Mediastinum - Ectopic hamartomatous thymoma

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Mediastinum

Thymoma and related entities

Ectopic hamartomatous thymoma

Reviewer: Hanni Gulwani, M.D. (see Reviewers page)
Revised: 27 February 2013, last major update December 2012
Copyright: (c) 2003-2013, PathologyOutlines.com, Inc.

General
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● Rare, supraclavicular-sternal soft tissue mass

Epidemiology
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● Usually adult men

Clinical features
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● May arise from ectopic tissue of third branchial pouch
● Benign behavior

Case reports
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● 19 year old man with supraclavicular subcutaneous mass (J Cutan Pathol 2006;33:369)
● 59 year old man whose tumor had CD99+ lymphocytes and a low proliferation index (Arch Pathol Lab Med 2003;127:e378)

Gross description
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● Well circumscribed, firm, yellow-white, variable microcysts

Gross images
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Yellow-white solid tumor

Micro description
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● Sheets of spindled epithelial cells resembling neurogenic or fibroblastic tumors
● Also epithelial nests, thin anastomosing cords and epithelial lined cysts
● May have focal adipose tissue, small lymphocytes
● No atypia, no necrosis, no mitotic figures

Micro images
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Various images

CD99, CD3, CD20

Positive stains
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● Keratin
● Lymphocytes: CD3, CD20, CD99

Electron microscopy description
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● Tonofilaments, well developed cell junctions

Electron microscopy images
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Tonofilaments and cell junctions

Differential diagnosis
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● Angiomyolipoma: HMB45+
● Biphasic synovial sarcoma: more cellular, more atypia, more mitotic figures
● Glandular MPNST: resembles fibrosarcoma, S100+, cytokeratin-
● Mixed tumor: chondromyxoid, S100+
● Myoepithelioma of soft tissue: chondromyxoid, S100+
● Thymolipoma: mediastinum, not neck, no spindle cells

End of Mediastinum > Thymoma and related entities > Ectopic hamartomatous thymoma

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