Mucoepidermoid Carcinoma of Salivary Gland: Limitations and Pitfalls on FNA

Abstract

Mucoepidermoid Carcinoma (MEC) is the most common malignant neoplasm of salivary gland origin. However, its morphologic heterogeneity poses difficulty in interpretation. In the present series we discuss the morphologic features of MEC, limitations and pitfalls in its diagnosis on Fine Needle Aspiration Cytology (FNAC). Fourteen cases of suspected MEC were evaluated cytologically followed by histopathological examination for confirmation. A definite cytological diagnosis was rendered in nine cases; three of the remaining five were underdiagnosed as abscess, pleomorphic adenoma and mucus cyst. Of the remaining two cases, one case each of sebaceous carcinoma and sialadenitis was mislabeled as MEC on cytology. A satisfactory aspirate composed of intermediate cells, mucin secreting cells and squamous cells in a mucinous background may not be obtained in all cases of low grade MEC. High grade MEC can be classified as squamous cell carcinoma. Hence, awareness of confounding factors with clinicopathologic correlation and judicious use of frozen section can help in minimizing errors.of false positive cases included one each of sebaceous carcinoma and chronic sialadenitis [Table/ Fig-1].

The key radiologic and cytomorphologic features of oncocytic and oncocytoid lesions of the salivary gland

“…The cell types that can be seen include epidermoid cells demonstrating dense cytoplasm, intermediate cells showing less abundant and clearer cytoplasm, and mucus‐secreting cells in a background of mucin and muciphages. High‐grade lesions, on the other hand, are dominated by the epidermoid cells that often feature pleomorphism and mitoses in a background of necrosis . Mature keratinization and keratin debris are only rarely seen …”

“…Several benign and nonneoplastic entities enter the differential diagnosis for low grade MEC by FNAC, leading to a relatively high false‐negative rate compared to other malignancies . The most common misdiagnosis is as a benign cyst or mucocele, as the mucocytes of MEC can be mistaken for histiocytes in the presence of abundant mucin . It is because of this pitfall that the Milan System recommends that specimens consisting purely of mucin without an epithelial component be classified as atypical as opposed to benign or nondiagnostic .…”

“…It is because of this pitfall that the Milan System recommends that specimens consisting purely of mucin without an epithelial component be classified as atypical as opposed to benign or nondiagnostic . Another recognized pitfall is misdiagnosis as pleomorphic adenoma, with intermediate and epidermoid cells being misinterpreted for the epithelial and myoepithelial cells, respectively, dense mucin misinterpreted as chondromyxoid matrix, and mucocytes again misinterpreted as histiocytes . At the high‐grade end of the spectrum, a number of different entities enter the differential, including salivary duct carcinoma and metastatic squamous‐cell carcinoma.…”

“…This gene fusion is present in roughly two-thirds of tumors. 56,63,64 The radiologic appearance of MECs differs depending on tumor grade ( Figure 5 67 Mature keratinization and keratin debris are only rarely seen. 68 The differential diagnosis for MEC is particularly wide among the oncocytoid salivary gland neoplasms.…”

“…28 The most common misdiagnosis is as a benign cyst or mucocele, as the mucocytes of MEC can be mistaken for histiocytes in the presence of abundant mucin. 67 It is because of this pitfall that the Milan System recommends that specimens consisting purely of mucin without an epithelial component be classified as atypical as opposed to benign or nondiagnostic. 10 Another recognized pitfall is misdiagnosis as pleomorphic adenoma, with intermediate and epidermoid cells being misinterpreted for the epithelial and myoepithelial cells, respectively, dense mucin misinterpreted as chondromyxoid matrix, and mucocytes again misinterpreted as histiocytes.…”