Evidence-based pathology and the pathologic evaluation of thymomas: The world health organization classification can be simplified into only 3 categories other than thymic carcinoma

BACKGROUND. The clinical validity and applicability of the World Health Organization (WHO) histopathologic classification of thymomas ('classification') has been questioned. Evidence-based pathology promotes the use of systematic reviews and analysis of data with meta-analysis rather than subjective reviews of the literature. METHODS. The authors performed a review of the English literature from 1999 to the present to identify 'best evidence' regarding the use of the 'classification.' The data were analyzed with meta-analysis software. RESULTS. To the authors' knowledge, only Level-3 or -4 evidence published in retrospective cases series is currently available regarding the use of the 'classification.' Meta-analysis demonstrated that only 3 WHO categories of thymomas are associated with significant survival differences: A/AB/B1, B2, and B3. It also indicated significant heterogeneity with regard to the results published in different studies. To the authors' knowledge there is no current evidence to determine whether thymoma types are significant prognostic features for patients previously stratified by stage. CONCLUSIONS. There is a lack of randomized clinical trials evaluating the prognosis of patients with thymomas and the effects of various treatment modalities. The WHO classification of thymomas needs revision and could most likely be simplified into fewer classes with significant prognostic value. Future studies are needed to evaluate the prognostic and/or predictive value for thymoma patients previously stratified by stage. The latter information is important to help select those patients who may benefit from neoadjuvant chemotherapy or postoperative radiotherapy and other modalities.