Additional reporting of diffuse and homogeneous ROS-1 SP384 immunoreactivity enhances prediction of ROS1 fusion-positive non-small cell lung cancer

Objectives ROS-1 immunohistochemistry (IHC) is a common method for screening ROS1 fusion in the clinical management of non-small cell lung cancer. The interpretation criteria for ROS-1 SP384 IHC, however, remain unestablished.Methods Sixty-five non-small cell lung cancer cases underwent AmoyDx ROS1 fusion real-time polymerase chain reaction (PCR) study and ROS-1 SP384 IHC tests, which were retrieved for analysis. ROS-1 IHC tests were interpreted based on the established classifiers as well as the presence of diffuse homogeneous immunoreactivity. The diagnostic accuracies of these ROS-1 IHC interpretation methods were evaluated by comparing them with the ROS1 real-time PCR results.Results Previous ROS-1 IHC classifiers demonstrated high sensitivity for positive ROS1 real-time PCR results (100%), but they showed low specificities (25%-50%) and overall accuracies (58%-72%). In contrast, the diffuse homogeneous ROS-1 immunoreactivity predicted positive ROS1 real-time PCR results with much higher specificity (94%) and overall accuracy (95%), albeit with a slightly lower sensitivity (97%). Some cases that showed discrepancy between diffuse homogeneous ROS-1 immunoreactivity and real-time PCR results involved rare ROS1::LDLR fusion and suboptimal IHC staining.Conclusions A 3-tier reporting system for ROS-1 SP384 IHC testing combining previous interpretation criteria and diffuse and homogeneous immunoreactivity may better predict ROS1 fusion status without decreasing specificity.