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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. Thymoma classification. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/mediastinumthymomaclassification.html. Accessed May 22nd, 2019.
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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Various images
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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Images hosted on other servers:
Various images
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
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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
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Case of Week #99:
Various images
CD3
CD20
Keratin
CD68
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Various images
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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Images hosted on other servers:
Various images
Abundant epithelial cells with
irregular nuclei (arrowheads)
and fewer lymphocytes
(arrows) than B1
WHO type B3
Microscopic (histologic) images
Images hosted on other servers:
Images hosted on other servers:
Various images
Perivascular spaces,
sheets of large polygonal
epithelial cells with nuclear
atypia, few lymphocytes
Polygonal epithelial cells (arrows) with irregular nuclei
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