Problems in the classification of thymoma

The histologic classification of thymoma has posed difficulties for pathologists for many years and continues to be a focus of controversy in the speciality today. In recent years, the World Health Organization (WHO) has assembled a panel of experts with the purpose of devising a histologic classification system of thymoma that will be widely adopted for common use by pathologists and clinicians throughout the world. This objective has been largely accomplished with the introduction of the WHO histologic classification of tumours of the thymus. Despite the widespread acceptance of this classification, however, several controversial issues have remained unaddressed that still require attention. Herein we review briefly some of the issues that pose a challenge and require improvement in our current classification of thymomas, including issues related to ease of applicability of this classification and interobserver reproducibility in daily practice, the lack of a unifying, underlying biologic principle, the limitations for the stratification of patients for clinical management and the accuracy of some of the assumptions behind the definitions for some of the different proposed histologic types.