Subtitle: Thymoma types in the WHO 2021 classification: type A, AB, B1, B2 and B3, micronodular, metaplastic and lipofibroadenoma, key histology, TdT and IHC, GTF2I, myasthenia gravis, thymic carcinoma and 10 MCQs.
Author: PathologyMCQ Editorial Board
Category: Lung and Thoracic Pathology
Last Updated: October 2026
Read Time: 7 minutes
At-a-Glance
- Thymomas are typed by epithelial cell shape (spindle vs polygonal) and the number of immature TdT+ T cells.
- Type A and AB carry GTF2I mutations; type A has the best outcome.
- B1 → B2 → B3: epithelial cells increase and lymphocytes decrease; B3 is most aggressive.
- Rare types: micronodular, metaplastic (YAP1::MAML2) and lipofibroadenoma.
- Thymic carcinoma: CD5 and CD117 positive, no TdT+ lymphocytes.
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Summary Chart
The Types One by One
Type A: bland spindle or oval epithelial cells in solid, storiform or rosette-like patterns, with few or no TdT+ lymphocytes. An atypical type A variant shows necrosis or increased mitoses.
Type AB: type A areas mixed with lymphocyte-rich areas full of TdT+ T cells (more than 10% of the tumour).
Type B1: thymus-like (organoid), lymphocyte-rich cortex with medullary islands and sometimes Hassall corpuscles; epithelial cells are single, not clustered.
Type B2: still lymphocyte-rich, but epithelial cells are more numerous and form clusters of 3 or more contiguous cells.
Type B3: sheets of polygonal, mildly atypical epithelial cells with few lymphocytes, perivascular spaces and palisading (arrow).
Metaplastic thymoma: biphasic anastomosing epithelial islands and bland spindle cells (dark and light areas); YAP1::MAML2 fusion.
Micronodular thymoma with lymphoid stroma: nodules of spindle epithelial cells in a lymphoid stroma rich in B cells with follicles. Lipofibroadenoma: fibroadenoma-like with hyalinised stroma, fat and thin epithelial strands.
Comparison Table
| Type | Epithelial cells | TdT+ lymphocytes | Key point |
|---|---|---|---|
| A | Spindle or oval | Absent or few | GTF2I; best outcome |
| AB | Spindle plus lymphocyte-rich areas | Many in B-like areas (>10%) | GTF2I |
| B1 | Single scattered, plump | Abundant | Thymus-like; medullary islands |
| B2 | Clusters ≥3 cells, plump | Abundant | Myasthenia common |
| B3 | Sheets of polygonal cells | Few | Palisading; most aggressive thymoma |
| Micronodular | Spindle nodules | Few inside nodules | B-cell follicles in stroma |
| Metaplastic | Biphasic epithelial and spindle | Few | YAP1::MAML2 |
| Lipofibroadenoma | Thin strands | Few | Fibroadenoma-like with fat |
IHC and Thymic Carcinoma
| Marker | Thymoma | Thymic carcinoma |
|---|---|---|
| CK19, p63/p40, PAX8 | Epithelial cells positive | Positive |
| TdT, CD1a, CD99 (lymphocytes) | Immature T cells present (except A) | Absent |
| CD5, CD117 (KIT) | Usually negative | Positive in most squamous types |
| GLUT1, MUC1 | Negative or weak | Often positive |
Myasthenia gravis occurs in about a third of thymomas (mostly B2, B3, AB) but is rare in thymic carcinoma.
Case Question
Test Yourself: 10 MCQs
Thymoma types: 10 MCQs
A, AB, B1–B3, rare types and thymic carcinoma, with instant explanations.
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Exam Pearls
- A = spindle, few lymphocytes, GTF2I; AB = A + lymphocyte-rich areas.
- B1 thymus-like; B2 epithelial clusters ≥3; B3 epithelial sheets with palisading.
- Thymic carcinoma: CD5 and CD117 positive, no TdT+ T cells.
- Metaplastic thymoma: YAP1::MAML2.
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References
- WHO Classification of Tumours Editorial Board. Thoracic Tumours. 5th ed. IARC; 2021.
- Marx A, et al. The 2021 WHO classification of tumors of the thymus and mediastinum: what is new in thymic epithelial, germ cell, and mesenchymal tumors? J Thorac Oncol. 2022;17:200–13.
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