Classification of Thymic Neoplasms

Composed of thymic epithelial elements

  1. 1.Thymoma (Muller-Hermelink - medullary/cortical/organoid/mixed)

    1. Encapsulated (equivalent to Masaoka stage T1)

    2. Minimally invasive (equivalent to Masaoka stage T2 - T2a gross invasion, T2b - microscopic invasion)

    3. Wide local invasion including pleural or pericardial implants (Masaoka stages T3 - invasion and T4a - dissemination)

    4. Metastasizing (Masaoka stage 4b - N1, N2 and M1)

  2. 2.Thymic carcinoma

    1. Squamous

    2. Non-keratinizing (Muller-Hermelink - well diffentiated thymic carcinoma)

Composed of other elements

  1. Thymic carcinoids and neuroendocrine tumours

  2. Germ cell tumours

  3. Hodgkin’s and non-Hodgkin’s lymphomas

WHO classification of thymomas

A: A tumor composed of a population of neoplastic thymic epithelial cells having spindle/oval shape, lacking nuclear atypia, and accompanied by few or no nonneoplastic lymphocytes.

AB: A tumor in which foci having the features of type A thymoma are admixed with foci rich in lymphocytes.

B1: A tumor that resembles the normal functional thymus in that it combines large expanses having an appearance practically indistinguishable from normal thymic cortex with areas resembling thymic medulla.

B2: A tumor in which the 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 pallisading effect may be seen.

B3: A type of thymoma predominantly composed of epithelial cells having a round or polygonal shape and exhibiting no or mild atypia. They are admixed with a minor component of lymphocytes, resulting in a sheetlike growth of the neoplastic epithelial cells.

C: A 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 other organs. Type C thymomas lack immature lymphocytes; whatever lymphocytes may be present are mature and usually admixed with plasma cells.