Keratinizing pleomorphic adenoma: A rare variant in an unusual location.

Pleomorphic adenoma is the most common benign salivary gland tumour characterized by great histologic diversity. The presence of extensive squamous metaplasia and numerous keratin pearls is mostly uncommon in the microscopic study and can signify a potential pitfall in the histopathological diagnosis Pleomorphic adenoma can show the presence of squamous metaplasia with keratin pearls as a rare finding and is encountered most often in the parotid gland (84%) and 6% in the minor salivary gland. Here we present a case report of a rare histopathological variant of pleomorphic adenoma with exuberant squamous metaplasia and keratin pearl formation of the minor salivary gland in an unusual location. Copyright:

INTRODUCTION

Among all salivary gland tumors, Pleomorphic adenoma is the most common benign tumor and also accounts for a majority of all salivary gland neoplasms. Willis coined the term Pleomorphic adenoma and it derives its name due to its light microscopic findings presenting with architectural pleomorphism. It is composed of epithelial and stromal/mesenchymal components and thus derives its name mixed tumour.[1] The incidence of pleomorphic adenoma in major salivary gland is 80%–85% and 40%–50% among intraoral minor salivary glands.[23] The morphological patterns has wide variations including presence of squamous cells, mucous cells, oncocytes, sebaceous cells, bone, adipose tissue, and crystalline materials.[4] Pleomorphic adenoma occur at any age, favouring males slightly more than females. They are most prevalent in the fourth to sixth decades of life. Generally, they are mobile except when they occur in the hard palate. In vast majority of cases, they are firm, painless slow growing swelling with absence of ulceration in the overlying mucosa. The palate is the most common intraoral site, and can occur less commonly in the upper lip and buccal mucosa.[5] Here we present a case of an unusual histopathological variant of pleomorphic adenoma in an uncommon location.

CASE REPORT

A 36-year-old male patient reported to a private dental practitioner with the chief complaint of painless growth over the left cheek region for past 1 year [Figure 1]. The swelling was initially small but gradually increased to present size. Asymptomatic presentation led to a delay in reaching the hospital for diagnosis and treatment. Patient had the habit of smoking cigarettes for past 5 years with no other evidence of any other systemic complications. On examination, the intraoral growth measured 5 cm × 2 cm, circular, nontender and was not fixed to the underlying structures. A provisional differential diagnosis of a benign salivary gland tumour, lipoma or fibroma was considered. Wide local excision of the growth in the left buccal mucosa was done under local anaesthesia and the specimen was submitted to the Department of Oral Pathology for histopathological examination. The gross specimen comprised of an encapsulated soft tissue mass, measuring 1.5 cm × 2 cm × 1.5 cm, oval, grayish-white, and firm. The cut surface of the specimen was capsulated, cream to yellowish with tan in few areas [Figure 2]. The hematoxylin and eosin stained histopathological section of the given specimen under low magnification revealed a well circumscribed encapsulated lesion composed of numerous keratin–filled multicystic spaces of variable size and shapes. Under higher magnification, the epithelial components are seen forming ducts and small cysts that contain eosinophilic coagulum [Figure 3]. The glandular epithelial cells are seen arranged in the form of nests, sheets and chords in the myxoid stroma. Numerous area show foci of keratinizing squamous cells and keratin pearl formation. The myoepithelial cells exhibits varying morphology and some cells are rounded with eccentric nuclei and hyalinised eosinophilic cytoplasm [Figure 4]. Foci of hyalinization, bone, chondroid and fat cells can also be noticed. A histopathological diagnosis of Keratinizing pleomorphic adenoma was given.

Figure 1

Intra oral photograph of a growth in the left cheek region

Figure 2

he gross specimen comprised of a of an encapsulated soft tissue mass, measuring 1.5 cm × 2cm × 1.5 cm, oval, gray white, and firm.

Figure 3

Scanner view (×4) shows lesion composed of superficial and deep areas with numerous Keratin filled multicystic spaces

Figure 4

The glandular epithelial cells forming ducts and l cysts that contain eosinophilic coagulum. Numerous area show foci of keratinizing squamous cells and keratin pearl formation (H&E ×10)

DISCUSSION

The WHO in 1972 defined Pleomorphic adenoma as a circumscribed tumor characterized by its pleomorphic or mixed appearance with clearly recognizable epithelial tissue being intermingled with tissue of myxoid, chondroid or mucoid appearance.[6] The histogenesis of pleomorphic adenoma, relates to a dual proliferation of cells with ductal or myoepithelial features separating it from monomorphic adenomas composed of only one cell type. The myoepithelial cell assumes an important role in determining the overall composition and appearance of mixed tumors. A range of cell types and microscopic patterns are seen in these tumours–those composed almost completely of epithelial cells at one end of a spectrum and those composed of completely myoepithelial cells at the other end. Between these two extremes, less well developed cells with features of both myoepithelial and ductal elements may be seen. Alternatively, it has been theorized that rather than simultaneous proliferation of neoplastic epithelial and myoepithelial cells, a single cell with the potential to differentiate toward either epithelial or myoepithelial cells may be responsible for these tumors.[5]

The myoepithelial component often predominates with spindled to ovoid cells with wispy pink cytoplasm embedded in a pale chondromyxoid blue gray stroma. These myoepithelial cells are often seen streaming off the ductal component blending into the stroma.[6] Cartilaginous differentiation is often seen. Occasionally, osseous metaplasia and even lipomatous metaplasia may be present. The tumors can show various epithelial differentiation such as oncocytic, squamous, mucinous, and sebaceous metaplasia. Among the varied patterns described, this case showed extensive squamous metaplasia with keratin pearl formation. If squamous metaplasia is predominating it can be mistaken for squamous cell carcinoma or mucoepidermoid carcinoma, if the overall architecture and configuration is not taken into consideration.[7]

Pathogenesis of squamous metaplasia and keratin pearl formation in pleomorphic adenoma has been proposed as the dedifferentiation of the acinar cells and subsequent hyperplasia of acinar, duct luminal, and myoepithelialcells.[8] Focal squamous metaplasia in PA can be related to ischemia, repair following infarction and necrosis of the salivary gland and may be found in about 25% of the pleomorphic adenoma.[9] However, there was no history of trauma nor ischemia in our case. Keratin pearls in pleomorphic adenoma are frequently seen involving the minor salivary glands than the major glands. Hence, we suggest that the epithelium of the minor salivary gland duct exposed to various source of irritants may be responsible for the presence of keratin pearls in pleomorphic adenoma. The source of irritation was the habit of smoking in our case. However, the exact mechanism is not clear.[10]

CONCLUSION

The awareness towards keratinizing pleomorphic adenoma with extensive squamous metaplasia is important as it can mimic malignancy. The presentation of tumor in a patient with habits and unusual location can pose diagnostic dilemma. Careful approach in diagnosis is of paramount importance for a proper treatment and management as misdiagnosis can lead to overtreatment of these lesions with more aggressive surgical approach or radiation therapy.

Declaration of patient consent

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Conflicts of interest

There are no conflicts of interest.