A pulmonary mucinous cystic tumour of borderline malignancy

We report a well-documented case of pulmonary mucinous cystic tumour of borderline malignancy involving the left lower lobe. The lesion was found incidentally by chest radiograph and CT scan with a provisional diagnosis of bronchioloalveolar carcinoma. The tumour was 4 cm in its greatest dimension, cystic and filled with gelatinous mucus. Microscopically, the neoplastic mucinous epithelium was composed of cuboidal cells with focally nuclear stratification and mild to moderate nuclear atypia. The patient has remained free from recurrence or metastases for 6 years. Pulmonary mucinous cystic tumour of borderline malignancy is a rare, recently described neoplasm, which spans a spectrum of tumours with malignant potential. The recent World Health Organization classification of lung tumours does not recognize this entity, which has a very good prognosis, and as such should be distinguished from classic pulmonary adenocarcinoma. Histological diagnosis can be difficult to distinguish from cystic bronchioloalveolar carcinoma or metastatic mucinous adenocarcinoma.