An unusual cause of facial pain: malignant change in a calcified pleomorphic adenoma in the deep lobe of the parotid gland.

Abstract

We describe a case of a malignant change in a calcified pleomorphic adenoma of the deep lobe of the parotid gland in a patient whose initial symptom was facial pain. We also discuss the relative merits of the different imaging modalities as they apply to the parotid gland.

Introduction

Pleomorphic adenomas are the most common of the benign parotid tumors, and most are confined to the superficial lobe. (1) These tumors typically appear as slowly growing, painless lesions in middle-aged patients. Pleomorphic adenomas of the deep lobe of the parotid gland are the most common tumors of the parapharyngeal space. (2) Like most parapharyngeal masses, they tend to be clinically occult and they are often not diagnosed until late because of the lack of early symptoms. These lesions are often discovered as an incidental finding during clinical examination or radiologic imaging. As these tumors enlarge, they displace the lateral pharyngeal wall medially or compress the structures in the parapharyngeal space thereby causing hoarseness, dysphagia, or dysarthria. The presence of pain or cranial nerve palsy is highly suggestive of malignancy. (3)

In this article, we describe a case of a malignant change in a calcified pleomorphic adenoma of the deep parotid lobe that manifested as facial pain.

Case report

A 60-year-old woman sought evaluation for a 3-month history of left-sided facial pain. The pain was generally dull, but it did flare intermittently as a sharp pain. She had no other symptoms referrable to the ears, nose, larynx, or pharynx.

Examination of her face and neck did not detect any swelling. Her mouth, pharynx, larynx, nose, ears, and temporomandibular joints were normal, and her facial nerve was intact. Computed tomography (CT) of the paranasal sinuses was obtained to investigate the possibility of pathology there. CT detected a 3 x 2.5-cm soft-tissue density abutting the deep lobe of the parotid gland in the left parapharyngeal space (figure 1). The mass contained multiple calcific foci peripherally and did not enhance significantly following the administration of intravenous contrast material.

[FIGURE 1 OMITTED]

The patient proceeded to magnetic resonance imaging (MRI) for further evaluation, and MRI demonstrated an intermediate signal mass on T1- and T2-weighted imaging with low signal foci of calcification peripherally (figure 2). The margins of the mass were poorly defined. The mass displaced the lateral pharyngeal wall toward the midline and was closely related to the pterygoid muscles anteriorly, which appeared thickened. Preservation of the parapharyngeal fat plane suggested that the mass had a parotid origin. No lymphadenopatby was noted, and there was no significant enhancement of the mass following the administration of intravenous gadolinium-DTPA (diethylenetriamine-penta-acetic acid). The imaging features were consistent with calcification in a long-standing pleomorphic adenoma of the deep lobe of the left parotid gland. The poorly defined mass margins and the appearance of the adjacent pterygoid muscles suggested the possibility of malignant change.

[FIGURE 2 OMITTED]

The mass was not amenable to percutaneous biopsy because of its location, so the decision was made to excise the lesion. The mass was removed via a transparotid approach with preservation of the facial nerve. Histopathology confirmed that the tumor was an adenocarcinoma arising in a long-standing calcified pleomorphic adenoma, and there was evidence of invasion of the adjacent pterygoid muscles.

Discussion

Ultrasonography (US) is the initial imaging modality of choice for the assessment of palpable parotid lesions. Pleomorphic adenomas on US are typically rounded, well circumscribed, and hypoechoic with associated distal acoustic enhancement. (4) If US demonstrates a probable pleomorphic adenoma confined to the superficial lobe, and if the diagnosis is confirmed following US-guided fine-needle aspiration or biopsy, then no further imaging is required. (5) The deep lobe of the parotid gland, however, is not amenable to US visualization because it is largely obscured by overlying structures. In such cases, CT or MRI is required for further assessment. (6)

On CT, pleomorphic adenomas generally appear to be well circumscribed, and they exhibit homogeneous enhancement following intravenous contrast administration. Dystrophic internal calcifications occur rarely in pleomorphic adenoma; when they do, they are an indicator of chronicity. Calcifications are best visualized with CT (7); they may be occult on MRI.

In light of its multiplanar capacity and superior spatial resolution, however, MRI does have advantages over CT in the further evaluation of parotid tumors and the parapharyngeal space. On MRI, pleomorphic adenomas can have either a homogeneous or heterogeneous appearance. They are of intermediate to low signal intensity on T1 weighting and hyperintense on T2 weighting. The high signal on T2 weighting has been attributed to the presence of myxoid tissue in the tumor. (8) Pleomorphic adenomas are better delineated on T2- than on T1-weighted MRI because the signal from both the tumor and the parotid gland on T1 weighting may be of intermediate intensity. Pleomorphic adenomas usually exhibit homogeneous enhancement following gadolinium-DTPA administration, although inhomogeneous enhancement--particularly in large lesions or in areas of nonenhancement attributable to the presence of cystic foci--may be observed. (9) Longstanding lesions may not exhibit these imaging features, as in our case.

In our patient, CT and MRI detected dystrophic calcification in a deep lobe parotid mass. Although the imaging features were not typical, the diagnosis of possible pleomorphic adenoma was made because tumoral calcification is extremely rare in any other parotid tumor. (7) Although CT and MRI are useful in the characterization of lesions in the parapharyngeal space, they cannot accurately predict histology; biopsy is required. (10) When a lesion is palpable or can be seen perorally, preoperative tissue can be obtained by fine-needle aspiration or biopsy immediately following radiologic imaging. Our patient's tumor was not palpable, and we deemed it hazardous to attempt to biopsy it. Therefore, we proceeded to surgery.

In patients with atypical facial pain, a parapharyngeal space lesion should be considered in the differential diagnosis. This area should be imaged by either CT or MRI, depending on availability. Pleomorphic adenomas are the most common tumors of the parapharyngeal space. Even with apparently benign imaging features, malignant change should still be considered in the presence of pain.

References

(1.) Spiro RH. Salivary neoplasms: Overview of a 35 year experience with 2807 patients. Head Neck 1986;8:177-84.

(2.) Carr RJ, Bowerman JE. A review of turnouts of the deep lobe of the parotid salivary gland. Br J Oral Maxillofac Surg 1986;24:155-68.

(3.) Westra WH. The surgical pathology of salivary gland neoplasms. Otolaryngol Clin North Am 1999;32:919-43.

(4.) Bradley MJ, Durham LH, Lancer JM. The role of colour flow Doppler in the investigation of the salivary gland tumour. Clin Radiol 2000;55:759-62.

(5.) Buckland JR, Manjaly G, Violaris N, Howlett DC. Ultrasound-guided cutting-needle biopsy of the parotid gland. J Laryngol Otol 1999;113:988-92.

(6.) Nettle WJ, Orell SR. Fine needle aspiration in the diagnosis of salivary gland lesions. Aust N Z J Surg 1989;59:47-51.

(7.) Som PM, Shugar JM, Sacher M, et al. Benign and malignant parotid pleomorphic adenomas: CT and MR studies. J Comput Assist Tomogr 1988;12:65-9.

(8.) Tsushima Y, Matsumoto M, Endo K, et al. Characteristic bright signal of parotid pleomorphic adenomas on T2-weighted MR images with pathological correlation. Clin Radiol 1994;49:485-9.

(9.) Vogl TJ, Dresel SH, Spath M, et al. Parotid gland: Plain and gadolinium-enhanced MR imaging. Radiology 1990;177:667-74.

(10.) Freling NJ, Molenaar WM, Vermey A, et al. Malignant parotid tumors: Clinical use of MR imaging and histologic correlation. Radiology 1992;185:691-6.

From the Department of Otolaryngology (Dr. Kesse and Dr. Violaris and the Department of Radiology (Dr. Howlett), Eastbourne District General Hospital, Eastbourne, U.K.

Reprint requests: Dr. D.C. Howlett, Department of Radiology Eastbourne District General Hospital, Kings Dr., Eastbourne BN21 2UD. UK. Phone: [phone]: fax: [phone]; e-mail: [e-mail]