Table 2.

Results of image guidance accuracy for pelvis phantom

CBCT imaging position	Phantom location on couch top	Actual physical shifts	Difference between actual and calculated shifts	Residual errors based on verification CBCT images
			Translation (mm)	Rotation (°)	Translation (mm)	Rotation (°)
2	CCV	5 mm & 0.5°	0.75	0.40	0.41	0.20
2	CCV	10 mm & 1°	0.42	0.30	0.37	0.40
2	CCV	20 mm & 2°	0.41	0.30	0.22	0.20
2	CCV−30	5 mm & 0.5°	0.22	0.10	0.22	0.10
2	CCV−30	10 mm & 1°	0.62	0.10	0.00	0.10
2	CCV−30	20 mm & 2°	0.51	0.30	0.76	0.00

CBCT, cone beam CT; CCV, centre of the calibrated volume; CCV, CCV−30, translation error, and rotation error are defined as in Table 1. The CCV +50 location was not tested, because it is unlikely to position the patient’s pelvis near the superior end of the couch top.

Scan protocol parameters were 360° acquisition, 125 kV, 42 mA, 20 ms, 10 frames per seconds, 1024 × 1024 reconstruction matrix, 0.52-mm pixel size, and 1-mm slice thickness.