Analysis of lower neck setup errors and planning target margin by CBCT for breast cancer radiation with breast bracket immobilized

Objective To evaluate the setup errors for lower neck with cone beam CT (CBCT) in breast cancer patients immobilized by breast bracket, and to probe the margins from supraclavicular clinical target volume (CTV) in 3 directions. Methods A total of 14 breast cancer patients with supraclavicular lymph node radiation were enrolled. All patients were immobilized by breast bracket, and each patient would undergo CBCT at the first, tenth and twentieth treatment after positioning, respectively. Then these CBCT images were registered to the planning CT to determine setup errors in translational and rotational direction, and evaluated correlation between them. At last, CTV margins were calculated from the systematic and random errors. Results The setup errors on x (left-ring), y (superior-inferior), z (anterior-posterior) translational directions were (2.89±2.52), (3.96±2.97), (4.21±2.24) mm and on θ (pitch degree), φ (roll degree), ψ (yaw degree) rotational direction were (2.38±1.97)°, (1.60±1.63)°, (1.91±1.54)°, respectively. The margins from CTV were 8.08, 8.13, 6.30 mm in x, y and z direction. On y and z directions translational errors were correlated significantly with φ and ψ degree rotational errors(Pearson=-0.515, -0.509, P<0.05). In inter-fraction only on z direction the setup changes were considered as correlative with ψ degree (Pearson=-0.583, P<0.05). Conclusions For supraclavicular region irradiation breast cancer patients immobilized with breast bracket, the margins from CTV were recommended as not less than 8.08, 8.13, 6.30 mm in x, y, z directions, respectively. The position immobilized method and the positioning workflow should be further improved in order to reduce the influence of the neck rotational on setup errors. Key words: Breast cancer; Setup error; Cone beam CT