Cone beam CT in analysis of set-up errors with thermoplastic device immobilization / 中华放射肿瘤学杂志

ABSTRACT

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 errors 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°± 0.77°,0.87°± 1.13° and 0.68°±0.89°. The margins from CTV to PTV were 0.37 cm,0.38 cm and 0.31 cm,respectively. In thoraci region,the shift errors in R-L,S-I and AP 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°±1.45° ,0.85°±1.23° and 0.78°±1.08°. 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°±1.56°, 1.05°± 1.44°and 0.98°± 1.24°. 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.