Kirchner et al (23) introduced a category of well-differentiated thymic carcinoma
, defined as TET with incomplete loss of organotypic differentiation, tightly packed epithelial cells with slight to moderate atypia, and some immature CD1-positive T cells.
Table 1: World Health Organization classification of thymoma Type Histologic description A Medullary thymoma AB Mixed thymoma B1 Predominantly cortical thymoma B2 Cortical thymoma B3 Well-differentiated thymic carcinoma
C Thymic carcinoma Table 2: Masaoka staging system of thymoma Stage 1 Encapsulated tumor with no gross or microscopic invasion Stage 2 Macroscopic invasion into mediastinal fat or pleura Stage 3 Invasion of pericardium, great vessels, or lung Stage 4 Pleural or pericardial metastatic spread Stage 5 Lymphatic or hematogenous spread
(2) Muller-Hermelink et al (3) classified thymomas into 6 categories based on resemblance of tumor to cortex or medulla of normal thymus (from well-differentiated to poorly differentiated tumors): medullary thymomas; mixed thymomas; predominantly cortical (organoid) thymomas; cortical thymomas; well-differentiated thymic carcinomas
; and high-grade thymic carcinomas.