Mediastinum

Last revised 4 November 2007

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Table of contents

Primary references, general

Inflammatory disorders: acute mediastinitis, chronic mediastinitis, sclerosing mediastinitis

Thyroid/parathyroid: thyroid nodular hyperplasia, parathyroid adenoma

Thymus: normal, acute thymic involution, ectopic thymus, ectopic tissue in thymus, myasthenia gravis, thymic dysplasia, thymic follicular hyperplasia, true thymic hyperplasia

Cystic lesions: general, bronchogenic, enteric, lymphangioma, meningocele, pancreatic pseudocyst, parathyroid, pericardial, seminoma, teratoma, thymic, thymoma-cystic

Tumors: general

Thymoma and related entities: thymoma, staging, classification, ectopic hamartomatous thymoma, ectopic thymoma, thymolipoma

Other benign/low grade tumors: elastolipoma, ependymoma, fibromatosis, ganglioneuroma, hemangioma, inflammatory myofibroblastic tumor, lipoma, lipomatosis, meningioma, neurofibroma, paraganglioma, schwannoma, solitary fibrous tumor

Thymic carcinoma: general, adenocarcinoma, adenosquamous, basaloid squamous cell, clear cell, lymphoepithelioma-like, mucoepidermoid, parathyroid, sarcomatoid, squamous cell-keratinizing, nonkeratinizing

Other mediastinal carcinoma: metastatic, neuroendocrine, small cell

Hematological neoplasms/lesions: Burkitt’s lymphoma, Castleman’s, diffuse large B cell lymphoma, extramedullary hematopoiesis, granulocytic sarcoma, Hodgkin’s lymphoma, Langerhans cell histiocytosis, lymphoblastic lymphoma, MALT lymphoma, NK lymphoma, nodal inclusions, plasmacytoma

Other mediastinal malignancies: carcinoid, chordoma, follicular dendritic cell tumor, ganglioneuroblastoma, germ cell, hemangioendothelioma, leiomyosarcoma, liposarcoma, malignant peripheral nerve sheath tumor, melanoma, mesothelioma, metastases, neuroblastoma, PNET, reticulum cell, rhabdomyosarcoma, thymoliposarcoma

Miscellaneous: features to report, grossing

 

Primary references

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American Journal of Surgical Pathology (AJSP), Jan 1999 to Nov 2003

Archives of Pathology and Laboratory Medicine (Archives), July 1998 to Nov 2003

Human Pathology (Hum Path), Jan 1999 to Oct 2003

Modern Pathology (Mod Path), Jan 1999 to Nov 2003

Rosai, J: Ackerman’s Surgical Pathology (8th Ed); Mosby-Year Book, Inc., 1996

Sternberg, S: Diagnostic Surgical Pathology (3rd Ed); Lippincott Williams & Wilkins, 1999

 

Please refer to these primary references for more detailed discussions

 

General

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Mediastinum is thoracic cavity between pleural cavities, from sternum to spine, thoracic inlet to diaphragm

Symptomatic tumors are usually 2-3 cm or more; most masses are detected incidentally, and undergo needle biopsy

Superior mediastinum: thymomas, thymic cyst, lymphoma, thyroid lesions, parathyroid adenoma

Anterior mediastinum: ventral to anterior cardiac border and aortic root - thymic epithelial tumors and cysts, germ cell neoplasms, lymphoproliferative lesions, retrosternal thyroid glandular proliferations, parathyroid lesions, aorticopulmonary-type paragangliomas, lymphangioma, hemangioma, lipoma

Middle mediastinum: between (a) anterior cardiac silhouette and aortic root and (b) posterior tracheal carinii - pericardial cyst, bronchogenic cyst, lymphoma

Posterior mediastinum: dorsal to large conducting airways - neurogenic tumors: schwannoma, neurofibroma, ganglioneuroma, ganglioneuroblastoma, MPNST, neuroblastoma, paraganglioma, gastroenteric cysts

Images: middle and posterior mediastinum

 

Acquired hypogammaglobulinemia or pure anerythrogenesis: thymoma, leukemia, lymphoma

Myasthenia gravis: thymic hyperplasia, thymoma

Superior vena cava syndrome: usually indicates malignancy, most commonly metastatic lung carcinoma and lymphoma in adults and lymphoma and acute leukemia in children; also fibrous mediastinitis

Systemic symptoms (weight loss, fever, night sweats): lymphoma, Castleman’s disease, microcystic anemia

 

 

Inflammatory disorders

Acute mediastinitis

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Usually in posterior mediastinum, due to traumatic perforation of esophagus or descending infection along prevertebral fascia

Initial lesion may be a neck abscess

Often causes mediastinal abscess which requires surgical drainage

Other causes: chest wall infection or post-cardiac surgery, often due to CMV

Micro images: fibrinoinflammatory infiltrate #1, #2

 

Chronic mediastinitis

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May compress superior vena cava and simulate malignancy

Usually anterior to tracheal bifurcation

Some cases may represent fibrosing mediastinitis

Micro: granulomas, fibrosis; may be fungus, Histoplasma (with thick fibrous capsule), mycobacteria (thin fibrous capsule)

 

Sclerosing mediastinitis

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Also called idiopathic mediastinal fibrosis

A fibroinflammatory lesion of mediastinum associated with other idiopathic fibrosing conditions such as retroperitoneal fibrosis, sclerosing cholangitis, Riedel’s struma, inflammatory pseudotumor of orbit

Also associated with pulmonary or mediastinal nodal infection due to Histoplasma or other fungi; trauma, syphilis, methysergide treatment, phlebitis

Usually anterosuperior mediastinum, presenting at any age, often with superior vena cava syndrome or cardiorespiratory compromise

Xray: asymmetric mediastinal widening with projection of mass into upper lung field

Treatment: steroids, surgical excision

Gross: dense white fibrosis, well demarcated from surrounding tissue

Micro: different patterns - cellular fibrous reaction with polymorphic inflammatory infiltrate with plasma cells and eosinophils; dense, fibrohyaline tissue with focal spindle cells and inflammatory cells or scattered lymphoid follicles with occasional dystrophic calcification

DD: fibrotic component of Hodgkin’s lymphoma, mesothelioma, metastatic carcinoma

References: Mod Path 1999;12:257

 

 

Thyroid/parathyroid lesions

Thyroid nodular hyperplasia

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Presents as either a large mass causing compression or multiple independent nodules

 

Parathyroid adenoma

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7% are found in superior mediastinum, grow larger than counterparts in neck

 

 

Thymus

Thymus-normal

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Embryology: primordial thymus appears at week 6 from third branchial (pharyngeal) pouch

Descends from high in neck with inferior parathyroid glands; lymphocytes invade at 10 weeks

Has critical role in development of cell mediated immunity and T cell differentiation

Grows until puberty, then involutes and undergoes fatty replacement, although still present in adult prepericardial or retrocarinal fat

Gross: lobulated organ covered by a capsule, with cortical (outer and deep) and medullary compartments; may resemble lymph nodes (if no epithelial cells) or tumors (if no lymphocytes)

Gross drawing: fetal thymus

Micro: composed of epithelial cells (endoderm, important for T lymphocyte differentiation), Hassall’s corpuscles (regressed epithelial cells), T cells of varying phenotypes; B cells in thymic medulla and thymic perivascular space (increasing with age, Hum Path 2001;32:926); also interdigitating reticulum cells, Langerhans cells, mast cells, eosinophils, stromal cells

Micro images: image, islands of lymphoid tissue and fat

Positive stains: epithelial cells - keratin, HLA-DR

Virtual slides: normal thymus gland (bottom slide)

 

Acute thymic involution

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Due to stress (chronic debilitating disease), HIV or other infections

Seen in newborn infants with chorioamnionitis and sepsis

Micro: preservation of lobular architecture and Hassall’s corpuscles, but marked lymphocyte depletion (particularly with HIV); vessels are large compared to size of lobules

HIV patients also have effacement of corticomedullary junction and inconspicuous Hassall’s corpuscles

DD: thymic hyperplasia, thymic dysplasia

References: Hum Path 2000;31:1121

 

Ectopic thymus

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Remnants, implants or accessory nodules may appear from angle of mandible to thyroid gland, most commonly at level of thyroid gland

Usually an incidental finding during thyroid surgery in preteens, very rare in adults due to thymic involution

Rarely becomes hyperplastic or neoplastic

Case reports: anterior neck mass in 22 year old woman (Archives 2001;125:842), infant with neck mass (Archives 2001;125:278)

Xray images: MRI of neck mass

Micro: normal appearing thymic tissue

Micro images: ectopic thymus adjacent to thyroid and parathyroid tissue, FNA of neck mass, keratin

 

Ectopic tissue in thymus

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Usually parathyroid tissue or sebaceous glands

 

Myasthenia gravis (MG)

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Defect in nicotinic acetylcholine receptor present in subsynaptic membrane of neuromuscular junction (at motor end plate), due to circulating autoantibodies to receptor

Acetylcholine receptor also present in normal thymus, in myoid type cells

MG may be due to T cells attacking myoid cells, then T cells induce B cells to produce autoantibodies; physiological connection with thymomas is unclear

12% of MG patients have other autoimmune diseases, including Graves’ disease, rheumatoid arthritis

MG patients with thymomas may have autoantibodies to titin or other striated muscle antigens

65% of cases have thymic hyperplasia, 25% normal thymus, 10% thymomas

Present or develops in 30-45% of patients with thymomas, usually months/years after excision of thymoma

Lymphoid follicles in thymoma or adjacent thymus indicates higher risk for MG

Risk factors for thymomas: male, initial MG symptoms after age 50 years

MG associated thymomas are morphologically similar to non-MG associated thymomas

Treatment: thymectomy (regardless of presence of thymoma)

DD: Lambert-Eaton syndrome (muscle weakness due to antibodies to neuronal calcium channel)

 

Thymic dysplasia

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Congenital thymic alteration due to developmental arrest

Associated with severe combined immunodeficiency syndrome, ataxia-telangiectasia, chromosomal instability syndromes, Nezelof syndrome, incomplete form of DiGeorge syndrome (complete form of DiGeorge syndrome has absent thymus)

Gross: small thymus size (< 5g)

Micro: tubules and rosettes of primitive appearing epithelium without segregation into cortical and medullary regions; no Hassall’s corpuscles, no/rare lymphocytes

DD: acute thymic involution

 

Thymic follicular hyperplasia

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Defined as substantial numbers of lymphoid follicles in thymus of adults

Thymus usually has normal size / weight

Present in 65% with myasthenia gravis; also associated with hyperthyroidism, Addison’s disease, SLE, early HIV, multilocular cysts, other immune-related diseases

Micro: follicles with germinal centers, medullary epithelial cells may be disordered or hypertrophied

DD: normal lymphoid follicles of infants/children (few present)

 

True thymic hyperplasia

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Thymus larger than normal limits for age, based on tables

Otherwise histologically unremarkable

Often in infants or children, or in adults after cancer chemotherapy

 

 

Cystic lesions

Cystic lesions-general

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10-15% of radiologically detected mediastinal masses

Often developmental

Thymic, pericardial, bronchogenic, enteric, parathyroid cysts or mixtures

 

Bronchogenic cyst

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Due to developmental defect from fusion of tracheoesophageal septum

Along tracheobronchial tree, usually posterior to carina

Childhood or middle life

Usually asymptomatic

Xray: round/oval mass that molds to adjacent structures; wall may contain linear calcifications; may have independent vascular supply

Gross: unilocular or multilocular with internal septation; contain viscous or turbid fluid

Micro: resemble normal bronchi; lined by respiratory-type epithelium (pseudostratified columnar, often ciliated) with underlying fascicles of smooth muscle and mature cartilage; no cholesterol granulomas; may have extensive squamous metaplasia

DD: mature teratomas

 

Enteric cyst

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Posterior mediastinum of children or teens

Due to developmental defect from fusion of tracheoesophageal septum

Associated with vertebral anomalies (hemivertebrae, spina bifida)

Leaky cysts are associated with pleural effusion or pulmonary consolidation

Paraesophageal cysts are associated with dysphagia or subnormal weight gain

Gastroesophageal cysts are associated with cough, vomiting, fever, pneumonia, empyema

Gastric cysts may produce acid and rupture or hemorrhage

Gross: 2-10 cm, rounded or irregular, fibromuscular wall of variable thickness, usually unilocular but may be multiloculated; smooth inner lining; often mucoid contents

Micro: squamous, simple columnar, pseudostratified columnar or mixed epithelial lining, usually with some gastric glandular mucosa, overlying a double layer of smooth muscle; no cholesterol granulomas, no cartilage

 

Lymphangioma

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Also called cystic hygroma

Benign proliferation of lymphatic channels

Common; in anterior, middle or posterior mediastinum or soft tissue of neck

Usually in children, often with cervical component

Large lesions may compress lungs, heart, nerves, but most lesions are asymptomatic and found on Xray

Xray: appear to infiltrate mediastinal soft tissue, variegated appearance

Gross: gray-white masses, edematous appearing, variable sized cystic cavities, serous type fluid, smooth inner lining

Micro: large, irregular vascular spaces lined by flattened, bland epithelial cells with fibroblastic or collagenous stroma; variable lymphocytic infiltrates; no specialized epithelium, no cholesterol granulomas

 

Meningocele - cystic

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Cystic lesions occur in infants or children in posterior mediastinum

Communicate with meninges, usually through a defect in vertebral bodies

Contain clear/amber cerebrospinal fluid

Usually incidental findings identified prior to surgery

Micro: thick fibrous wall, lined by flattened arachnoid cells; variable neural tissue, calcification

 

Pancreatic pseudocyst

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Rarely is present in mediastinum

 

Parathyroid cyst

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Derived from third and fourth branchial pouch, as is thymus

Cysts may be present in low cervical or anterosuperior mediastinum

Pure cysts are present at any age; contain high levels of parathyroid hormone; can diagnose by FNA via PTH in fluid, AJCP 1986;86:776

Patients usually are normocalcemic and present with an asymptomatic mass on Xray

Gross: 1-10 cm, unilocular, thin walled, clear fluid, no nodules

Micro: lined by flattened parathyroid chief cells, oxyphils, clear cells; cyst wall has uniform thickness; may contain granular material resembling colloid; no cholesterol granulomas, no cartilage, no smooth muscle

Positive stains: glycogen, parathyroid hormone, chromogranin A

Negative stains: thyroglobulin

DD: parathyroid adenomas with secondary cystic change or heterotopic salivary gland like tissue, AJSP 2000;24:837

 

Pericardial cyst

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Usually at right cardiophrenic angle, adherent to pericardium and diaphragm; may communicate with pericardial cavity

Usually asymptomatic patients of all ages anywhere in mediastinum; rarely associated with dyspnea or chest pain

Due to failure of one of multiple disconnected lacunae to merge with the others

Xray: irregular, adjacent to cardiac contour

Gross: thin fibrous wall, unilocular, collapses when opened, smooth cyst lining, serous fluid contents

Micro: fibrous tissue lined by bland mesothelium, rarely with papillary hyperplasia, no cholesterol granulomas, no smooth muscle, no cartilage, no specialized epithelium

Positive stains: keratin

 

Seminoma - cystic

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Cysts are due to degenerative changes

Usually in anterosuperior mediastinum

Associated with Klinefelter’s syndrome (XXY), hematologic abnormalities

Gross: variable cystic contents, unilocular or multilocular

Micro: seminoma cells present in cyst wall, but may not be numerous; have glycogen rich cytoplasm with marked retraction, coarse chromatin with prominent nucleoli, variable mitotic figures; subdivided by fine fibrous trabeculae; associated with germinal centers, giant cells; no cholesterol granulomas (may have other granulomas), no cartilage, no smooth muscle

Positive stains: PAS, PLAP

 

Teratoma - cystic

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Common - 10-20% of mediastinal lesions

In anterosuperior mediastinum, usually children or young adults (mean age 20 years)

Neoplastic, not a developmental malformation

Usually contain tissue derived from at least 2 of 3 germ cell layers - endoderm, mesoderm, ectoderm

50% have symptoms of cough, dyspnea, chest pain

May be associated with Klinefelter’s syndrome (XXY)

Tumors adhering to lung, pericardium or blood vessels are either malignant or ruptured mature teratomas with inflammatory reaction

Rarely associated with acute myelogenous leukemia

Gross: cystic with variable solid component; malignant tumors are often adherent to adjacent structures or have necrosis; benign tumors often have keratinous debris, cartilage or mucus

Micro: mature squamous epithelium with cutaneous appendages (commonly), GI columnar epithelium, neuroglia, bone, cartilage, fat, striated muscle; also choroid plexus, hepatocytes, pancreas, retinal-type tissue; may contain immature neuroepithelial tissue (see below); no cholesterol granulomas

 

Immature teratoma

Contains immature neuroepithelial tissue such as embryonic tubules

Benign behavior in patents under age 15

 

Mature teratoma with malignant transformation

Very rare

Contain overtly malignant tissue

Tumors with germ cell components are classified as malignant mixed germ cell tumors

 

Thymic cyst

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Derived from third and fourth branchial pouch, as is parathyroid gland

Usually ages 20-50 years, present as incidental masses in anterosuperior mediastinum

Congenital (unilocular) or acquired (multilocular)

Xray: rounded, circumscribed masses in anterior mediastinum, may have peripheral rim of calcification

May be associated with mediastinal Hodgkin’s lymphoma, but not non-Hodgkin’s lymphomas

Rarely occur post-operatively

Gross: up to 18 cm; unilocular with thin wall and serous fluid or multilocular with turbid, cheesy or hemorrhagic material, thick wall and fibrous adhesions; either centered in thymus or connected to it by a small pedicle

Micro:

unilocular cysts: have thin wall with a few layers of bland squamoid cells and thymic tissue in wall, no inflammation, no cholesterol granulomas, no hemorrhage

 

multilocular cysts: may have more layers of squamoid, cuboidal, columnar, micropapillary or mixed glandular epithelium; may have pseudoepitheliomatous hyperplasia; usually cholesterol granulomas; commonly lymphocytes, granulation tissue, hemorrhage; cysts separated by thick fibrous septae; 50% have Hassall’s corpuscles or other thymic tissue, but not in cyst wall; no cartilage or smooth muscle is present

DD: cystic degeneration in Hodgkin’s lymphoma, seminoma, thymomas; cystic lymphangioma

 

Mixed multilocular thymic cyst

Parathyroid or salivary gland tissue present

 

Proliferating multilocular thymic cyst

Resembles cutaneous proliferating epidermoid cyst and proliferating trichilemmal cyst

Micro: pseudoepitheliomatous hyperplasia of cyst lining cells (narrow tongues of squamoid epithelium extending deeply into fibrous cyst wall) with reactive changes but no dysplasia; typical mitotic figures present

DD: squamous cell carcinoma (extremely rare in thymic cysts)

 

Thymomas - cystic

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Cystic degeneration of thymomas in anterosuperior mediastinum

Associated with paraneoplastic syndromes

Gross: encapsulated, uniloculated or multiloculated; variable cystic contents

Micro: bland proliferation of thymic epithelium (spindle or polygonal cells) present in cyst wall; usually no cholesterol granulomas, no cartilage, no smooth muscle

DD: thymic cyst (no thymic epithelial cells in cyst wall)

 

 

Tumors

Tumors-general

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Macroscopic invasion into lung, pericardium or vessels is usually associated with the potential for invasive growth, although not necessarily malignancy (thymomas, fibromatosis, fibrosing mediastinitis)

Hemorrhage and necrosis are usually associated with malignancy

Encapsulation is associated with indolent processes (benign cysts, thymomas)

Stromal bands are associated with thymomas (bands intersect at right angles) or lymphoma (bands intersect obliquely)

Sternberg’s approach to mediastinal tumors: cystic or not, atypical cells or not, if malignant-either small cells, large polygonal cells, mixed small and large polygonal cells, spindle/pleomorphic cells, myxoid-adipose

 

 

Thymoma and related entities

Thymoma

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Most common primary anterior mediastinal neoplasm

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

Commonly ages 49-62 years in adults

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

Spindle cell histologic patterns have indolent behavior, may be associated with hematologic malignancies

Non-spindle cell thymomas are also called cortical thymomas

Site: usually anterosuperior mediastinum; rarely posterior mediastinum, neck, thyroid, pulmonary hilum, lung parenchyma, pleura

Poor prognostic factors: high stage, B3 or C classification (see below), positive margin, invasion of capsule (AJSP 2002;26:1605)

Hemorrhage and necrosis in well encapsulated, noninvasive thymomas don’t affect prognosis (AJSP 2001;25:1086)

Case reports: Case of the Week #99 (B1 type), with SLL/CLL (Archives 2003;127:E76), invasive thymoma with cancer associated retinopathy (Hum Path 2003;34:717)

Treatment: surgical excision; possibly chemotherapy or radiation

Gross: 80% encapsulated, 20% are infiltrative into surrounding structures; usually large unless incidental with coronary bypass surgery; cystic degeneration common; multinodular, yellow-gray; sharp lobulations due to fibrous bands with some nodules having pointed ends

Gross images: thymoma

Micro: 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

Micro images: thymoma and SLL/CLL; see also subtypes below

Virtual slides: thymoma

Positive stains: keratin (epithelial cells), CEA, CD3 (lymphocytes outside lymphoid follicles are T cells), S100 (interdigitating reticulum cells), Ki-67, EMA (some tumors)

Negative stains: vimentin, CD70 (AJSP 2000;24:742)

Cytogenetics: rarely alterations of #6 (Archives 2000;124:1714)

EM: branching tonofilaments, complex desmosomes, elongated cell processes, basal lamina

DD: thymic cyst, thymic carcinoid (well formed rosettes), lymphoma, seminoma, solitary fibrous tumor (see also below)

 

Thymoma classification

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Recently discussed by Suster and Moran (Am J Clin Pathol 2006;125:542)

For prognostic purposes, must distinguish types with benign behavior (types A, AB, B, B1 and B2), from those with atypical behavior (type B3, Ann Thorac Cardiovasc Surg 2005;11:367) and those that are cytologically malignant (type C, also called thymic carcinoma, AJSP 2002;26:1605)

 

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 non-neoplastic 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

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 - carcinoma

 

Thymoma subtypes (some)

Lymphocyte predominant

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WHO type B1

Micro: 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

B1 type - #1#2#3#4#5CD3CD20keratinCD68 

Positive stains: keratin demonstrates finely arborizing network of interconnecting epithelial cell processes; CD1, CD2, CD3, CD99, bcl2, TdT

EM: well-formed intercellular junctions between epithelial cell processes, numerous tonofilaments

DD: thymic lymphoid hyperplasia (normal cortical and medullary glandular distinction is maintained, well-formed germinal centers present, does not produce a mass), Castleman’s 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), Burkitt’s lymphoma (HIV+, different nuclear histology)

 

Spindle cell

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WHO type A

Excellent prognosis

May be associated with hematologic malignancies

Rosai believes composed of nonfunctional, postmature thymic epithelial cells that match epithelial cells of involuted thymus in adult life, not cortical or medullary cells

Gross: usually encapsulated or minimal capsular invasion

Gross images: well circumscribed with lobulated cut surface

Micro: epithelial predominant with fusiform epithelial tumor cells; gland like spaces, storiform patterns are common; no/rare lymphocytes

Micro images: spindle cell thymoma 

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 CAM5.2+, keratin+, CD20-; in one study, all were incidental on chest Xray or during coronary artery bypass surgery; not associated with autoimmune disorders

Negative stains: CD5 (in epithelial cells)

References: AJSP 2001;25:111, AJSP 1999;23:955

 

Microscopic thymoma

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Normal sized or enlarged glands removed from adults for myasthenia gravis or as incidental findings during coronary bypass surgery

Micro: nodular aggregates of bland cells subdivided by fibrous bands

 

Thymoma staging

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Staging (Masaoka)

I-totally encapsulated grossly and microscopically (includes microscopic invasion into but not through the capsule)

IIA-microscopic invasion through capsule but totally excised

IIB-transcapsular infiltration into thymus, mediastinal soft tissue or pleura

III-invades pericardium, great vessels, lungs

IVA-seeds pericardial or pleural surfaces multifocally

IV-distant metastases

 

Substaging (Haniuda)

II-p0: no adhesion to pleura