Abstract:

Objective To investigate the clinicopathological characteristics of micronodular thymoma with lymphoid stroma (MNT). Methods A retrospective analysis was conducted on five MNT patients who underwent surgical resection at the Department of Thoracic Surgery, Ruijin Hospital, Shanghai Jiao Tong University School of Medicine between January 2020 and July 2023. Their clinical symptoms, pathological features, immunophenotypes, clinical treatments, and prognosis were evaluated, along with a literature review. Results Among the five MNT patients, three were male and two were female, aged 55 to 68 years. Four cases had tumors located in the anterior mediastinum and one in the superior mediastinum, all incidentally discovered by chest CT. The tumor diameters ranged from 1.3 to 4.5 cm. Three cases had intact capsules with clear boundaries and solid, gray-white, and fine-textured cut surfaces. Two cases exhibited cystic-solid morphology, while two cases showed capsule invasion. Microscopically, all cases demonstrated unique histological features of MNT: tumor primarily consisted of epithelial nodules and lymphocytes. The epithelial cells showed short spindle or oval shapes, with mild atypia, indistinct nucleoli, rare mitotic figures, and no necrosis. Lymphocytes were interspersed between the nodules, accompanied by visible lymphoid follicle formation. Immunohistochemical staining showed that tumor cells in all five cases were positive for AE1/AE3, CK19, P63, and Bcl-2. In one case, epithelial lining cells of the cystic area exhibited EMA expression but were negative for P63 and Bcl-2. Lymphocytes in all five cases showed expression of CD20, CD3, CD5, and TdT (focal +), follicular germinal centers were positive for CD10, and Langerhans cells within the nodules expressed Langerin, S100, and CD1α. EBER in situ hybridization was performed in three cases, all of which were negative for both epithelial and lymphoid cells. During a follow-up period ranging from 7 to 39 months after surgical resection, no recurrence was observed. A literature review of newly reported MNT cases from 2019 to 2023 in PubMed, Medline, China National Know-ledge Infrastructure, and Wanfang databases, along with summarized cases from 1999 to 2018 in the literature was conducted. A total of 206 MNT cases were identified. Among them, 195 were elderly patients (>45 years), 9 were young adults (18-45 years old), and 2 were children (<18 years old). The majority had tumors located in the mediastinum, although some were found in the neck. No significant gender preference was observed. Most cases were incidentally detected during physical examinations. Cystic features were more common on cut surface. Only one case experienced recurrence after surgery. Conclusion MNT is a rare tumor predominantly occurring in the mediastinum of middle-aged and elderly individuals. It demonstrates favorable prognosis after surgical resection. The pathological features of MNT include scattered epithelial nodu-les within a rich lymphoid stroma, with visible lymphoid follicle formation. The nodules contain scattered Langerhans cells and may exhibit cystic changes. A definitive diagnosis can be made based on the tumor’s location, histopathological features, and immunohistochemical markers. However, it still requires differentiation from other lesions.

Key words: Micronodular thymoma with lymphoid stroma, Immunohistochemical staining, Molecular genetics, Origin