Unilocular cystic mucoepidermoid carcinoma: 6 cases reported and literature review.

Unilocular cystic mucoepidermoid carcinoma (UCMEC) is a rare and diagnostically challenging variant of mucoepidermoid carcinoma, frequently misdiagnosed preoperatively as a benign cystic lesion. We retrospectively analyzed six cases of UCMEC treated between January 2021 and May 2025. The cohort included three males and three females, with a mean age of 55.66 years (range: 24-77). The tumors were located in the parotid gland (n=4) and palate (n=2), with one palatal lesion exhibiting bony extension. The mean maximum tumor diameter was 2.5 cm. Histologically, all cases showed a predominant unilocular cystic architecture. Immunohistochemistry was positive for P40, P63, and CK7, supporting epithelial differentiation. Mucin production was confirmed by Alcian Blue-Periodic Acid Schiff (AB-PAS) staining. According to the AFIP grading system, five cases were low-grade and one was high-grade. Molecular analysis identified MAML2 gene fusion in five cases (83.3%), all of which were low-grade tumors. Surgical resection is the cornerstone of treatment. The detection of MAML2 fusion is a valuable diagnostic and prognostic marker, being strongly associated with low-grade histology and a favorable outcome. This case series aims to elucidate the clinicopathological and molecular characteristics of UCMEC to improve diagnostic accuracy. Accurate preoperative or intraoperative distinction from benign lesions and correct grading are paramount for determining the appropriate surgical scope and optimizing patient prognosis.