A Rare Histologic Variant of Thymoma: Diagnostic Challenges of Micronodular Thymoma With Lymphoid Stroma

Micronodular thymoma with lymphoid stroma (MNT) is a rare subtype of thymoma characterized by multiple epithelial micronodules embedded within abundant lymphoid stroma containing prominent germinal centers. It generally exhibits an indolent clinical course and a favorable prognosis after surgical excision. We report the case of a 64-year-old woman who presented with an anterior mediastinal mass. Histopathological examination demonstrated micronodular proliferation of spindle-to-oval epithelial cells associated with dense lymphoid stroma and reactive germinal centers. Immunohistochemical study showed positivity of tumor cells for cytokeratin, while CD20-positive B lymphocytes were predominantly localized within follicles, and TdT-positive immature T lymphocytes surrounded the epithelial nests. The tumor cells were negative for CD5 and CD20. Based on the morphological and immunophenotypic findings, a diagnosis of micronodular thymoma with lymphoid stroma was established. This report highlights the characteristic histopathological features of this uncommon entity and emphasizes the importance of differentiating it from other thymic epithelial tumors and micronodular thymic carcinoma. Complete surgical excision remains the treatment of choice and is usually associated with an excellent prognosis.