Interobserver variation in the classification of thymic tumours – a multicentre study using the WHO classification system E T Verghese, M A den Bakker, 1 A Campbell, 2 A Hussein, 3 A G Nicholson, 4 A Rice, 5 B Corrin, 6 D Rassl, 7 G Langman, 8 H Monaghan, 9 J Gosney, 10 J Seet, 11 K Kerr, 12 S K Suvarna, 13 M Burke, 14 P Bishop, 15 S Pomplun, 16 S Willemsen 17 & B Addis 18 Department of Pathology, Leeds Teaching Hospital NHS Trust, Leeds, UK, 1 Departments of Pathology and 17 Epidemiology & BioStatistics, Erasmus MC, The Netherlands, 2 Department of Pathology, Hull Royal Infirmary, Hull, UK, 3 Department of Pathology and Cytology, SU ⁄ Sahlgrenska, Sweden, 4 Department of Pathology, Royal Brompton Hospital, Royal Brompton & Harefield NHS Trust, 5 Department of Pathology, Imperial College Healthcare NHS Trust and 6 Department of Pathology, Imperial College, London, 7 Department of Pathology, Papworth Hospital NHS Foundation Trust, Cambridge, 8 Department of Pathology, Birmingham Heartlands Hospital, Birmingham, 9 Department of Pathology, Royal Infirmary of Edinburgh, Edinburgh, 10 Department of Pathology, Royal Liverpool and Broadgreen University Hospitals NHS Trust, Liverpool, 11 Department of Pathology, St George’s Healthcare Trust, London, 12 Department of Pathology, Aberdeen Royal Infirmary, Aberdeen, 13 Department of Pathology, Northern General Hospital, Sheffield, 14 Department of Pathology, Harefield Hospital, Royal Brompton & Harefield NHS Trust, London, 15 Department of Pathology, University Hospital of South Manchester Foundation Hospital NHS Trust, Manchester, 16 Department of Pathology, King’s College Hospital NHS Foundation Trust, London, and 18 Department of Pathology, Southampton University Hospitals NHS Trust, Southampton, UK Date of submission 12 December 2007 Accepted for publication 25 February 2008 Verghese E T, den Bakker M A, Campbell A, Hussein A, Nicholson A G, Rice A, Corrin B, Rassl D, Langman G, Monaghan H, Gosney J, Seet J, Kerr K, Suvarna S K, Burke M, Bishop P, Pomplun S, Willemsen S & Addis B (2008) Histopathology 53, 218–223 Interobserver variation in the classification of thymic tumours – a multicentre study using the WHO Classification system Aims: To test the reproducibility of the current World Health Organization (WHO) classification of thymic epithelial tumours and to determine the level of interobserver variation within a group of pathologists, all with experience and expertise in thoracic pathology. Methods and results: Ninety-five thymic tumours were circulated to a group of 17 pathologists in the UK and The Netherlands over a 1-year period. Participants were asked to classify them according to WHO criteria. The diagnoses were subjected to statistical analysis and j values calculated. The overall level of agreement was moderate (j 0.45). When the categories were reduced in number by creating two groups, (A + AB + B1 + B2 and B3 + C), the level of agreement increased to 0.62. An alternative grouping (A + AB + B1 and B2 + B3 + C) increased it slightly further. The best agreement was in tumour types A and AB. Difficulties arose in distinguishing B1 tumours from B2 tumours and B2 tumours from B3 tumours. Conclusions: Although the WHO system describes a number of well-defined tumour types with clear diag- nostic criteria, the overall level of agreement was moderate and improved if some groups were amal- gamated. Keywords: interobserver agreement, kappa, thymic carcinoma, thymoma, WHO classification Abbreviations: CI, confidence interval; WHO, World Health Organization Address for correspondence: Dr Bruce Addis, Department of Histopathology, Level E, South Block, Southampton General Hospital, Tremona Road, Southampton, SO16 6YD, UK. e-mail: bruce.addis@suht.swest.nhs.uk Thymic tumour panel (2006): A. Campbell, A. Hussein, A. G. Nicholson, A. Rice, B. Addis, B. Corrin, D. Rassl, G. Langman, H. Monaghan, J. Gosney, J. Seet, K. Kerr, K. Suvarna, M. Burke, M. den Bakker, P. Bishop, S. Pomplum Additional cases provided by: S.Muller, D. Snead Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Publishing Limited. Histopathology 2008, 53, 218–223. DOI: 10.1111/j.1365-2559.2008.03088.x