AB047. Mixed type A thymoma and micronodular thymoma with lymphoid stroma: a case report of an unusual tumor and literature review

Abstract

Background: Micronodular thymoma with lymphoid stroma (MNTLS) is a rare subtype of thymoma and accounts for only 1–5% of all thymoma. Though the histogenesis of MNTLS is not yet elucidated, it is postulated that type A thymoma and MNTLS shared a common histogenesis with suspected medullar origin. To date, only five cases of mixed thymoma composed of type A thymoma and Micronodular thymoma has been adequately described in the literature. Case Description: A 76-year-old woman presented with a mediastinal mass incidentally found on routine chest radiograph during a regular health check-up. There was no evidence of myasthenia gravis. On further evaluation by chest computed tomography, an 8.3 cm mediastinal mass was identified in the left anterior mediastinum. The mass had a lobulated contour, suggested a thymic epithelial tumor. He received extended thymectomy via median sternotomy. On cut sections, the tumor measured 9 cm × 7 cm and was enclosed by a thin fibrous capsule. On histological examination, two different types of thymoma were noted, showing partly infiltrative growth into the fibrous capsule. Large nodules seen were composed of epithelial cells with sparse lymphocytes. Spindled tumor cells arranged in short fascicles. These characteristics, together with positivity for cytokeratin, were compatible with type A thymoma. The type A area showing gradual transition into the rest of the tumor, which harbored a mixture of epithelial and lymphoid components. Epithelial tumor nodules were segregated by lymphoid stroma. Tumor cells were strongly positive for cytokeratin. Lymphoid stroma was positive for CD1a. The patient was diagnosed ultimately with mixed type A thymoma and MNTLS. Conclusions: In the present case, epithelial components of type A thymoma and MNTLS showed similar histologic and immunohistochemical profiles with areas of gradual transition, suggesting type A thymoma and MNTLS share a common histogenesis.