To evaluate the impacts of slice thickness and registration frame range on the accuracy of cone-beam computed tomography (CBCT) image-guided head and neck (HN) radiotherapy, and to provide a basis for positioning correction in image-guided radiotherapy.

A planned CT scan was performed for an anthropomorphic HN phantom with slice thickness of 1 mm and 3 mm and simulated positioning errors in x, y, and z directions on the accelerator. CBCT scan and reconstruction were performed with slice thickness of 1 mm and 3 mm. Two different registration frame ranges were used (range 1: from C7 to superior orbit; range 2: from C7 to calvaria). Automatic bony registration was performed for CBCT and planned CT images with slice thickness of 1 mm and 3 mm. The registration accuracy was evaluated.

For range 1, the registration errors in x, y, and z directions with a slice thickness of 1 mm were significantly lower than those with a slice thickness of 3 mm (0.5±0.2 mm vs. -0.7±0.2 mm, P=0.00; 0.5±0.3 mm vs. 1.0±0.3 mm, P=0.00; -0.1±0.5 mm vs. 1.5±0.5 mm, P=0.00). For range 2, the registration errors in x, y, and z directions with a slice thickness of 3 mm were-0.4±0.2 mm, 0.5±0.2 mm, and 0.7±0.4 mm, respectively.

Engagement of calvaria in registration range can substantially enhance the registration accuracy in CBCT or CT images for HN. The registration error with slice thickness of 1 mm can be controlled within 1 mm.

Scanning thickness; Registration volumes; Bone-based registration; Accuracy