203 When Rare Meets Rarer: A Mixed Thymoma Uniting the Most Uncommon Thymoma Types

Introduction/Objective Thymomas are very rare, with an incidence of 1.5 cases/million-population. Despite their indolent nature, thymomas harbor malignant potential, and their behavior correlates with the stage at presentation. Treatment is primarily surgical, with ∼10% of tumors relapsing after surgery. Thymomas are classified according to the 2021 WHO Classification into the following subtypes: A (conventional and atypical), AB, B (B1, B2, and B3), micronodular thymoma with lymphoid stroma, and metaplastic thymoma. This classification is based on histological and immunohistochemical features. Micronodular thymoma with lymphoid stroma (MTLS) accounts for <5% of all thymomas and typically manifests at early stages. Atypical type A thymoma is even scarcer, described only in small case series, and usually presents at higher stages compared to conventional type A thymoma. We present the case of a 68-year-old female with a thymoma exhibiting a combination of these uncommon subtypes, the second report of this association following Su and Zong’s 2020 report. Methods/Case Report A 68-year-old female with a history of smoking was found to have a 2.6 cm anterior mediastinal mass during an evaluation for chronic obstructive pulmonary disease. Over six years, the mass enlarged, requiring a thymectomy. On gross examination, a well-demarcated smooth and fleshy mass was identified, measuring 3.1 x 2.4 x 2.5 cm. Microscopic examination revealed a biphasic tumor. The dominant component (90%) consisted of nodules and interconnected cords of bland, oval-to-spindle epithelial cells within a lymphocyte-rich background, featuring prominent reactive germinal centers. The secondary component (10%) was a hypercellular nodule with elongated spindle-shaped cells showing moderate atypia and increased mitotic activity (up to 5 ­mitoses/2 mm²). By immunohistochemistry, all epithelial cells were positive for p63 and negative for CD5, CD117, and GLUT1. TdT stain highlighted scant thymocytes in a background of mature lymphocytes. Ki-67 proliferative index was 10% (500 cell count). These findings were diagnostic of mixed micronodular thymoma with lymphoid stroma and atypical type A thymoma. On follow-up, the patient is doing well and will continue to undergo imaging surveillance for up to 10 years to monitor for late recurrence. Results NA Conclusion We report the second case of mixed micronodular thymoma with lymphoid stroma and atypical type A thymoma, and the first documented in English literature. Thymomas are uncommon, these subtypes are even rarer, and their combination is exceptionally rare. Identifying thymomas with mixed histological patterns is important, as each subtype can have different clinical implications. Atypical type A thymoma is a new subtype in the WHO classification. Due to its rarity, clinical and pathological data, along with follow-up, are needed to better understand its biology and disease outcome. Documenting exceptional thymoma variants and maintaining interinstitutional tumor registries are essential for understanding this poorly characterized disease.