Ⅰ. INTRODUCTION

Pleomorphic adenoma (PA) is a benign mixed tumor composed of epithelial and myoepithelial cells arranged in various morphological patterns. [1] It can occur in the salivary glands, accounting for 54~76% of all salivary tumors. [2] PA located in the palatal area usually presents as a smoothsurfaced and dome-shaped mass. Being pleomorphic, histologically it can be composed of a mixture of epithelial cells and mesenchyme-like tissues. [3]

Epidermoid cysts are rare in the head and neck region. Only 1.6% of these occur in the oral cavity and about 7% occur in the head and neck region. [4] Histologically, the cysts are lined by stratified squamous epithelium and the lumen is filled with degenerating orthokeratin. [5]

Here we report a case of PA with an epidermoid cyst in the soft palate area of a 76-year-old female patient.

Ⅱ. CLINICAL REPORT

A 76-year-old female patient with a swelling on the soft palate was referred by a local dental clinic to the Oral and Maxillofacial Surgery department of Kyungpook National University Dental Hospital in August 2018. The swelling first appeared 4 years ago and was asymptomatic. An intraoral examination showed the presence of a dome-shaped, smoothsurfaced swelling, 2 cm in diameter on the right side of the soft palate. There was no history of trauma and no observable signs of inflammation around the area. On palpation, the lesion was firm and there was no pain. An unusual distribution of blood vessels was observed on the overlying palatal mucosa.

Panoramic radiographs did not show any changes in the maxilla. A computed tomography (CT) scan with contrast enhancement revealed a well-defined oval heterogeneous mass (20 × 14 × 18 mm) in the soft palate with a small calcification in the right upper aspect. (Fig. 1)

These findings suggested a diagnosis of PA. Under general anesthesia, the mass was completely excised and enucleated. The mass (24 × 23 × 12 mm) was relatively encapsulated, firm, and the cut surface showed a grayish-white color. (Fig. 2-A)

Histopathologic examination showed that the mass was relatively delineated by a fibrous capsule with diverse morphological patterns. A calcified lesion was also observed. These features represent the characteristics of PA. In addition, the presence of a cystic structure was noticeable. The cystic lining was composed of stratified squamous epithelium without skin appendages and the lumen was filled with degenerating orthokeratin debris, which are characteristic features of an epidermoid cyst. These histopathologic findings helped to make the final diagnosis of PA with an epidermoid cyst. (Fig. 2)

Following surgery, there were no complications and the postoperative healing was good.

Ⅲ. DISCUSSION

The etiology of PA is known. PA is usually asymptomatic and presents as a slow growing, well-demarcated tumor. PA occurs more frequently in 40 to 60-year-old females. [6] 85% of PAs are located in the parotid gland, while 10% occur in the minor salivary glands, and 5% occur in the submandibular glands. The most common site for PA of the minor salivary glands is the palate, followed by the lip, buccal mucosa, floor of the mouth, tongue, tonsil, pharynx, retromolar area, and nasal cavity. [7] Generally, in contrast-enhanced CT images, PA has characteristic features such as a well-defined smooth margin, heterogeneity, and high contrast between the tumor and the surrounding tissues. [8] In addition to these features, which were observed in our patient, a small calcification in the right upper aspect was also found. Therefore, using contrast-enhanced CT images, we made a preoperative diagnosis of PA. Histopathological examination showed morphologic diversity consisting of acini, cords, and thin strands of epithelial cells suspended in a stroma, which often had a myxomatous appearance. [9]

Epidermoid cysts present as nodular, fluctuant subcutaneous lesions that may or may not be associated with inflammation. Histologic examination of an epidermoid cyst typically reveals a cavity which is lined by stratified squamous epithelium resembling the epidermis. A well-developed granular cell layer is observed, and the lumen is filled with degenerating orthokeratin. [5] Not infrequently, the epithelial lining could be disrupted. When this occurs, a prominent granulomatous inflammatory reaction, including multinucleated giant cells, may be present in the cyst wall since the exposed keratin is recognized as a foreign material. [5] Although PA itself is characterized by morphologic diversity, PA with an epidermoid cyst has not been previously reported and it can be difficult to diagnose.

PA with squamous metaplasia has been previously reported by Lam et. al. and its histological feature included a lack of tumor cell growth outside the confines of the fibrous capsule. More than 95% of the epithelial element in the tumor was composed of sheets of squamous cells with the formation of a cyst. [10] In their report, multiple cystic formations were found, and there was no evidence of necrosis. Maji et. al. also previously reported PA with extensive cystic degeneration. They proposed 4 theories regarding the major mechanisms for the formation of cysts in PA. 1) focal necrosis caused by tumor cells with squamous metaplasia, 2) enlargement of duct-like structures by the increased secretion from tumor cells or salivary gland tissues, 3) hemorrhage, and 4) necrosis in the malignant tumor. [11,12] After fine needle aspiration (FNA), secondary necrosis or infarction have been reported in a few cases. [13,14] In this case, however, the tumor has no evidence of necrosis or previous trauma. On the other hand, we found one large cyst formation.

Differential diagnosis should be considered with minor salivary gland tumors, particularly mucoepidermoid carcinoma, as well as benign and malignant mesenchymal lesions such as neurofibroma or rhabdomyosarcoma. Encapsulation and mobility of nodules could be indicative of a benign tumor. [15] Nevertheless, an excisional biopsy must always be a requisite for the final diagnosis. If the incision biopsy or FNA biopsy is performed at the epidermoid cyst site, the lesion would show the presence of stratified squamous epithelium and no evidence of the stromal elements. This may lead to a misdiagnosis of squamous cell carcinoma or mucoepidermoid carcinoma.

Ⅳ. CONCLUSION

In summary, we report a very rare case of PA on the palate which contains an epidermoid cyst inside it. For instance, if the presence of epidermoid cystic element was confirmed using FNA, it could lead to a false diagnosis of malignant tumors such as squamous cell carcinoma or mucoepidermoid carcinoma. To prevent the misdiagnosis of PA with squamous components, an excisional biopsy with a cautious and systematic approach is needed.