| WHO type | Morphologic characteristics |
| Thymomas |
| A | - Bland oval/spindle, rarely polygonal epithelial cells growing in a fascicular, storiform, or hemangiopericytomatous pattern (less common patterns: rosettes with or without lumen, glandular or glomeruloid structures, hemangioma-like papillary projections in cystic spaces, meningioma-like whorls)
- Epithelial cells in general lack necrosis and have low mitotic activity (<4 mitoses/2 mm2)
- Rare ("easily countable") or no lymphocytes
|
Atypical A
(subtype of A) | - Type A thymoma with some degree of atypia such as:
- Increased mitotic activity
- Coagulative tumor necrosis
- Hypercellularity
|
| AB | - Types A+B1 or A+B2
- Comprised of at least 10% of B1 or B2 subtype
- Subtypes can be intermingled or separate
|
| B | - B1 → B3
- Increase in tumor cells (in relation to thymocytes)
- Emergence of atypia in tumor cells
- Presence (B1>B2) or absence (B3>B2) of medullary islands
|
| B1 | - Medullary islands
- Scattered epithelial cells (<3 contiguous epithelial cells)
|
| B2 | - Mixture of epithelial cells and lymphocytes; epithelial cells often form small clusters
- Medullary islands on occasion
- Perivascular spaces common
|
| B3 | - Polygonal epithelial cells, rare lymphocytes
- Interspersed perivascular spaces
|
| Uncommon types of thymoma |
| Metaplastic thymoma | - Biphasic morphology.
- Component A: Nests/islands of bland, darker, polygonal epithelial cells, variable nuclear atypia.
- Component B: Paler, more elongated, bland-appearing spindle cells reminiscent of fibroblasts.
- Paucity of lymphocytes.
|
| Micronodular thymoma with lymphoid stroma | - Nests/nodules of bland oval to spindle tumor cells, reminiscent of type A thymoma (→ nodular appearance of the tumor on low magnification).
- Abundant areas of reactive lymphoid cells without epithelial cells; predominantly of B-cell phenotype forming follicles with germinal centers; only scattered TdT-positive thymocytes along the interface to the tumor cell nests.
|
| Lipofibroadenoma | - Mixture of fibrous and adipose tissue. The fibrous tissue contains strands and nests of thymic gland tissue, at least in some areas in form of epithelial strands with only scattered lymphocytes. Overall reminiscent of fibroadenoma of breast.
|
| Thymic carcinomas |
- Distorted architecture; possibly single tumor cells
- Desmoplasia
- Usually more cytologic atypia and increased mitotic activity
|
| Thymic neuroendocrine neoplasms |
| Neuroendocrine tumors |
| Typical carcinoid tumor | - <2 mitoses/2 mm2
- No necrosis
|
| Atypical carcinoid tumor | - ≥2 to 10 mitoses/2 mm2 and/or
- Necrosis
|
| Neuroendocrine carcinomas |
| Small cell carcinoma | |
| Large cell neuroendocrine carcinoma | - >10 mitoses/2 mm2
- Includes neuroendocrine tumor with carcinoid morphology but mitotic count of >10 mitoses/2 mm2
|
