Rare Malignant Tumors of the Parotid Glands: Oncocytic Neoplasms

Salivary gland neoplasms are a rare group of tumors; the annual incidence rate is 1 in 100,000 individual, comprising about 3% of all head and neck neoplasms [4]. The mean age of patients with salivary gland tumors is 45 years, peaking in the sixth and seventh decades of life. Benign salivary gland tumors occur more frequently in females, while malignant tumors are slightly more frequent in males. The parotid gland is the most frequent site about 70% of cases. About 80% of parotid tumors are benign and 64 to 80% of all primary salivary gland epithelial tumors involve the parotid gland, mostly located in the superficial lobe [5]. Oncocytic neoplasms comprise a group of rare tumours of the parotid glands, and their incidence represents approximately 1% of parotid neoplasms [1]. Histologically they are classified according to the new World Health Organization classification in three distinct types, namely oncocytosis, oncocytoma and oncocytic carcinoma [2]. Oncocytomas usually occur in the elderly and affect the parotid glands in 80% [3]. Pathologically, oncocytoma is described as a well circumscribed mass, composed of layers of oncocytes (small round nucleus, micro-granular, eosinophilic cytoplasm). Oncocytes are large, granular, eosinophilic epithelial cells mainly found in glandular tissue, including that of the salivary glands and thyroid. Oncocytomas that originate from oncocytes are very rare neoplasms that account for less than 1% of all salivary gland tumors [4]. Oncocytic carcinomas are even more uncommon; they represent 11% of all oncocytic salivary gland neoplasms, 0.5% of all epithelial salivary gland malignancies and 0.18% of all epithelial salivary gland tumors [5, 10]. The terms oncocytic carcinoma, oncocytic adenocarcinoma, malignant oncocytoma and malignant oxyphilic adenoma are synonymous [6]. Besides these defined neoplasms, there are other entities that are not well identified oncocytic changes of the parotid gland, such as oncocytic metaplasia, diffuse oncocytosis, nodular oncocytosis and multifocal nodular oncocytic hyperplasia and also oncocytic metaplasia within other salivary gland tumors such as the Warthin’s tumor [7]. While the diagnosis of these lesions is usually straightforward, the histologic distinction between nodular oncocytosis and oncocytoma is admittedly rather arbitrary in certain cases.