Cone beam CT in analysis of set-up errors with thermoplastic device immobilization

Objective: To evaluate the systematic and random set-up errors in patients immobilized with thermoplastic device during radiotherapy, and to determine the proper margins extended from clinical target volume(CTV) or internal target volume (ITV) to planning target volume (PTV). Methods: From March 2007 to September 2007, 120 patients were included in this study, including 13 receiving head and neck irradiation, 67 thoracic irradiation and 40 abdominal irradiation. All patients were immobilized with thermoplastic device and received CT simulation and intensity modulated radiation therapy (IMRT). X-ray cone beam CT was regularly performed before treatment and the images were compared with the simulation CT images. The shift and rotation in right-left(R-L), superior-inferior(S-I) and anterior-posterior(A-P) directions were recorded and analyzed. The shift margin from CTV or ITV to PTV was calculated with the equation, margin = 2'mean + 0.7'standard deviation. Results: In head and neck region,the shift error's in R-L,S-I and A-P directions were (0.13 ± 0.15) cm, (0.13 ± 0.17) cm and(0.11 ± 0.14) cm, and the corresponding rotation errors were 1.05 degree ± 0.77 degree, 0.87 degree + 1.13 degree and 0.68 degree + 0.89 degree. The margins from CTV to PTV were 0.37 cm, 0.38 cm and 0.31 cm, respectively. In thoracic region, the shift errors in R-L, S-I and A- P directions were (0.20±0.27) cm, (0.34 ± 0.44) cm and(0.25 ± 0.31) cm, and the corresponding rotation errors were 1.06 degree ± 1.45 degree, 0.85 degree ± 1.23 degree and 0.78 degree ± 1.08 degree. The shift margins from ITV to PTV were 0.59 cm, 1.00 cm and 0.72 cm. In abdominal region, the shift errors in R-L, S-I and A-P directions were (0.23 ± 0.30) cm, (0.37 ± 0.45 )cm and (0.27 ± 0.34) cm, and the corresponding rotation errors were 1.22 degree ± 1.56 degree, 1.05 degree ± 1.44 degree and 0.98 degree ± 1.24 degree. The shift margins from CTV or ITV to PTV were 0.66 cm, 1.05 cm and 0.78 cm. Conclusions: Cone beam CT can be used in the precise measure of set-up errors, which can provide institution-specific margins for PTV designing in patients immobilized with thermoplastic device . (authors)