A Rare Case of Mixed Type A Thymoma and Micronodular Thymoma with Lymphoid Stroma

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Micronodular thymoma with lymphoid stroma (MNTLS) is a rare subtype of thymoma and accounts for only 1%–5% of all thymoma. 1 Histologically, MNTLS shows characteristic segregating epithelial components in rich lymphoid stroma. Though the histogenesis of MNTLS is not yet elucidated, it is postulated that type A thymoma and MNTLS are common in origin. Here we report a case of mixed thymoma composed of type A thymoma and MNTLS. A 63-year-old man presented with a mediastinal mass incidentally found on routine chest radiograph during a regular health checkup. He had a history of hypertension and diabetes treated with medication. There was no evidence of myasthenia gravis. On further evaluation by chest computed tomography (CT), a 7.3-cm mediastinal mass was identified in the right anterior mediastinum (Fig. 1A). The mass had a lobulated contour , and intratumoral septation suggested a thymic epithelial tumor. No enlargement of mediastinal lymph nodes or pleural seeding was found on CT. He received extended thymectomy via median sternotomy. During the operation, there was no pleu-ral adhesion or invasion into surrounding structures. On cut sections, the tumor measured 7×6 cm and was enclosed by a thin fibrous capsule. Vague, tan-colored nodules averaging 1.5 cm in size were identified. These were partly separated by thin fibrous septa and areas of numerous tiny cobblestone like micronodules (Fig. 1B). On histological examination, two different types of thymoma were noted, showing partly in-filtrative growth into adjacent fat tissue (Fig. 1C). Large nod-ules seen were composed of epithelial cells with sparse lympho-cytes. Spindled tumor cells formed an organoid pattern, arranged in short fascicles and a solid sheet. The tumor cells had elongated , bland nuclei with fine chromatin and inconspicuous small nucleoli. These characteristics, together with positivity for cyto-keratin (CK) and Bcl-2, were compatible with World Health Organization (WHO) type A thymoma. The type A area was ill-defined, showing gradual transition into the rest of the tumor , which harbored a mixture of epithelial and lymphoid components (Fig. 1D). Multiple small epithelial nodules were embedded in the lymphoid stroma. The tumor cells from the smaller nodules were spindled, bland-looking and formed pseudoro-settes that had elongated nuclei with fine chromatin and inconspicuous nucleoli, similar to type …