Introduction: Primary pulmonary mucinous cystic neoplasia comprises a group of tumors, from benign cystadenoma to mucinous cystadenocarcinoma.
Case presentation: We report a case of primary pulmonary mucinous cystadenocarcinoma in a 75-year-old woman who was found to have a right hilar mass on a routine chest X-ray. A lobectomy was performed and the resection specimen revealed a multicystic mucinous tumor. Microscopically, the tumor was composed of confluent mucin-filled cystic spaces lined by columnar mucin-secreting cells which ranged from cytologically bland to moderately atypical with 'bronchioloalveolar pattern' invasion into the adjacent parenchyma. Immunohistochemically, tumor cells were positive diffusely for Cytokeratin 7, and focally for Cytokeratin 20 and Thyroid Transcription Factor-1.
Conclusion: This case highlights the continuous spectrum of pulmonary mucinous cystic neoplasia from benign mucinous cystadenoma to malignant mucinous cystadenocarcinoma, and the probable existence of a 'borderline' mucinous cystic tumor. Although molecular data are lacking to substantiate progression from benign to malignant in these neoplasms, the importance of recognizing the morphologic continuum lies in alerting pathologists to thoroughly examine specimens to rule out invasive foci in tumors with 'borderline' morphology.