Background: British Thoracic Society guidelines recommend 18F Fluorodeoxyglucose (F-FDG) PET/computed tomography (CT) for solitary pulmonary nodule (SPN) follow-up in high-risk individuals or if the CT Brock score is >10%. Nodule tracer uptake is assessed visually in comparison to the surrounding lung tissue and mediastinal blood pool (Herder score). This score is used to calculate the risk of malignancy and guide patient management. Despite its widespread use, there have been no studies to date looking at interobserver agreement using the Herder scale.
Patients and methods: F-FDG PET/CT studies of 100 consecutive patients imaged for the evaluation of SPN were retrospectively analysed. Anonymized images were reviewed independently by three Consultant Nuclear Medicine Radiologists and the Herder score was documented, along with a confidence score graded 1-3, where 1 indicated 'not at all confident' and 3 indicated 'very confident'. Interobserver agreement was assessed using interclass correlation coefficient modelling.
Results: There was complete reviewer agreement in 81% cases, and interclass correlation with Cronbach's alpha was excellent at 0.973 (95% confidence interval, 0.962-0.981). The agreement between pairs of reviewers was good and confidence scores using the Herder scale were high, with reviewers giving a confidence score of 3 in an average of 78% of cases.
Conclusion: Our study suggests excellent interobserver agreement for use of the Herder scale in evaluating SPNs. Reviewer confidence scores were high reflecting high confidence in the use of the Herder scale for evaluating SPN.