Mediastinum - Thymoma

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Mediastinum

Thymoma and related entities

Thymoma

Reviewer: Hanni Gulwani, M.D. (see Reviewers page)
Revised: 24 March 2013, last major update December 2012
Copyright: (c) 2003-2013, PathologyOutlines.com, Inc.

General
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● Most common primary anterior mediastinal neoplasm

Epidemiology
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● Commonly ages 49-62 years
● Childhood thymomas are rare (usually are lymphoblastic lymphomas), usually near puberty, similar behavior and morphology as adult tumors, but only occasionally associated with myasthenia gravis

Sites
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● Usually anterosuperior mediastinum
● Rarely posterior mediastinum, lung hilum or parenchyma, neck, pleura, thyroid

Clinical features
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● Associated with myasthenia gravis [MG] (10% with MG have thymoma, 30-45% with thymoma develop MG, higher risk for MG if lymphoid follicles are present in thymoma or adjacent thymus)
● Associated with other immune-mediated disorders: acquired hypogammaglobulinemia (12%), aplastic anemia, pure anerythrogenesis, dermatomyositis, leukemia, lymphoma, lymphopenia, motor neuropathy, mucocutaneous candidiasis, myeloma, myocarditis, myositis, relapsing polychondritis, rheumatoid arthritis, scleroderma, Sjogren’s disease, syndrome of inappropriate antidiuretic hormone secretion, systemic lupus erythematosus
● Patients with thymomas have increased risk of developing additional malignancies, especially thymomas with predominantly cortical component (Histopathology 2012;60:437)
● All thymic tumors, regardless of histology, are associated with invasion and metastases (Mod Pathol 2012;25:370)

Prognostic factors
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● Poor prognostic factors: high stage, B3 or C classification, positive margin, invasion of capsule (Am J Surg Pathol 2002;26:1605)
● Prognostic factors for OS were age, WHO histology, Masaoka stage and recurrence, while pleural involvement, WHO histology and Masaoka stage had significant impacts on DFS (Oncol Rep 2008;19:1525)
● Weakly prognostic: podoplanin overexpression may predict lymph node metastasis and poor clinical outcome (Hum Pathol 2011;42:533)
● Not prognostic: hemorrhage and necrosis in well encapsulated noninvasive thymomas (Am J Surg Pathol 2001;25:1086)
● Controversial: transcapsular invasion (Arch Pathol Lab Med 2008;132:926, Arch Pathol Lab Med 2008;132:1859)

Case reports
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● 60 year old man with invasive thymoma and paraneoplastic retinopathy (Hum Pathol 2003;34:717)
● 61 year old man with metaplastic thymoma (Int J Surg Pathol 2009;17:51)
● 62 year old man with composite thymoma and CLL/SLL (Arch Pathol Lab Med 2003;127:E76)
● 70 year old man with a calcified mediastinal mass (Case of the Week #99)
● Two cases of thymoma arising within cardiac myxoma (Am J Surg Pathol 2005;29:1208)

Treatment
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● Surgical excision
● Possibly chemotherapy or radiation

Gross description
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● 80% encapsulated, 20% infiltrative into surrounding structures
● Usually large unless incidental with coronary bypass surgery
● Multinodular, yellow-gray
● Sharp lobulations due to fibrous bands with some nodules having pointed ends
● Cystic degeneration common

Micro description
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● Spindle cell histologic patterns have indolent behavior, may be associated with hematologic malignancies
● Non-spindle cell thymomas are also called cortical thymomas
● Cytologically bland epithelial cells and non-neoplastic lymphocytes
● Capsule may be thick and calcified
● May have prominent vasculature, microcystic and pseudopapillary patterns, extensive sclerosis
● Rarely has marked plasma cell infiltrate, amyloid, rosettes without central lumina
● Usually no well formed Hassall’s corpuscles
● Thymoma with pseudosarcomatous stroma: highly cellular spindle cell proliferation without nuclear atypia (Am J Surg Pathol 1997;21:1316)

Micro images
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AB: medullary and spindle cells and lymphocytes	B1: cellular lobules of lymphocytes with scattered medullary cells	B3: epithelial cells with nuclear atypia
Various stains

Virtual slides
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Thymoma

Positive stains
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● CEA, CD3 (lymphocytes outside lymphoid follicles are T cells), EMA (some tumors), keratin (epithelial cells), Ki-67, S100 (interdigitating reticulum cells),
● CD205, Foxn1 (Am J Surg Pathol 2007;31:1038)
● PAX8 (Am J Surg Pathol 2011;35:1305)
● XIAP (Am J Clin Pathol 2009;131:689)

Negative stains
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● CD70, vimentin (Am J Surg Pathol 2000;24:742)

Electron microscopy description
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● Branching tonofilaments, complex desmosomes, elongated cell processes, basal lamina

Molecular / cytogenetics description
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● Rarely alterations of #6 (Arch Pathol Lab Med 2000;124:1714)

Differential diagnosis
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● Thymic cyst
● Thymic carcinoid: well formed rosettes
● Lymphoma
● Seminoma
● Solitary fibrous tumor

End of Mediastinum > Thymoma and related entities > Thymoma

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