SU-E-J-28: Assessment of Rotational Misalignment in Cervical Spine Treatments and Potential for 6DOF Couch Corrections




To quantify the magnitude of angular misalignments that may arise during spinal radiosurgery for metastases to cervical vertebrae and assess the ability of six-degree of freedom (6DOF) alignment systems to apply appropriate corrections.


A retrospective analysis of the distributions of rotational misalignments of cervical vertebrae based on planning CT and CBCT acquired for verification of patient positioning was performed. Fifteen patients being treated for cancers of the head and neck were immobilized using Type-S head-only thermoplastic masks mounted to an overlay board. Rigid registration was performed using commercially available software for individual vertebrae of each patient. Distributions of rotations were tabulated across all patients and vertebrae, modeled with a normal distribution, and 95% confidence intervals were estimated. Dosimetric consequences of maximal and 6DOF correctable misalignments are simulated.


Across all patients and vertebrae, 2/105 pitch rotations were >3° (1.9%), including 2/15 (13%) patients. Upper and lower 95% confidence intervals for pitch, yaw, and roll misalignments were [−2.3°, 3.3°], [−1.8°, 1.4°], and [−2.6°, 2.2°], respectively. Rotations for each vertebra were also considered. The greatest need for rotation appears to be for pitch correction of vertebra C4, with misalignment of 0.2° ± 1.9° observed, which predicts that a rotation >3° would be necessary 11.5% of the time that this vertebra is of interest.


Empirical and model data presented here suggest that approximately 2–4% of vertebrae would require a rotation >3° in the sagittal plane, with some vertebrae more prone to rotational misalignments than others. Uncorrected rotational misalignment has the potential to alter dose distributions with respect to both treatment volumes and organs at risk and rotations >3° may require physical repositioning of the patient. Implementation of six-degree of freedom couch systems may facilitate clinical flow after rotational misalignment is detected, and allow expeditious, automated setup corrections.

Varian and Dartmouth-Hitchcock Medical Center are parties to a customer acceptance evaluation agreement regarding the Varian 6 degree of freedom couch.