Oral Clear Cell Tumors

Introduction:

The oral cavity is a potential site for the development of benign and malignant, or metastatic tumors. These tumors can form anywhere within the oral cavity, the salivary glands, or even the jaw bone, but salivary clear cell carcinomas are the most common clear cell tumors. The spread of primary metastasis through oral cancer is always to the lungs in males or the breasts in female patients.

What Are Clear Cells?

Clear cells are polyhedral cells with clear cytoplasm and with distinct nuclei. It was originally called Merkel cells, named after Merkel in 1875. They are also called Helle Zellen cells, and because they cannot be stained by hematoxylin or eosin stains, they came to be known as clear cells. These cells are composed of proliferating lesions or groups of tumors that may result from the accumulation of cytoplasmic water content, cellular organelles, zymogen granules, intermediate filaments, or even glycogen accumulation. These clear cells may often be found incidentally upon histological examination of a lesion, and they occur both in benign and malignant tumors. However, the hematoxylin and eosin stains cannot stain these clear cells, and hence the tumor cannot be detected by these two stains histologically.

The clear cells may arise out or are primarily derived from four different components, such as:

• Epithelial.

• Hematopoietic.

• Mesenchymal.

• Melanocytic.

Apart from the clear cell tumors of salivary glands, it is also essential to differentiate or identify the clear cell metastatic cancers that occur due to the proliferation from distant cancers like kidney, liver, thyroid, prostate gland, or bowel and metastasize to the oral cavity. In these cancers, the malignant cells have the ability to form clear cell tumors in the maxillofacial region as well. Though some lesions may be benign in nature, there are varying subtypes of tumor groups, and significant cases of patients affected invariably report the lesions of malignant or metastatic or neoplastic nature.

How Are Clear Cell Tumors Classified?

Clear cell tumors of the oral cavity can be classified into:

1) Clear Cell Odontogenic Lesions - These odontogenic lesions may be either cysts or tumors that arise from the derivative or remnants that form the tooth structure or periodontal apparatus.

A. Odontogenic Cysts:

• Gingival cyst of adult

• Lateral periodontal cyst

• Clear cell calcifying odontogenic cyst.

B. Odontogenic Tumors:

• Clear cell odontogenic carcinoma.

• Clear cell odontogenic calcifying epithelial tumor.

• Odontogenic ghost cell tumor.

2) Clear Cell Variant Tumors - These tumors may be benign in origin that needs to be crosschecked histologically by the pathologist. These can occur anywhere in the oral cavity and are rarer in incidence.

Clear Cell Salivary Gland Tumors:

• Clear cell myoepithelioma.

• Clear cell mucoepidermoid carcinoma.

• Clear cell oncocytoma.

• Clear cell acinic carcinoma.

• Hyalinizing clear cell carcinoma.

• Epithelial, myoepithelial carcinoma.

• Clear cell myoepithelial carcinoma.

What Is the Histopathology of Oral Clear Cell Tumors?

Clear cell tumors can be differentiated from other primary salivary gland tumors only by this histopathologic examination.

In the case of salivary gland clear cell carcinomas that are more common, the pathologist can observe nests of clear cells divided by vascular tissues and thin fibrous septa. The periodic acid Schiff stain (PAS) is one of the important and highly useful diagnostic stains that detects or tests positive for mucin, polysaccharides, and glycogen. Calponin-based immunohistochemical staining or other immunohistochemical staining methods may be needed to assess or differentiate and identify clear cell tumors of the maxillofacial region like those tumors of renal cell carcinomas that usually show a strong focal cytokeratin sensitivity (Ck positivity ).

In the case of minor salivary gland clear cell carcinomas, the Ck positivity is diffusely differentiating it from renal cell carcinomas. Similarly, the immunohistochemical staining with P63 can distinguish mucoepidermoid carcinoma from clear cell tumors. In a similar analogy, certain myoepithelial markers like actin, apart from epithelium-specific markers and epithelium membrane antigens (EMA's), are used for differentiating clear cell tumors from other benign or malignant lesions of the oral cavity.

What Are the Etiology and Clinical Features of Oral Clear Cell Tumors?

Patients may complain of slow-growing mass, oral discomfort, glandular swelling, the mass of the affected site, and pain. The mean age of prevalence of these heterogeneous tumors is usually in the sixth decade of life and is a slightly more common predisposition in females with a 2:1 ratio. In the salivary glands affected, 80% of these tumors can be found in:

• The base of the tongue.

• Palate.

• The floor of the mouth.

• Buccal mucosa.

• Oropharynx.

Though these lesions are rare and less reported in the major salivary glands, the parotid gland is more affected in comparison to the submandibular salivary glands. When these lesions are seen in the salivary glands, they mostly tend to be malignant, except only for two salivary gland benign lesions or subtypes. They are:

• Oncocytoma.

• Myoepithelium.

The recurrence rate of salivary gland lesions is usually around 11% as per research, and this cancer's clinical course and prognosis are generally good or fair if treated on time. If left untreated or upon clinical presentation, the lesions usually metastasize to regional lymph nodes. With the distinctive primary metastatic tendency, the lesions may infiltrate, destroy and invade local or surrounding tissues starting from the oral cavity at the site of occurrence and eventually metastasize.

How Are Oral Clear Cell Tumors Managed?

Four major factors influence the treatment planning of clear cell cancers by the oral and maxillofacial surgeon. They are:

• Positive margins.

• The histological grade.

• The invasion capacity of extent, that is, either neural or vascular.

• The presence of positive neck nodes or lymph node metastasis.

Surgical excision remains the treatment of choice, although if the metastasis has occurred to primary or distant sites by invasive and locally destructive cell behavior. Then neck dissection and radiotherapy are accordingly performed to prevent further recurrence of the lesions due to distant metastasis or nodal spread. In distant metastasis, the long-term prognosis may be poor, however as clear cell carcinomas are generally low-grade malignancies, timely treatment will assure the patient a good long-term prognosis.

Conclusion:

Clear cell tumors are challenging diagnostically. The histopathologic assessment by the pathologist is crucial to differentiate them from other tumors. Also, as it is low-grade cancer, timely management can yield a good prognosis in these patients and prevent distant metastasis and nodal spread.