TU‐H‐CAMPUS‐JeP2‐02: Interobserver Variability of CT, PET‐CT and MRI Based Primary Tumor Delineation for Lung Cancer

Abstract

Purpose:Target delineation in lung cancer radiotherapy has, in general, large variability. MRI has so far not been investigated in detail for lung cancer delineation variability. The purpose of this study is to investigate delineation variability for lung tumors using MRI and compare it to CT alone and PET‐CT based delineations.Methods:Seven physicians delineated the primary tumor volumes of nine patients for the following scenarios: (1) CT only; (2) post‐contrast T1‐weighted MRI registered with diffusion‐weighted MRI; and (3) PET‐CT fusion images. To compute interobserver variability, the median surface was generated from all observers’ contours and used as the reference surface. A single physician labeled the interface types (tumor to lung, atelectasis (collapsed lung), hilum, mediastinum, or chest‐wall) on the median surface. Volume variation (normalized to PET‐CT volume), minimum distance (MD), and bidirectional local distance (BLD) between individual observers’ contours and the reference contour were measured.Results:CT‐ and MRI‐based normalized volumes were 1.61±0.76 (mean±SD) and 1.38±0.44, respectively, both significantly larger than PET‐CT (p<0.05, paired t‐test). The overall uncertainty (root mean square of SD values over all points) of both BLD and MD measures of the observers for the interfaces were not significantly different (p>0.05, two‐samples t‐test) for all imaging modalities except between tumor‐mediastinum and tumor‐atelectasis in PET‐CT. The largest mean overall uncertainty was observed for tumor‐atelectasis interface, the smallest for tumor‐mediastinum and tumor‐lung interfaces for all modalities. The whole tumor uncertainties for both BLD and MD were not significantly different between any two modalities (p>0.05, paired t‐test). Overall uncertainties for the interfaces using BLD were similar to using MD.Conclusion:Large volume variations were observed between the three imaging modalities. Contouring variability appeared to depend on the interface type. This study will be useful for understanding the delineation uncertainty for radiotherapy planning of lung cancer using different imaging modalities.Disclosures: Research agreement with Phillips Healthcare (GH and EW), National Institutes of Health Licensing agreement with Varian Medical Systems (GH and EW), research grants from the National Institute of Health (GH and EW), UpToDate royalties (EW), and none (others). Authors have no potential conflicts of interest to disclose.