Sci-Thurs PM: Planning-05: Comparison of 4DCT Contouring on Exhalation + Inhalation Phases and Maximum Intensity Projection (MIP) for Lung Cancer

Abstract

4DCT has been widely used in radiation therapy planning to determine target motion and generate the internal target volume (ITV). However, contouring on all respiratory phases created by 4DCT may not be the most efficient way to generate ITV. To develop a practical contouring protocol of adequate accuracy, we compared in this study two available approaches: one is to combine the target contours from the maximum exhalation and inhalation phases only; the other is to contour on the derived Maximum Intensity Projection (MIP). In this study, our physicians contoured the target for 11 lung‐cases (Stage Ia to IIIb), all scanned using 4DCT on a Philips 16‐slice big bore. For the 2 of the 11 cases without the target abutting other structures, the combined contour from the exhale and inhale phases (ITV_ex+in) was entirely encompassed by the contour from MIP (ITV_MIP). However, 28% (6.3cc) of ITV_MIP was outside ITV_ex+in in one case. This underestimation of the target volume by ITV_ex+in might be partially due to the curved trajectory of the target motion. In the rest 9 cases, all with the target abutting other structures, 5 cases had 2.5∼6.8cc ( median =4.4 cc ) of ITV_ex+in outside ITV_MIP. The underestimation of the target volume by ITV_MIP was due to portions of the target in one respiratory phase being shadowed in MIP by abutting structures from another phase. Our results indicate that for determining ITV in lung on 4DCT it is necessary to combine contours from all three sets including MIP, exhale and inhale phases.