Comparative clinicopathological and immunohistochemical study of micronodular thymoma and micronodular thymic carcinoma with lymphoid stroma

AIM: To elucidate the clinicopathological and immunohistochemical characteristics of micronodular thymomas (MNTs) and micronodular thymic carcinomas (MNCs) with lymphoid stroma. METHODS: We examined four cases of MNTs and three cases of MNCs pathologically and immunohistochemically. RESULTS: There were prominent cystic changes infive of the seven cases. The neoplasms contained epithelial tumour cells arranged in a micronodular growth pattern lined by cystic walls and separated by abundant lymphoid stroma. Only the tumour cell component of MNCs showed signs of malignancy characterised by cytological atypia and increased mitotic activity. Neoplastic MNC epithelial cells showed strong positivity for CD5 and CD117. However, no immature lymphocytes (TdT-positive and CD99-positive) were present in and around the tumour nodules. None of the patients died or suffered from disease due to MNTs or MNCs. CONCLUSION: MNTs and MNCs are rare and less aggressive forms of thymic tumours and can be differentially diagnosed by immunohistochemistry.