Problem areas and inconsistencies in the WHO classification of thymoma
Section snippets
WHO schema for the classification of thymic epithelial neoplasms
Two major types of thymoma were identified in the WHO schema, depending on whether the neoplastic epithelial cells and their nuclei showed a spindle or oval shape (designated as type A), or a round epithelioid appearance (designated as type B)12 (Figure 1A and B). Tumors with a combination of these two cell types were designated as type AB. Type B thymoma was further subdivided on the basis of the proportional increase and emergence of atypia of the neoplastic epithelial cells into three
Problem areas and inconsistencies in the WHO classification
Problem areas with the current WHO classification of thymic epithelial neoplasms center mainly around the following issues: (1) problems associated with the histopathologic criteria for diagnosis of the various subtypes; (2) the existence of rare and unusual morphologic types of thymoma that cannot be fit into any of the WHO categories; (3) problems associated with lack of interobserver reproducibility; and (4) conflicting claims regarding the clinical significance and prognostic value of the
Conclusion
There is no question that the current WHO schema is a step forward from what existed previously in the field of thymoma histopathology, and that it has significantly advanced our understanding of these lesions. However, with continuous progress in our understanding of the biology of these tumors, and with an honest acknowledgment of the problems and limitations imposed by the current WHO schema, it is perhaps time to heed the words of the authors of the original WHO monograph who stated in
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2017, Lung CancerCitation Excerpt :Thymic carcinoma on the contrary is a highly aggressive tumor with frequent lymphatic and hematogenous metastasis [3]. However, its prognostic significance ability to guide further treatment is controversial [4–7]. Additionally, recurrence can occur even in low grade (WHO subtype A, AB, or B1) thymomas in up to 15% of the cases [8].
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