The impact of four dimensions CT simulation on planning target volume in radiotherapy for primary lung cancer.

Abstract

e20091 Background: 4D CT simulation has been evolved to estimate the internal body motion and considered as a useful tool for intra-thoracic tumor definition. This study aimed to evaluate the impact of using 4D simulation on the planning target volume (PTV) for primary lung tumor. Methods: Patients who underwent CT simulation for primary lung cancer radiotherapy between 2012-2016 using 3D- (free breathing) and 4D- (respiratory gated) institutional protocol were included in this retrospective review. For each patient, gross tumor volume (GTV) was contoured in free breathing scan (3D-GTV), exhale scan (e-GTV) and inhale scan (i-GTV). The corresponding CTVs (3D-CTV, e-CTV and i-CTV) were created by adding 1 cm in all directions. 3D-internal target volume (3D-ITV) was generated by 0.5 cm cranio-caudal expansion of 3D-CTV, while 4D-ITV was created by combination of e-CTV and i-CTV. Subsequently, a 0.5 cm margin was added to generate the 3D-PTV and 4D-PTV respectively. The volumes of 3D-PTV and 4D-PTV were compared to examine the impact of 4D CT simulation on changes in the volume of PTV. Univariable and multivariable analysis were performed to test the impact of volume and location of GTV on the changes of PTV volume by more than 10 % between free breathing and respiratory gated scans. Results: A total of 10 patients were identified. The median [range] GTV, i-GTV, e-GTV volumes were 13.55 [1.44-628.66], 13.17 [1.77-627.36], 12.85 [1.34-630.25] cc respectively. The 3D-CTV, i-CTV, e-CTV volumes were 86.37 [23.76-1209], 84.97 [25.5- 1220.4], 83.40 [23.36-1224.12] cc respectively. 3D-ITV and 4D-ITV median volume was 106.06 [3.99-1422.8], 88.02 [20.51-1338.18] cc respectively. 3D-PTV was significantly larger than the 4D-PTV; median [range] volumes were 182.79 [58.65- 1861.05] vs. 158.21 [52.76-1771.02] cc, p = 0.0068). On multivariable analysis, neither the volume of GTV (p = 0.4917), nor the location of the tumor (peripheral, p = 0.4914 or lower location, p = 0.9594) had an in impact on PTV differences between free breathing and respiratory gated scans. Conclusions: The use of 4D simulation reduces the PTV for primary lung cancer, and it should be routinely implemented in clinical practice regardless the tumor volume or location.