What is a perifissural nodule? Low inter-observer agreement in NLST data

Purpose/Objectives: Pulmonary nodules on chest CT classified by radiologists as perifissural nodules (PFN) have been shown to have a negligible chance of malignancy. We studied the inter-observer variability for classifying nodules as PFNs in National Lung Screening Trial (NLST) data. Methods and Materials: Out of a sample of 5819 low-dose CT scans of slice thickness <=2mm from the NLST, we detected and annotated 3669 non-calcified solid nodules with diameters of 5 to 10mm. 359 nodules were selected for the observer study. With definitions provided, six radiologists independently classified these nodules as either "typical PFN,? "atypical PFN,? "non-PFN,? or "not applicable.? A "typical PFN? has a lentiform, triangular, or polygonal shape, is located on or within 10 mm of the visceral pleura or lung fissure, and has extending linear densities. An "atypical PFN? lacks one of the three key criteria defining a typical PFN. A "non-PFN? showed at least one of the following features: spiculation, irregular shape, un-sharp borders, and distortion of the pleura or fissure. Adherence to these definitions was not required. Opacities deemed to be subsolid, completely calcified, not nodular, or not visible were considered "not applicable?; when rated as such by at least three radiologists, these were excluded (n=43). This left 316 nodules for analysis using descriptive statistics, Mann-Whitney U tests, and Fleiss' kappa (k). Results: Fifty-six of 316 nodules (17.7%) were classified by all six radiologists as either typical or atypical PFNs. More than four times the number, 229/316 nodules (72.5%), were classified by at least one radiologist as PFNs. k was 0.50; when distinguishing PFN subgroups, k decreased to 0.30. Only 7/316 nodules (2.2%) were unanimously classified as typical PFNs; none were atypical PFNs by unanimity. Compared to non-PFNs, nodules classified by at least three readers as either typical or atypical PFNs were smaller on average and were more often located in the lower lobes and attached to a fissure (p<0.001). Pleural attachment was not a good distinguishing factor between PFNs and non-PFNs (p=0.54). Conclusion: There is only a fair to moderate inter-radiologist agreement when classifying pulmonary nodules as PFNs. More than two-thirds of all nodules in our sample were considered to be a PFN by at least one radiologist; less than one-fifth were PFNs by full consensus, and considerably less when distinguishing PFN subgroups. This suggests that better criteria for identifying PFNs need to be developed and adhered to.