Comparison of 2D Radiographic Images and 3D Cone Beam Computed Tomography for Positioning Head-and-Neck Radiotherapy Patients

Purpose: To assess the positioning accuracy using two-dimensional kilovoltage (2DkV) imaging and three-dimensional cone beam CT (CBCT) in patients with head and neck (H&N) cancer receiving radiation therapy. To assess the benefit of patient-specific headrest. Materials and Methods: All 21 patients studied were immobilized using thermoplastic masks with either a patient-specific vacuum bag (11 of 21, IMA) or standard clear plastic (10 of 21, IMB) headrests. Each patient was imaged with a pair of orthogonal 2DkV images in treatment position using onboard imaging before the CBCT procedure. The 2DkV and CBCT images were acquired weekly during the same session. The 2DkV images were reviewed by oncologists and also analyzed by a software tool based on mutual information (MI). Results: Ninety-eight pairs of assessable 2DkV-CBCT alignment sets were obtained. Systematic and random errors were <1.6 mm for both 2DkV and CBCT alignments. When we compared shifts determined by CBCT and 2DkV for the same patient setup, statistically significant correlations were observed in all three major directions. Among all CBCT couch shifts, 4.1% ≥ 0.5 cm and 18.7% ≥ 0.3 cm, whereas among all 2DkV (MI) shifts, 1.7% ≥ 0.5 cm and 11.2% ≥ 0.3 cm. Statistically significant difference was found on anteroposterior direction between IMA and IMB with the CBCT alignment only. Conclusions: The differences between 2D and 3D alignments were mainly caused by the relative flexibility of certain H&N structures and possibly by rotation. Better immobilization of the flexible neck is required to further reduce the setup errors for H&N patients receiving radiotherapy.