Pulmonary invasive mucinous adenocarcinoma mimicking pulmonary actinomycosis

BACKGROUND: Primary pulmonary invasive mucinous adenocarcinoma is a rare and distinct subtype of lung adenocarcinoma. CASE PRESENTATION: A 72-year-old woman presented with productive cough for two months and fever for six days. Chest computed tomography (CT) showed a mass in the left lower lobe. Sputum culture tested negative for bacteria, but the sequence of Actinomyces meyeri was detected by metagenomic next generation sequencing from the bronchoalveolar lavage fluid. It was considered a pathogenic bacterium as the normalized number of DNA sequencing reads was 10 times higher than the normal level. The patient’s symptoms alleviated quickly, and the chest CT lesion shrank to a third of the original size following treatment with penicillin for two months. However, a repeat chest CT performed after four months of treatment revealed that the lesion had expanded. Positron emission tomography/CT revealed that fluorodeoxyglucose metabolism was increased in the mass with surrounding ground glass density of the left lower lobe. Furthermore, CT-guided percutaneous lung biopsy was performed, and hematoxylin–eosin staining showed columnar tumor cells with abundant mucin in the cytoplasm with a basal nucleus. Finally, the patient was diagnosed with pulmonary invasive mucinous adenocarcinoma and agreed to undergo a thoracoscopic surgery. CONCLUSIONS: Pulmonary invasive mucinous adenocarcinoma is a subset of lung adenocarcinoma with low incidence rate. The clinical features and CT findings are non-specific. A histopathological diagnosis is of fundamental importance in preventing misdiagnosis.