Oncocytoma Presenting as a Fat-Containing Intraparotid Mass

Introduction

Neoplasms of the parotid gland represent 80% of salivary gland tumors, with 80% of these masses of benign etiology. ¹ Oncocytoma signifies approximately 1% of all salivary gland tumors, with a majority residing in the parotid gland. ² Pending tumor location, complete surgical excision via superficial or deep lobe parotidectomy with facial nerve preservation is the standard of care. ¹

Case Report

An 87-year-old woman with a history of breast cancer status post lumpectomy presented with a lump under her left ear, which she noticed 3 months prior while applying skin cream. She was asymptomatic and initially deferred further workup.

Upon developing transient ischemic attack symptoms, she underwent a head computed tomography (CT; Figure 1) at an outside facility which demonstrated enlargement of the superficial and deep lobes of the left parotid gland, with mixed soft tissue and fat attenuation. Physical examination revealed left-sided parotid fullness without discrete borders to it. There was no cervical adenopathy. Magnetic resonance imaging (MRI) demonstrated extensive, heterogenous T1 hyperintensity (Figure 2). There was suppression of these corresponding regions on fat-saturated T2 imaging and post-gadolinium imaging. The soft tissue components enhance with gadolinium relative to normal parotid parenchyma. The mass demonstrated diffusion restriction. Fine needle aspiration was performed and demonstrated groups of oncocytic cells.

Figure 1.

Axial computed tomography images on soft tissue windows demonstrate a heterogeneous mass involving the superficial and deep parotid containing both macroscopic fat and soft tissue elements. The parapharyngeal fat is medially displaced and partially obliterated. Bone windows (not shown) did not demonstrate any evidence of erosion of the adjacent mandible.

Figure 2.

Magnetic resonance imaging films demonstrate a T1 heterogenous mass with T1 areas of hyperintensity. There is loss of the normal fat signal, which appears more ill-defined and “hazy.” The mass contains internal fat signal that suppresses with frequency-specific fat saturation. The soft tissue components enhance on postcontrast images (not shown).

A total parotidectomy and parapharyngeal space removal via a cervical–parotid approach (Figure 3) was performed, and the pathology specimen revealed an oncocytoma in a background of extensive oncocytic metaplasia (Figure 4); a histochemical stain for mitochondria (phosphotungstic acid - hematoxylin stain) and immunohistochemical stain (Figure 5) for peripheral tumor cells (p63) confirmed the diagnosis.

Figure 3.

Intraoperative view shows macroscopic fatty metaplasia of the deep lobe parotid mass, located medial to the facial nerve. Of note, the pes anserinus (near probe tip) is lengthened medially due to the slow-growing pressure of the mass.

Figure 4.

Low-power microscopic examination reveals nodules of tumor with interspersed adipocytes (hematoxylin & eosin ×40).

Figure 5.

An immunohistochemical stain for p63 shows a peripheral tumor cell staining pattern, a classic feature of oncocytoma.

The patient is doing well after surgery. At 2-month follow-up visit, she has improving mild left-sided facial weakness, particularly in the mid and upper divisions.

Discussion

Oncocytoma of the salivary glands on CT imaging is characterized by solitary or multiple solid masses with sharp, well-delineated margins showing homogeneous enhancement. ³ On MRI, lesions appear T1 hypointense but isointense to the native parotid gland on fat-saturated T2 and postcontrast T1 imaging. ⁴ Oncocytoma with intratumoral fat has been described in the kidney; however, in a review of 9 cases of parotid gland oncocytomas, there is no note of extensive intratumoral fat. ^4,5 Additionally, a report of 10 parotid gland oncocytomas does not report a single case of significant fat metaplasia. ⁶ The presence of fat in CT and MRI images may suggest a lipomatous tumor such as lipoma and liposarcoma. This represents a diagnostic dilemma, as preoperative planning for a benign oncocytoma is notably different from the more aggressive liposarcoma. Physicians should be cognizant of this pathologic entity in determining surgical strategy while providing patient counseling.