Micronodular thymoma with lymphoid stroma: two cases of report and literature review

Purpose To investigate the clinico-pathological characteristics and differential diagnosis of micronodular thymoma with lymphoid stroma. Methods Clinical manifestations,pathological features,immunohistochemistry,treatment and prognosis were analyzed in two cases of micronodular thymoma with lymphoid stroma with a review of the literature. Results Patient one is a 62-year-old woman and Patient two is a 56-year-old man. Both patients were asymptomatic and their thymic tumors were discovered grossly partially cystic and located in the anterior mediastinum. Histologically,the tumors were characterized by a proliferation of small neoplastic nodules separated by abundant lymphoid stroma with prominent germinal germinal centers. The nodules were composed of neoplastic spindle cells containing bland,oval nuclei without mitotic activity or necrosis. The adenoid growth pattern was noted in one tumor. Immunohistochemical studies showed strong positivity of the spindle tumor cells for CKpan,CK19,p63 and CK5 /6. Stains for EMA were negative in the spindle tumor cells. Cystic lining cells showed a strong linear immunoreativity for of EMA and foci of glandular differentiation. Many lymphocytes in the stroma,including germinal centres,were positive for CD20. The BCL-2 immunoreactivity was also detected in many lymphocytes in the stroma,especially in the mantle zone of lymphoid follicles. In contrast,many lymphocytes positive for CD3 and CD5 were evenly dispersed in the stroma. The reticulin stain decorated tumor nests,but not individual neoplastic cells,surrounded by reticulin fibers. Conclusion Micronodular thymoma with lymphoid stroma is very rare but can be diagnosed with a characteristic morphology and immunoreactivity to p63 and CK5 /6. We speculate MNT is derived from the thymic medulla.