A Case of Pleomorphic Adenoma Originating from Accessory Parotid Gland

Case Report

A 65-year-old male patient presented to Department of Oral and Maxillofacial Surgery Okayama University hospital with a complaint of a slowly growing cheek mass for 2 years. He has a medical history of hypertension treated with calcium channel blocker for 10 yeras and subarachnoid hemorrhage occurred in 60-year-old. The clinical examination revealed a mass that was 2.8 × 2.0 cm in the medio-distal and cranial-caudal directions, respectively. The mass was well defined, rounded margins, free from the skin and underlying structures. There were no palpable lymph nodes in the neck. The mass did not elicit pain. The salivary flow from the Stensen’s duct was decreased.

On computed tomography, the mass had a well-defined margin. A major portion was liquid in consistency, with a solid component observed at only one part (Fig. 1a). T1-weighted magnetic resonance imaging showed a region of homogenous hypointensity with well-defined margins (Fig. 1b). Short T1 inversion recovery (STIR) imaging showed a region of heterogenous hypointensity to hyperintensity, while contrast-enhanced T1-weighted imaging showed heterogenous enhancement, with the part observed as hypointensity on STIR imaging showed strong enhancement (Fig. 1c, d). The mass was located 1.5 cm anterior to the original parotid gland.

Fig. 1.

a Horizontal computed tomography images at the accessory parotid gland. b Magnetic resonance images. T1-weighted imaging. c Magnetic resonance images. Short T1 inversion recovery (STIR) imaging. d Magnetic resonance images. Contrast-enhanced T1-weighted imaging

Fine-needle aspiration cytology (FNAC) was performed (Fig. 2b) and did not indicate malignancy. Surgery was conducted under general anesthesia. 2.5-cm incision at the right buccal mucosa was made parallel to the anterior border of the mandible ramus. The identified anatomic layers included the mucosa and the buccinator muscle. A horizontal incision parallel to the buccinator muscle fibers was performed, and the tumor was found adjacent to the lateral part of the buccinator muscle. The tumor was encapsulated with connective tissue. It was easily separated from the muscle and was mainly composed of parotid gland connective tissue containing buccal fat pads (Fig. 2c). The tumor was ablated with extracapsular dissection, and the duct was excised, and duct orifice was expanded to the buccal mucosa (Fig. 2d). During operation, the facial nerve was monitored with a nerve stimulator to avoid nerve injury. The patient was discharged 4 days after surgery. There have been no signs of recurrence at 48 months postoperatively.

Fig. 2.

a Extra oral examination. b Fine-needle aspiration cytology. c Buccal incision. Tumor was easily distinguished from surrounding connective tissue. d After tumor enucleation

Histopathologically, the majority of the tumor comprised of a mucus-containing cystic structure, with the remaining part as a solid structure showing a multilobular growth pattern (Fig. 3a). The tumor cells were composed mainly of duct-like cells and myoepithelial-like cells with myxoid stroma (Fig. 3b).

Fig. 3.

a Tumor showing encapsulation and solid structure with multilobular growth pattern. b Cellular area composed of modified myoepithelial cells and ducts. c Myoepithelial cells positive for S-100 protein. d A focus of prominent squamous metaplasia

Immunohistochemically, the myoepithelial-like cells were positive for S-100 protein (Fig. 3c). Squamous metaplasia and keratin pearls were partially observed (Fig. 3d). The pleomorphic adenoma was diagnosed.

Discussion

Pleomorphic adenoma is the common tumor of the major salivary glands. It is more commonly found at the parotid gland than in the other major salivary glands (approximately 80%) [1]. The accessory parotid glands were found in approximately 21% of individuals [2]. Tumors arise at these glands comprising only 1% of all salivary tumors [3]. An estimated 50% of accessory parotid gland tumors are malignant [4]. Histologically, pleomorphic adenoma is the most common benign tumor of accessory parotid glands, with mucoepidermoid carcinoma being the most common malignant tumor [5]. Thus, all patients presenting with a mid-cheek mass should be suspected of having a malignant accessory parotid gland tumor. FNAC makes an especially important contribution to decisions regarding surgical approach.

Various surgical approaches for the primary treatment of accessory parotid solid tumors have been established: (1) a standard parotidectomy incision; (2) the intraoral approach; and (3) direct skin incision. Of these, the standard parotidectomy incision has been the most common and remains the recommended approach for complete tumor excision and prevention of facial nerve and Stensen’s duct injury, regardless of whether the tumor is benign or malignant. However, this approach is associated with a skin scar. The intraoral incision approach has the advantage of having no skin incision on the face. However, intraoral incisions must be strictly limited for use on accessory parotid gland tumors that are less than 30 mm and benign. Facial nerve injury is the most important complication to avoid the intraoral incision approach. Surgeons should use facial nerve stimulator for preservation of facial nerve function during accessory parotid gland tumor resection by the intraoral incision approach [6].

Conclusion

FNAC is a useful inspection to decide surgical approach for tumor at the accessory parotid gland.

Compliance with Ethical Standards

Conflict of interest

All authors declare that they have no conflict of interest.

Informed Consent

Informed consent was obtained from all individual participants included in the study.