Mediastinum
Last revised 4 November 2007
Copyright (c) 2003-2007, PathologyOutlines.com, Inc.
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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
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
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
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
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)
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
Presents as either a large mass causing compression or multiple independent nodules
7% are found in superior mediastinum, grow larger than counterparts in neck
Thymus
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)
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
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
Usually parathyroid tissue or sebaceous glands
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)
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
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)
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
10-15% of radiologically detected mediastinal masses
Often developmental
Thymic, pericardial, bronchogenic, enteric, parathyroid cysts or mixtures
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
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
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
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
Rarely is present in mediastinum
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
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
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
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
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)
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
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
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)
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
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; #5; CD3; CD20; keratin; CD68
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
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
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
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