Primary Pulmonary Mucoepidermoid Carcinoma: Histopathological and Moleculargenetic Studies of 26 Cases

INTRODUCTION Pulmonary mucoepidermoid carcinoma (PMEC) is an uncommon neoplasm of the lung and the main salivary gland-type lung carcinoma. The aims of this study were to review the clinicopathological and immunohistochemical features of PMEC and characterize the genetic events in PMEC. METHODS We reviewed the pathology cases in our hospital and found 34 initially diagnosed PMEC cases, 26 of which were confirmed as PMEC after excluding 8 cases of MEC-like pulmonary carcinoma. The clinicopathological characteristics of the 26 PMEC cases and the 8 cases of MEC-like pulmonary carcinoma were retrospectively reviewed. MAML2 rearrangement was detected by fluorescence In Situ Hybridization (FISH). Immunostains of ALK, calponin, collagen IV, CK7, EGFR, HER2, Ki-67, Muc5Ac, p63, p40, and TTF-1 were performed. DNA was extracted from 23 cases of PMEC. Mutation profiling of the EGFR, KRAS, BRAF, ALK, PIK3CA, PDGFRA, and DDR2 genes were carried out using next-generation sequencing (NGS), Sanger sequencing, and quantitative polymerase chain reaction (QPCR) in 9 successfully amplified cases. RESULTS Twenty-six cases of PMEC (18 low-grade, 8 high-grade) included 13 men and 13 women aged 12-79 years. Twenty-two cases had a central/endobronchial growth pattern, and 4 cases had a peribronchial growth pattern. Immunohistochemically, CK7, Muc5Ac, p40, and p63 were positive in all cases (26/26);EGFR was positive in 11 cases (11/26); TTF-1, Calponin, HER2 and ALK were negative in all cases (0/26). MAML2 rearrangement was identified in 12 of 18 PMEC cases. No mutations were detected in any of the 7 genes in the 9 cases that qualified for mutation analysis. Twenty-three PMEC patients had follow-up information with a median interval of 32.6 months. Both the 5- and 10-year overall survival rates (OS) were 72.1%, and a high-grade tumor was an adverse prognostic factor in PMEC. There were 8 cases of MEC-like pulmonary carcinoma aged 36-78 years: 2 cases were located in the bronchus, and 6 cases were located in the lung. p63 and TTF-1 were positive in all cases (8/8), p40 was positive in 5 cases (5/8), and ALK was positive in 5 cases (5/8). No cases of MAML2 rearrangement were detected, but there were 5 cases of ALK rearrangement. CONCLUSIONS PMEC is a primary malignant pulmonary tumor with a relatively good prognosis that is historically characterized by the presence of mucous cells and a lack of keratinization. There are distinct differences between PMEC and MEC-like pulmonary carcinoma in tumor location preference, immunophenotype, and molecular genetics, and the differential diagnosis is critical due to the therapeutic and prognostic considerations.