Gross appearance of the cut surface of a nephrectomy specimen containing a renal oncocytoma. Note the rounded contour, the mahogany colour and the central scar.
An oncocytoma is a tumor made up of oncocytes, epithelial cells characterized by an excessive amount of mitochondria, resulting in an abundant acidophilic, granular cytoplasm.  The cells and the tumor that they compose are often benign but sometimes may be premalignant or malignant.
Oncocytoma of the Salivary Gland. This lesion presented as a lateral anterior neck mass. At surgery, it was found to be a soft 3.0 x 2.1 x 1.8 cm tumor of the submandibular salivary gland. The photo shows the characteristic dark color of an oncocytoma, a rare type of benign neoplasm, at the left side of the image (the normal lobulated salivary gland tissue is to the right).
The salivary gland oncocytoma is a well-circumscribed, benign neoplastic growth also called an oxyphilic adenoma. It comprises about 1% of all salivary gland tumors. The histopathology is marked by sheets of large swollen polyhedral epithelial oncocytes, which are granular acidophilic parotid cells with centrally located nuclei. The granules are created by the mitochondria.
Thyroid oncocytomas can be benign (adenomas) or malignant (carcinomas). Grossly, oncocytic adenomas are encapsulated, solid nodules with a characteristic brown cut surface. The gross appearance of a minimally invasive oncocytic carcinoma is indistinguishable to that of an adenoma, while widely invasive oncocytic carcinomas are obviously invasive macroscopically and display pervasive vascular invasion with multifocal involvement of the thyroid gland. There are no reliable cytologic features which distinguish oncocytic adenomas from carcinomas and the only criteria for a diagnosis of malignancy is the identification of transcapsular and/or vascular invasion.
Patients with thyroid oncocytomas present with a thyroid nodule, usually with normal thyroid function. If the tumor is big or invasive, there may be other symptoms such as difficulty swallowing or talking.