Ectopic pleomorphic adenoma.

We describe a case of pleomorphic adenoma of minor salivary glands occurring in the retromolar area. A 58-year-old male patient reported with complaint of a small mass in the left lower retromolar region of the mouth. It has grown slowly to the present size. Based on the history and clinical findings, differential diagnosis of fibroma, minor salivary gland tumor, and mucocele were considered. The microscopic findings revealed the presence of minor salivary gland tissue and the lesions showed the presence of myoepithelial and epithelial cell proliferation. The cells were arranged in ductal pattern. There were areas where the epithelial cells showed proliferation in sheets, cords, and strands. The patient was followed up for a period of 6 months and there was no evidence of recurrence. The retromolar area being affected as in the case presented is rare for this condition. Surgical excision along with surrounding normal tissue is the treatment of choice.

Introduction

Minor salivary gland tumors constitute less than 20% of all salivary gland neoplasms. Over 50% of intraoral salivary gland tumors occur on the palate. Pleomorphic adenoma is one of the most common benign tumor affecting salivary glands. It accounts for 53–77% of parotid tumors, 44–68% of submandibular tumors, 6.4% occur in the minor salivary glands. Pleomorphic adenoma arising from minor salivary glands in the nasopharynx and parapharyngeal space have also been reported although in rare instances.[1] It appears as a unilateral painless, slowly growing, firm mass typically in adults from the 3rd to 5th decades and 60% of the cases are reported in females.[23] The other intraoral sites are lips, buccal mucosa, tongue, floor of the mouth, and retromolar trigone (0.7%).[4]

Case Report

A 58-year-old South Indian male patient came to the Department of Oral Medicine and Radiology, with the complaint of a small mass in the left lower retromolar region of the mouth. It was present from the past 6 months and has grown slowly to the present size. The patient's medical history was noncontributory. The patient also gave a history of occasionally biting on the mass causing an ulcer.

On intraoral examination, there was a 2×2 cm dome-shaped mass on the left retromolar region. The surface was smooth and the covering mucosa showed an ulcer in the anterior part of the lump. On palpation, the lesion was firm, nontender, and mobile over the underlying tissues.

No discharge was present. The overlying mucosa color was normal except on the traumatized area [Figure 1].

Figure 1

Intraoral appearance of the swelling in the left mandibular retromolar region. Note the traumatized area on the surface of the swelling

Based on the history and clinical findings, differential diagnosis consisting of fibroma, minor salivary gland tumor, and mucocele were considered. Wide excision of the lesion with primary closure was carried out. The excised specimen was sent for histopathological examination. The microscopic findings revealed the presence of myoepithelial and epithelial cell proliferation. The cells were arranged in ductal pattern [Figure 2]. There were areas where the epithelial cells showed proliferation in sheets, cords, and strands. Hyalinized areas were also seen. Minor salivary gland tissue was observed and the lesion showed the presence of a capsule. A diagnosis of pleomorphic adenoma of minor salivary glands (retromolar area) was rendered. The patient was followed up for a period of 6 months during which satisfactory healing of the surgical site was observed and no evidence of recurrence was noticed.

Figure 2

Photomicrograph showing epithelial and myoepithelial cells in ductal pattern with mucin

Discussion

The minor salivary glands are small, independent, predominantly mucous glands that are found in every part of the upper respiratory tract. The majority of the minor salivary glands are located in the palate, while the others are found in the submucosa of the uvula, inner surface of the lips, around the opening of the parotid duct, in the mucous membrane of the cheek, floor of the mouth, palatoglossal folds, superior pole of the tonsils (Weber's glands), on the inferior surface of the tongue, near the frenulum of tongue, and within the palatine tonsil.[5]

Pleomorphic adenomas are benign heterogenous tumors of salivary gland origin. They are made up of myoepithelial and epithelial components. This tumor of the salivary glands is also known as a benign mixed tumor. In most cases, these tumors arise in the major salivary glands (e.g., the parotid). When they occur in the minor salivary glands, the most common site is the palate, followed by the lip, buccal mucosa, floor of mouth, tongue, tonsil, pharynx, retromolar area, and nasal cavity.[5] Pleomorphic adenoma arising in the retromolar area is of rare occurrence.[6] The palate has the highest concentration of minor salivary glands in the upper aerodigestive tract, and it is the most common site for benign and malignant minor salivary gland tumors.[7] Intraoral pleomorphic adenoma usually presents as a mobile slowly growing, painless firm swelling that does not cause ulceration of the overlying mucosa.[8] It usually presents as a firm, rubbery submucosal mass.[3] In our case an ulcer was noticed on the surface of the mass due to trauma from the opposing tooth.

A study conducted on a Mexican sample, showed that it was 64.2% of benign neoplasms and only 35.8% were malignant. Usual location was palate and cases reported in the retromolar area are malignant and submandibular tumor were benign with a slight female predominance.[9] Variants of pleomorphic adenoma include pleomorphic adenoma with lipomatous change, myxolipomatous pleomorphic adenoma, pleomorphic adenoma with squamous differentiation, and benign metastasizing mixed tumor.[10] There are three histologic subtypes, myxoid (80% stroma), cellular (myoepithelial cells predominant), and mixed (classic).[3] Lipomatous or myxomatous tissue was not observed in our case.

Pleomorphic adenoma is known to produce recurrence either due to spillage, inadequate removal or enucleation at the time of operation, but is not known to produce distant metastasis.[11] A recurrence rate of 2–44% has been reported in the literature.[3] The ideal treatment of choice for pleomorphic adenoma is wide local excision with the removal of periosteum or bone if they are involved.[3]

Rarely, a malignant tumor may arise within this tumor, a phenomenon known as carcinoma ex-pleomorphic adenoma. The malignant transformation in the pleomorphic adenoma has been linked to recurrence and multiple excisions. This has been reported to occur in 2-7% of cases. The most reliable features to determine malignancy include an infiltrative growth pattern, vascular permeation, perineural invasion, and marked cytologic atypia with abnormal mitotic figures.[12] There is a second class of tumors, which are called metastasizing benign mixed tumors.[13] Pleomorphic adenoma is not an uncommon salivary gland tumor. This affects more commonly the parotid gland. However it can also arise from minor salivary glands.