Background: Mucinous cystadenocarcinoma of the lung is an uncommon tumor. Because it contains relatively few neoplastic cells relative to the amount of mucin produced, diagnosis of this entity, particularly on small specimens, is exceedingly difficult.
Case: The diagnosis of adenocarcinoma was made on transthoracic fine needle aspiration from a patient with a right upper lobe lung mass. Abundant mucoid material suggested a mucin-producing neoplasm. Histopathology revealed a mucinous cystadenocarcinoma with focal mucinous bronchoalveolar carcinoma.
Conclusion: The presence of copious extracellular mucin in fine needle aspirates from the lung otherwise diagnostic of adenocarcinoma should raise the possibility of a mucinous tumor. In particular, the diagnosis of mucinous cystadenocarcinoma may be suggested in cases that have a cystic appearance on imaging studies.