Muller-Hermelink classification of thymic tmours

Mixed thymoma

  1. two epithelial components

  2. one element is medullary thymoma.

  3. other component is lymphocyte-rich–mimicking cortical areas of the normal thymus

Organoid thymoma / Predominantly cortical / Lymphocyte-rich

  1. both cortical and medullary areas of the normal thymus including Hassall’s corpuscles, hence the alternative term organoid thymoma

  2. often lack a complete fibrous capsule and may invade adjacent organs by their plump, pushing periphery

  3. organoid thymomas have the highest lymphocyte content

  4. the few epithelial cells are small but elongated and correspond to those normally found in the corticomedullary junction

Cortical thymomas

  1. lobulated tumors

  2. interlobular fibrous septa

  3. often incomplete capsules and invasion of neighboring structures

  4. lymphocyte count is less than in organoid thymoma

  5. squamous differentiation is encountered occasionally at the periphery of the tumor.

  6. predominant immunophenotype of intratumoral lymphocytes indicates an immature CD1+ T cell, as usually found in the normal thymic cortex.

Well-differentiated thymic carcinomas (WDTC)

  1. lymphocyte-poor tumors

  2. growing in solid epithelial strands

  3. infiltrating extrathymic organs

  4. palisading phenomena around perivascular spaces is a distinct morphologic feature of WDTC.

  5. in contrast to cortical thymoma, epithelial tumor cells are smaller and nucleoli are inconspicuous.

  6. cytologic atypia and mitotic figures are regularly present.

  7. in approximately half of these tumors, a component of cortical or organoid thymoma can be found

Endocrine carcinoma

  1. neoplasm of the diffuse endocrine system, differing from thymomas

  2. primary site in the thymus is rare

  3. incomplete capsule

  4. often exhibits local invasiveness, metastases in draining lymph nodes, and, less frequently, distant sites.

  5. typical histo-architectural patterns include trabecular, acinar, glandular, and tubular formations

  6. the defining cytologic features are the granular cytoplasm and finely granular so-called salt and pepper chromatin

  7. carcinoid tumors express neuroendocrine antigens, most regularly synaptophysin, chromogranin, neuron specific enolase, and CD56


  1. Marino M, Muller-Mermelink HK: Thymoma and thymic carcinoma. Relation of thymoma epithelial cells to the cortical and medullary differentiation of thymus. Virchows Arch [A] 407:119,1985

  2. Close PM, Kirchner T, Uys CJ, Muller-Hermelink HK: Reproducibility of a histogenetic classification of thymic epithelial tumours. Histopathology 1995 Apr;26(4):339-43