ABSTRACT
Purpose To investigate the clinicopathologic features and differential diagnosis of oncocytic variant of medullary thyroid carcinoma (MTC).Methods Morphological,immunohistochemical findings on 3 oncocytic variant of MTC cases and electron microscopic findings on 2 of these 3 cases were studied,with review of the relevant literatures.Results The cytoplasm was abundant,eosinophilic and granular with defined margins.The nuclei were enlarged,round to oval.Prominent nucleoli were identified in some cells.Tumor cells formed sheets,trabeculae or follicles with infiltrative pattern.Immunohistochemically,thyroglobulin (TG) was negative in all 3 cases.Calcitonin was positive in 2 cases and negative in 1 case.Ultrastructurally,a large number of mitochondria and various neuroendocrine granules were found in the 2nd and 3rd cases.Conclusion Oncocytic variant of MTC is very rare with variable histopathologic appearances.It should be considered in diagnosing oncocytic lesions of thyroid.A definite diagnosis can be rendered based on comprehensive findings of the immunohistochemistry,serology study and electron microscopy.It needs to avoid misdiagnosis and missed diagnosis.