Background: To assess the position accuracy of the six-degree-of-freedom (6-DoF) couch based on cone beam computed tomography (CBCT) and exploit the correlation of the six degrees errors. Methods: CT scans of an anthropomorphic phantom and patients were obtained at 3 mm slice thicknesses using a Philips scanner at the head, first supine and prostrate positions. An Eclipse Treatment Planning System was used to create a treatment plan. Different levels of known position errors were introduced to simulate patient position status for the anthropomorphic phantom. CBCT datasets for each position were acquired and registered to original CT datasets to evaluate the accuracy of the 6-DoF couch and determine the setup errors of patients. The setup errors of 200 CBCT datasets from 14 patients with pelvic tumors were analyzed. The correlations between six degrees position errors were finally extracted. Results: For the phantom study, the difference between known introduced errors and the setup errors were almost negligible. The deviation (meant one standard deviation) in registration methods were (0.01 +/- 0.02) cm, (0.04 +/- 0.075) cm, (0.02 +/- 0.004) cm, (0.01 +/- 0.04)degrees, (0.1 +/- 0.08)degrees, (0.03 +/- 0.05)degrees and (0.01 +/- 0.01) cm, (0.03 +/- 0.007) cm, (0.01 +/- 0.01) cm, (0.05 +/- 0.06)degrees, (0.08 +/- 0.08)degrees, (0.04 +/- 0.05)degrees for supine and prone position, respectively. The deviation in positions were (0.07 +/- 0.10) cm, (0.16 +/- 0.02) cm, (0.08 +/- 0.06) cm, (0.54 +/- 0.46)degrees, (0.24 +/- 0.16)degrees, (0.09 +/- 0.09)degrees and (0.06 +/- 0.09) cm, (0.19 +/- 0.09) cm, (0.09 +/- 0.07) cm, (0.49 +/- 0.49)degrees, (0.16 +/- 0.08)degrees, (0.1 +/- 0.13)degrees for bone and soft tissue registration methods, respectively. For patient data, the setup errors were (-0.07 +/- 0.22) cm, (0.14 +/- 0.35) cm, (-0.12 +/- 0.4) cm, (0.79 +/- 1.6)degrees, (0.41 +/- 0.71)degrees, (-0.03 +/- 0.8)degrees for supine position and (0.16 +/- 0.27) cm, (0.19 +/- 0.48) cm, (-0.05 +/- 0.34) cm, (1.1 +/- 1.49)degrees, (0.65 +/- 1.00)degrees, (-0.23 +/- 0.75)degrees for prone position, respectively. There is a significant moderate correlation between the longitudinal and pitch directions and between the vertical and pitch directions when the patient is in the supine position. Conclusions: The six-dimensional couch positioning verification system based on CBCT has high accuracy and can meet the requirements of precise radiotherapy for pelvic tumors. There is a certain correlation between translation direction and rotation direction.
