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 κ values calculated. The overall level of agreement was moderate (κ 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 diagnostic criteria, the overall level of agreement was moderate and improved if some groups were amalgamated.

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Erasmus MC: University Medical Center Rotterdam

Verghese, E.T, den Bakker, M.A, Campbell, A.P, Hussein, A, Nicholson, A.G, Rice, A, … Addis, B.J. (2008). Interobserver variation in the classification of thymic tumours - A multicentre study using the WHO classification system. Histopathology, 53(2), 218–223. doi:10.1111/j.1365-2559.2008.03088.x