Importance of four-dimensional computed tomography simulation in locally advanced lung cancer radiotherapy: Impact on reducing planning target volume

Abstract

Background/Aim. Four dimensional (4DCT) simulation is a useful tool for motion assessment in lung cancer radiotherapy. Conventional Three dimensional (3D) - Free Breathing simulation is static, with limited motion information. The aim of this study was to compare clinically significant differences between the target volumes defined on 3D CT vs. 4D CT simulation and potential impact on the planning target volume (PTV). In addition, to quantify movements of primary tumour (GTV) during 4D CT simulation on three axis -Z-supero inferior (SI), X-mediolateral (ML), and Y-anteroposterior (AP). Methods. This retrospective study evaluated 20 lung cancer patients who underwent CT simulation for radical radiotherapy treatment. Free Breathing 3D CT and 4D CT simulation were acquired for each patient in accordance with our institutional protocol. Volumetric comparison radiation volumes defined on 3D CT vs. 4D CT simulation was done-Gross tumour volume GTV 3D vs. internal GTV- (iGTV 4D) and PTV 3D vs. iPTV 4D. Volumetric values expressed in cm3 and equivalent spherical diameter (ESD) expressed in cm were assessed. Comparison of GTV movement in the phase FB-GTV FB, phase 0-GTV0, phase 50-GTV 50, and phase Maximum intensity projection (MIP) -GTV MIP was made with GTV FB as the basic value. The evaluation was made in three axis. Results. Comparison volumetric values between GTV 3D vs. iGTV 4D was 63.15 vs.85.51 (p<0.001) respectively. iGTV 4D was significantly larger than GTV 3D (p<0.001). The mean value (ESD) PTV 3D vs.iPTV 4D was 8.44 vs. 7.82 (p<0.001) respectively, and mean value volume PTV 3D vs. iPTV 4D was 352.70 vs. 272.78 (p<0.001) respectively. PTV 3D was significantly larger than iPTV 4D (p<0.001). Statistically significant difference (p<0.05) was identified in the deviation related to Z axis between the upper and lower lobe. Conclusion. 4D CT simulation based delineation can reduce planning target volume compared to 3D simulation based radiation therapy and therefore, it is a prerequisite for high-quality and precise radiation therapy treatment.