Type A Thymoma

Predominantly composed of spindled to oval epithelial cells
Bland oval, occasionally round nuclei
- Evenly dispersed chromatin
- Inconspicuous nucleoli
- Infrequent mitotic figures
Scant to absent lymphoid component
- May have mature or immature T phenotype
Occasional features
- Gland-like structures / cysts
  - May have distinct lining layer but no specific glandular differentiation
- Hemangiopericytoma-like staghorn vessels
- Pseudo-rosettes without lumens
- Glomeruloid bodies projecting into cysts
Typically lacks broad fibrous bands
Reticulin fibers individually surround most cells
May infiltrate into fat but invasion of adjacent structures is rare

Epithelial cells
- Keratin positive
  - May be variable, especially in background of gland-like areas
    - Gland lining typically strongly stained
- EMA negative to focally positive
- CD20 focally positive in most cases
- CD5 negative
Lymphocytes
- Most mature T phenotype
- Few with immature T phenotype (CD1a, CD99, TdT)
Few B cells

Type A Thymoma	Type B3 Thymoma
Predominantly spindle cells	Predominantly polygonal cells
Lacks any epidermoid features	Frequently has epidermoid differentiation (but lacks overt keratinization and bridges)
Frequent glandular structures lined by cuboidal to flat cells	Frequent perivascular spaces surrounded by palisaded epithelial cells
Lacks cytologic atypia	Mild to moderate cytologic atypia
Epithelial cells may be CD20 positive	Epithelial cells CD20 negative
Epithelial cells EMA and CD5 negative	Epithelial cells may be focally positive for EMA or CD5
Prominent reticulin within epithelial areas	Lacks prominent reticulin around epithelial cells

Type A Thymoma	Solitary Fibrous Tumor
Plump spindle cells with appreciable cytoplasm	Thin spindle cells without appreciable cytoplasm on H&E stain
Keratin positive, CD34 negative	CD34 positive, keratin negative
Some immature T cells usually present	Any lymphocytes present are mature T cells

Both may have hemangiopericytoma like vasculature

Type A Thymoma	Spindle Cell Carcinoma
Mild cytologic atypia at worst	Cytologically atypical
Epithelial cell mitotic figures rare	Epithelial cell mitotic figures may be frequent
Some immature T cells usually present	Lymphocytes are mature T and B cells
Usually well circumscribed, with only microscopic invasion	Usually clearly invasive

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Dorfman DM, Shahsafaei A, Chan JK. Thymic carcinomas, but not thymomas and carcinomas of other sites, show CD5 immunoreactivity. Am J Surg Pathol. 1997 Aug;21(8):936-40.
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