Thymoma Staging 

Updated: Dec 30, 2015
  • Author: Quintessa Miller, MD; Chief Editor: Jules E Harris, MD, FACP, FRCPC  more...
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Staging and Classification of Thymoma

There is no standard system for staging thymoma; however, thymomas are most commonly staged according to the Masaoka system. The World Health Organization (WHO) histologic classification is also employed. [1, 2, 3]

Table. Modified Masaoka clinical staging of thymoma (Open Table in a new window)

Stage Definition
I Macroscopically and microscopically completely encapsulated
IIA Microscopic transcapsular invasion
IIB Macroscopic invasion into surrounding fatty tissue or grossly adherent to but not through mediastinal pleura or pericardium
III Macroscopic invasion into neighboring organs (ie, pericardium, great vessels, or lung)
IVA Pleural or pericardial dissemination
IVB Lymphogenous or hematogenous metastasis

WHO histologic classification...

Table. (Open Table in a new window)

WHO histologic classification Type Histologic description
A Tumor in which foci having features of type A thymoma are admixed with foci rich in lymphocytes


Tumor resembles normal functional thymus; combines large expanses having an appearance practically indistinguishable from that of normal thymic cortex with areas resembling thymic medulla


Tumor in which neoplastic epithelial component appears as scattered plump cells with vesicular nuclei and distinct nucleoli among a heavy population of lymphocytes; perivascular spaces are common and sometimes very prominent; a perivascular arrangement of tumor cells resulting in a palisading effect may be seen.


Thymoma predominantly composed of epithelial cells having a round or polygonal shape and exhibiting no or mild atypia; they are admixed with a mild component of lymphocytes, resulting in a sheetlike growth of the neoplastic epithelial cells


Thymic tumor exhibiting clear-cut cytologic atypia and a set of cytoarchitectural features no longer specific to the thymus, but rather analogous to those seen in carcinomas of the other organs; type C thymomas lack immature lymphocytes; whatever lymphocytes may be present are mature and usually admixed with plasma cells