Micronodular thymoma with lymphoid stroma: As benign as it sounds.

ABSTRACT: Micronodular thymoma (MNT) is a rare subtype of thymoma (reported incidence is approximately 1-5%). We report a case received as a core biopsy from the "right lung mass" of a 31-year-old female. CT scan showed an 8.1 cm large well-defined mass lesion in the right middle lobe, causing indentation and mild compression of the right atrium. Microscopically, the biopsy showed a thymic neoplasm comprised of multiple discrete and coalescing nodules of bland epithelioid tumor cells separated by an epithelial cell-free lymphocyte-rich stroma. On immunohistochemistry, cytokeratin highlighted the thymic epithelial cells. The lymphoid cells showed a naive T-cell phenotype (TdT+ CD 3+). It is worthwhile recognizing this rare variant of thymoma as almost all the patients present with localized low stage disease with rare/no reports of recurrences or distant metastases.