Variables altering the impact of respiratory gated CT simulation on planning target volume in radiotherapy for lung cancer.

Background: Respiratory gated CT simulation (4D-simulation) has been evolved to estimate the internal body motion. This study aimed to evaluate the impact of tumor volume and location on the planning target volume (PTV) for primary lung tumor when 4D simulation is used.

Methods: Patients who underwent CT simulation for primary lung cancer radiotherapy between 2012 and 2016 using a 3D- (free breathing) and 4D- (respiratory gated) technique were reviewed. For each patient, gross tumor volume (GTV) was contoured in a free breathing scan (3D-GTV), and 4D-simulation scans (4D-GTV). Margins were added to account for the clinical target volume (CTV) and internal target motion (ITV) in 3D and 4D simulation scans. Additional margins were added to account for planned target volume (PTV). Univariate and multivariate analyses were performed to test the impact of the volume of the GTV and location of the tumor (relative to the bronchial tree and lung lobes) on PTV changes by more than 10% between the 3D and 4D scans.

Results: A total of 10 patients were identified. 3D-PTV was significantly larger than the 4D-PTV; median volumes were 182.79 vs. 158.21 cc,  = 0.0068). On multivariate analysis, neither the volume of the GTV ( = 0.5027) nor the location of the tumor (peripheral,  = 0.5027 or lower location,  = 0.5802) had an impact on PTV differences between 3D-simulation and 4D-simluation.

Conclusion: The use of 4D-simulation reduces the PTV for the primary tumor in lung cancer cases. Further studies with larger samples are required to confirm the benefit of 4D-simulation in decreasing PTV in lung cancer.