fig1

Figure 1. WHO histologic classification of thymoma (A) Type A thymoma is characterized by a spindle cell proliferation with limited numbers of lymphocytes, (B) Type AB thymoma is characterized by a spindle cell population intimately admixed with increased numbers of lymphocytes, (C) Type B1 thymoma is characterized by sparse epithelioid cells in a dense immature lymphoid cell population, (D) Type B2 thymoma is characterized by an increased number of epithelioid cells with a relatively decreased proportion of lymphoid cells, (E) Atypical thymoma (WHO type B3 thymoma) is characterized by sheets of epidermoid appearing epithelioid cells with increased cytologic atypia and few lymphocytes, (F) Metaplastic thymoma is characterized by a pseudosarcomatous stromal component with an admixed epithelial component composed of neoplastic thymic epithelial cells. Note: Not shown here are numerous other subtypes of more esoteric thymomas that do not fit into any particular classification system and have limited documented molecular data.