Mediastinum
General
WHO classification
Author: Hanni Gulwani, M.B.B.S.
Topic Completed: 1 December 2012
Minor changes: 10 June 2020
Copyright: 2003-2020, PathologyOutlines.com, Inc.
PubMed Search: Thymoma[TI] classification[TI]
Table of Contents
WHO classification | Suster and Moran classification | WHO type A | WHO type AB | WHO type B1 | WHO type B2 | WHO type B3Cite this page: Gulwani H. WHO classification. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/mediastinumthymomaclassification.html. Accessed August 14th, 2020.
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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Table
- World Health Organization histologic classification has some prognostic value (Int J Surg Pathol 2009;17:255, Hematol Oncol Clin North Am 2008;22:543, J Cardiovasc Surg (Torino) 2006;47:89), although interobserver variation is common (Histopathology 2008;53:483) and other problems exist (Semin Diagn Pathol 2005;22:188)
Suster and Moran classification
Favorable prognostic categories: groups I - III
Group I
Group II
Group III
Unfavorable prognostic categories: groups IV - VI
Group IV
Group V
Group VI
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
- References: Am J Clin Pathol 2006;125:542
WHO type A
Definition / general
Clinical features
Prognostic factors
Gross description
Gross images
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Microscopic (histologic) description
Short spindled:
Long spindled:
Micronodular:
Microscopic (histologic) images
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Negative stains
- 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
- May have papillary and pseudopapillary features (Am J Surg Pathol 2011;35:372), adenomatoid-like appearance with signet ring cell morphology (Am J Surg Pathol 2010;34:1544)
- May be associated with hematologic malignancies
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+
Micronodular:
- 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
lymphocytes
(arrowheads)
Rosette-like
structures mimic
a neuroendocrine
neoplasm
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
Microscopic (histologic) description
Microscopic (histologic) images
Case of Week #99:
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Positive stains
Electron microscopy description
Differential diagnosis
- 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
CD3
CD20
Keratin
CD68
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Immature T cells
(arrows), epithelial
tumor cells with large
nuclei (arrowheads)
Positive stains
- Keratin demonstrates finely arborizing network of interconnecting epithelial cell processes
- T cells: CD1, CD2, CD3, CD99, BCL2, TdT
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
