ABSTRACT
【Objective】 To investigate setup and respiratory movement residual error with the guidance of online four-dimensional cone beam computed tomography (4DCBCT) and the impact on margins required to internal target volume (ITV) margin during stereotactic body radiotherapy (SBRT) of lung tumor in the middle or lower lobe. 【Methods】 Twenty SBRT treatment patients with lung tumor in the middle or lower lobe were enrolled for treatment residual error retrospective analysis. Thermoplastic masks were used as immobilization devices during SBRT treatment. ITV was determined by 4DCBCT simulation. A total of 76 4DCBCT scans before the treatment were recorded to determine the setup and respiratory residual error for ITV margins. 【Results】 The setup and respiratory movement error were significantly reduced with the guidance of online 4DCBCT during radiotherapy. The setup residual errors were respectively (0.07±0.12)cm, (0.03±0.29)cm and (0.04±0.14)cm in right-left (RL), superior-inferior (SI) directions and anterior-posterior (AP) directions. The respiratory movement residual errors were respectively (-0.06±0.07)cm, (0.02±0.26)cm and (0.02±0.11)cm in RL, AP, and SI directions. Based on setup and respiratory movement residual errors, the PTV margins of NSCLC were reduced from 1.13 cm, 2.15 cm and 0.90 cm to 0.50 cm, 0.59 cm and 0.56 cm in RL, AP and SI directions, respectively, calculated with recipe by VanHerk. 【Conclusion】 With the guidance of online 4DCBCT, the setup and respiratory movement residual error cannot be ignored during SBRT of lung tumor in the middle or lower lobe. The ITV margin required after online 4DCBCT correction for the patients enrolled in the study would be approximatively 0.6 cm around to ensure an accurate dose to the target tumor and reduce the dose to normal tissues.