Oncocytoma of the salivary glands: a clinicopathologic and immunohistochemical study

We present a clinicopathologic and immunohistochemical study of 21 Chinese patients with oncocytoma of salivary gland origin, a rare benign tumour composed exclusively of large epithelial cells with eosinophilic granular cytoplasm (oncocytes). The median age was 60.1 years with a male predominance (67%). All the tumours occurred in the parotid except one in the palate. A painless mass was the most common feature, although intermittent pain was complained of in four cases. All the patients were treated by superficial parotidectomy, with no recurrence or metastasis. Histologically, most tumours displayed an encapsulated nodular growth pattern, but one case presented with an aggressive growing tendency. Typical oncocytes were observed in all cases, with one clear cell variant found. The oncocytes were arranged in solid sheets, trabecular or duct-like structures. Rarely, small foci of hemorrhage or lymphoid stroma were observed, but germinal centres were always absent. Phosphotungstic acid hematoxylin staining illustrated dark-blue cytoplasmic granules, demonstrated as mitochondria by electron microscopy. All the tumours showed immunoreactivity for CK5/6, CK8/18, CK10/13, CK19 and EMA, but were negative for SMA or S-100. MIB-1 antibody, used to identify the dividing cells by staining of the nucleus, was found to stain the cytoplasm of the oncocytes. In summary, clinical diagnosis for an oncocytoma is challenging for its similar features to other benign tumours. Histopathological diagnosis is reliable with histochemical and electron microscopic conformation of the oncocytes, but differential diagnosis is still challenging. MIB-1 immunostaining might be considered as a diagnostic aid.