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Mediastinal mass.
Thymomas are neoplasms of the thymic epithelial cells. Thymomas usually occur in adults and are typically discovered incidentally or from mass effect on mediastinal structures. Thymomas are associated with myasthenia gravis and may be discovered on workup for the disease.
Broadly, the WHO classifies thymomas based on the morphologic characteristics of the thymic epithelial cells and based on the presence of immature thymic lymphocytes. The main subtypes of thymoma include Type A, AB, B1, B2, B3, and thymic carcinoma.
The present case represents a type A thymoma, which is characterized by oval to spindled cells growing in a predominantly fascicular and microcystic pattern. The nuclei are bland with inconspicuous nucleoli. Type A thymomas are also characterized by the absence of admixed immature lymphocytes (which can be identified by immunohistochemistry for TdT and/or CD1a).
Type A thymomas overall have excellent prognosis with a low risk of recurrence if the tumour can be completely resected.
See Related Content for references:
1) Detterbeck FC., et al. JTO 9:S65-72, 2014