Diagnostic reproducibility of thymic epithelial tumors using the World Health Organization classification: note for thoracic clinicians

Histopathological diagnosis of thymic epithelial tumors according to the current World Health Organization classification is not adequately reproducible; however, most thoracic clinicians are unaware of this. We illustrate this problem in practical settings to raise clinician awareness. An expert pathologist specialized in thymic pathology and a trained general pathologist independently diagnosed 158 resected thymic epithelial tumors. Assuming that the expert’s diagnoses were more accurate, the two pathologists’ diagnoses were judged to be concordant when tumor subtypes (thymoma) or categories (thymic carcinoma and neuroendocrine tumor) were in agreement. The concordance rates for different thymoma subtypes were 75 % (3/4), 30 % (11/37), 100 % (17/17), 80 % (39/49), and 53 % (9/17) for types A, AB, B1, B2, and B3, respectively. Discordant cases of type AB thymoma were mainly diagnosed as type B1 or B2 by the general pathologist. Discordant cases of type B2 thymoma were diagnosed as type AB, B1, or B3, and discordant cases of type B3 thymoma were diagnosed as type A, B2, or carcinoma. Discordant cases of thymic carcinoma were diagnosed as type A or B3 thymoma. Investigation of the concordant and discordant cases suggested that reasonable discrepancies can occur because of the noncommittal categorical boundaries inherent in this classification. Thoracic clinicians should consider this potential problem in daily practice.