Low-grade lymphoma derived from distinct native population of thymic postgerminal center stage parafollicular B cells

Believed to represent thymic counterpart of distinctive type of extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALToma)

Generally excellent prognosis with long-term survival

Some cases may give rise to development of diffuse large cell lymphoma

Indolent growth with protracted clinical course

Monotonous proliferation of small lymphoid cells in interfollicular distribution

Lymphoid cell population is admixed with reactive, hyperplastic lymphoid follicles

Lymphoid cells infiltrate residual epithelial structures (i.e., Hassall corpuscles), creating lymphoepithelial lesions

Residual thymic epithelium may undergo cystic degeneration resulting in multiple thymic cysts

Small lymphocytes can infiltrate surrounding perithymic fat at edges of lesion

Lymphoid cells display slightly irregular nuclei surrounded by clear cytoplasmic halos

Lymphoid cells may display plasmacytoid features

Rare mitotic figures may be present