Benign clear cell thyroid tumor with "balloon cell" change: evidence suggesting its pathogenesis

A unique benign clear cell tumor of the thyroid is described. The lesion was composed of swollen thyroid acinar cells distended by large membrane-bound vesicles. This picture was identical to the "balloon cell" change seen in nevi and melanomas. Only balloon cells with many relatively normal-appearing mitochondria but few membranous organelles were noted in all blocks of this tissue. On fine-needle aspiration biopsy smears, oncocytes and transitional cells with oncocytic and clear cell features were also observed. On the basis of the light and electron microscopic findings, the described vesicles are probably degenerative as in nevi and melanomas and are associated with membranous organelles. The observation of oncocytic areas indicates the strength of fine-needle aspiration biopsy as a sampling technique. There is no evidence that these vesicles are of mitochondrial origin as in some clear cell thyroid lesions. Clear cell lesions of the thyroid are a heterogeneous group and may be benign, malignant, or metastatic.