Primary pulmonary mucoepidermoid carcinoma: a case report

Primary pulmonary mucoepidermoid carcinoma(PMEC) is a rare salivary gland-type malignancy, accounting for fewer than 0.2% of all primary lung tumors. We report herein a 61-year-old male patient with PMEC who was admitted for a space-occupying lesion in the left upper lobe and had a 20-year smoking history. Contrast-enhanced chest CT and positron emission tomography-computed tomography (PET-CT) revealed a central space-occupying lesion in the left hilum with distal obstructive inflammation. Flexible bronchoscopy revealed a nodular mass in the left main bronchus. Biopsy histopathology combined with immunohistochemistry confirmed the diagnosis of PMEC. Immunohistochemistry showed CK7 (+), P63 (-), P40 (-), CD117 (focal+), TTF-1 (-), NapsinA (-), and Ki-67 (7%+). Molecular testing identified a CRTC1-MAML2 gene fusion, whereas no classic driver gene mutations (e.g., EGFR or KRAS) were detected. PD-L1 expression was negative, and the tumor mutational burden (TMB) was low. The low Ki-67 index and positive MAML2 fusion in this case suggest a low-grade PMEC with a relatively favorable prognosis. The present study discusses the clinicopathological features, imaging findings, molecular diagnosis, and therapeutic advances of PMEC, along with a review of the relevant literature, aiming to increase clinical awareness of this rare disease.