Low-grade mucoepidermoid carcinoma mimicking benign cystic lesions in the salivary gland: A diagnostic dilemma

Abstract

Mucoepidermoid carcinoma (MEC) is a common malignancy arising in the parotid gland. The diagnosis of MEC is typically based on its morphological features alone, characteristically containing mucocytes, intermediate cells and epidermoid cells. However, when cystic degeneration is diffuse, it is challenging to distinguish MEC from other benign cystic tumors. This is a case report of a 58-year-old Caucasian man who presented with a parotid mass. H&E sections of the mass reveal multiloculated cysts lined by bland-looking epithelium with only rare papillary architectures. The papillary proliferation contains mucocytes, and epidermoid cells highlighted by the p63 immunohistochemistry study. The diagnosis was confirmed by FISH result of positive MAML2 (11q21) rearrangement. Patient underwent parotidectomy and is disease-free 6 months post-surgery. MEC with cystic degeneration is a common diagnostic pitfall which can mimic many benign lesions in the salivary gland. We present a rare case with MEC with extensive cystic change, its molecular and pathologic findings and review the diagnostic features of MEC, its benign mimickers and useful tools for distinguishing these entities.

Keywords: Salivary gland tumor; acinic cell carcinoma; cystic degeneration; low grade mucoepidermoid carcinoma; rare tumors.

Figure 1.

Incidental finding on head MRI with coronal postcontrast fat saturated image of the brain (left) and T2 weighted image of the head (right) demonstrating avidly enhancing bilobed T2 hypointense lesion within the right parotid gland.

Figure 2.

The specimen was serially sectioned to reveal a multiloculated white to tan solid-cystic mass. The cut surface shows a cystic component (blue arrow) and the white nodular area (black arrow). The mass abuts the inked outer surface.

Figure 3.

Histologic examination of the parotid mass shows multicystic lesions lined with bland epithelium containing pink amorphous material in the lumen (A, 40x; B, 200x) and ample lymphocytic infiltration in the stroma (B, 200x). There are multiple papillary infoldngs along the cyst walls with most devoid of mucous cells (C, 200x). However, several foci of mucinous-rich epithelium arranged in fused papillary structures are noted (D, 200x).

Figure 4.

Tumor cells within an endothelial-lined structure is suspected, raising the possibility for lymphovascular invasion (A, 400x). The papilla with mucous cells also contains epidermoid cells on H&E (B, 200x) that stain positive for p63 (C, 200x) which supports the diagnosis of mucoepidermoid carcinoma.