Micronodular Thymic Carcinoma with Lymphoid Stroma: A Case Report

Background: Micronodular thymic carcinoma (MTC) with lymphoid hyperplasia is believed to be the malignant counterpart of micronodular thymoma (MT) with lymphoid hyperplasia. Since MT and MTC share a similar morphology, MTC is considered a malignant form of MT; there have been a few cases of malignant transformation from MT to MTC. We report a case of MTC with lymphoid hyperplasia.Case presentation: Our 53-year-old patient presented with an incidental tumor on chest X-ray. The resected tumor consisted of nodular epithelial nests and lymphoid tissue within a surrounding germinal center. Some epithelial nests showed apparent malignant morphology. Atypical epithelial cells with large vesicular nuclei formed nests, some of which showed comedo necrosis. These cells showed transition continuously to benign type B thymoma-like cells, demonstrating cord-like arrangements. Carcinomatous elements, showing a high Ki67 index, expressed Glut1, CD5, CD117, and BCL2; conversely, benign nests displayed attenuated expression of these markers and a low Ki67 labeling index. GTF2I point mutation and Langerhans/dendritic cells, which are indicators of favorable thymoma prognosis, were not detected. Due to pleural metastasis, the patient was treated with lenvatinib 27 months postoperatively. Conclusions: This is the first report of a partially benign, GTF2I-negative MTC. Histological and genetic findings might be predictive of patient prognosis.