WHO classification

Topic Completed: 1 December 2012

Minor changes: 15 October 2021

Copyright: 2003-2022,, Inc.

PubMed Search: Thymoma[TI] classification[TI]

Hanni Gulwani, M.B.B.S.
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Cite this page: Gulwani H. WHO classification. website. Accessed January 21st, 2022.
WHO classification
  • A: also called epithelial, spindle cell, medullary; atrophic, mimics adult thymus; homogenous population of neoplastic epithelial cells with spindle / oval shape, no nuclear atypia and accompanied by few or no nonneoplastic lymphocytes
  • AB: mixed thymoma; tumor in which foci having the features of type A thymoma are mixed with foci rich in lymphocytes; the segregation of the two patterns can be sharp or indistinct (Am J Surg Pathol 1999;23:955)
  • B: bioreactive, resembles thymus in fetus and infant
  • B1: lymphocyte rich; resembles normal functional thymus by combining large expanses having normal thymic cortical areas with those resembling thymic medulla
  • B2: cortical; neoplastic epithelial component appears as scattered plump cells with vesicular nuclei, distinct nucleoli; heavy population of lymphocytes, perivascular spaces are common
  • B3: epithelial cells with round / polygonal shape and mild atypia, mixed with minor component of lymphocytes; foci of squamous metaplasia and perivascular spaces common
  • C: thymic carcinoma

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Suster and Moran classification
Favorable prognostic categories: groups I - III
Group I
  • Encapsulated or minimally invasive thymoma
  • Completely excised
  • Equivalent to WHO histologic types A, AB, B1, B2

Group II
  • Encapsulated or minimally invasive thymoma
  • Completely excised
  • Equivalent to WHO histologic type B3

Group III
  • Widely invasive thymoma or thymoma with implants
  • Completely excised
  • All histologic types

Unfavorable prognostic categories: groups IV - VI
Group IV
  • Widely invasive thymoma or thymoma with implants
  • Incompletely excised
  • All histologic types

Group V
  • Widely invasive thymoma with or without intrathoracic metastases
  • Unresectable / biopsy only
  • All histologic types

Group VI
  • Widely invasive thymoma with distant metastases
  • Unresectable / biopsy only
  • All histologic types

WHO type A
Definition / general
  • Also called spindle cell thymoma
  • Rosai believes composed of nonfunctional, postmature thymic epithelial cells that match epithelial cells of involuted thymus in adult life, not cortical or medullary cells

Clinical features
Prognostic factors
  • Excellent prognosis

Gross description
  • Usually encapsulated or minimal capsular invasion

Gross images

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Well circumscribed with lobulated cut surface

Microscopic (histologic) description
  • Epithelial predominant with fusiform epithelial tumor cells
  • Gland-like spaces, storiform patterns are common
  • No / rare lymphocytes
  • May have atypical features (Am J Surg Pathol 2012;36:889)
  • Either short spindled (57%), long spindled (31%) or micronodular (12%) (Am J Surg Pathol 2001;25:111)
  • Desmoplastic spindle cell thymoma: extensive areas of young fibrocollagen and prominent fibroblastic proliferation with scattered areas of more conventional spindle cell thymoma (Hum Pathol 2013;44:623)
  • Histologic features may not correlate with invasion or encapsulation because all thymomas may be capable of invasion (Am J Clin Pathol 2010;134:793)

Short spindled:
  • 57%, often in hemangiopericytic or microcystic pattern
  • Epithelial cells often CD20+

Long spindled:
  • 31%, fibroblast-like epithelial cells resembling fibrohistiocytic neoplasms
  • Epithelial cells often CD20+

  • 12%, small nests of short spindle cells without atypia in lymphoid stroma with frequent germinal centers
  • No mitotic activity; epithelial cells are CAM5.2+, keratin+, CD20-
  • Often incidental findings on chest Xray or at coronary artery bypass surgery
  • Not associated with autoimmune disorders

Microscopic (histologic) images

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Thymic epithelial cells without atypia

Bland spindled
epithelial cells
with occasional
small T cells

Spindle epithelial
cells (arrows) with

structures mimic
a neuroendocrine

Negative stains
  • CD5 (in epithelial cells)
WHO type AB
Microscopic (histologic) images

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Immature T cells
and scattered
large, round thymic
epithelial cells

A (asterisk) and B (star) type lobules

WHO type B1
Definition / general
  • Also called lymphocyte predominant

Microscopic (histologic) description
  • 2/3 or more small lymphocytes
  • Lymphocytes efface thymic architecture
  • Thick fibrous capsule present, also fibrous septae that intersect at acute angles
  • Lymphocytes have folded nuclei (T cells) and mitotic figures
  • Lymphocytes mixed with bland thymic epithelial cells, may have perivascular serum "lakes," mast cells and focal medullary differentiation with loose aggregates of lymphocytes resembling thymic medulla

Microscopic (histologic) images

Case of Week #99:

Various images





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Immature T cells
(arrows), epithelial
tumor cells with large
nuclei (arrowheads)

Positive stains
Electron microscopy description
  • Well formed intercellular junctions between epithelial cell processes, numerous tonofilaments

Differential diagnosis
  • Burkitt lymphoma: HIV+, different nuclear histology
  • Castleman disease: not centered in thymus, "onion skinning" by lymphocytes, either fibrohyaline or plasma cell subtypes
  • Lymphoblastic lymphoma: usually teens / young adults, similar staining except negative for keratin but beware of positive staining of trapped epithelial cells
  • Thymic lymphoid hyperplasia: normal cortical and medullary glandular distinction is maintained, well formed germinal centers present, does not produce a mass
WHO type B2
Microscopic (histologic) images

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Abundant epithelial cells with
irregular nuclei (arrowheads)
and fewer lymphocytes
(arrows) than B1

WHO type B3
Microscopic (histologic) images

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Perivascular spaces,
sheets of large polygonal
epithelial cells with nuclear
atypia, few lymphocytes

Polygonal epithelial cells (arrows) with irregular nuclei

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