Thymoma classification

Mediastinum
Thymoma and related entities
Thymoma classification

Author: Hanni Gulwani, M.D. (see Authors page)

Revised: 19 June 2018, last major update December 2012

Copyright: (c) 2003-2018, PathologyOutlines.com, Inc.

PubMed Search: Thymoma[TI] classification[TI]

Table of Contents

Cite this page: Gulwani, H. Thymoma classification. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/mediastinumthymomaclassification.html. Accessed July 19th, 2018.

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

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

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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Various images

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

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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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Various images

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

WHO type B3

Microscopic (histologic) images

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Various images

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

Polygonal epithelial cells (arrows) with irregular nuclei