setup errors were significantly different.

T-test results demonstrate a statistically significant difference in local error when correcting for rotations vs. not, in all 6DOF. When examining the actual magnitude of residual error reduction using three-dimensional vectors of the average translational differences, an average reduction of 0.9 mm is observed when 6DOF corrections are applied.

Residual setup errors in each of the 6DOF were statistically significant; however, the magnitude depicts a very small reduction in residual error <1 mm. Clinically, this makes a very small difference because the planning target volume (PTV) presumably covers an expansion of 3-5 mm.