Hide-and-Seek With Spindle-Cell-Component-Poor Metaplastic Thymoma.

Metaplastic thymoma is a rare biphasic thymic tumor with indolent behavior and recurrent YAP1::MAML2 gene rearrangement. Although the diagnosis of this tumor is usually straightforward based on hematoxylin and eosin (H&E) findings alone, cases with scant spindle-cell ("pseudosarcomatous stroma") components can be easily confused with more commonly occurring type A thymoma. We present a case of metaplastic thymoma with a sparse stroma-like spindle-cell component, discussing its histological and immunohistochemical hints and drawing attention to the visual similarity to type A thymoma. This is also the first published case of metaplastic thymoma with associated psoriasis.