Large cell neuroendocrine carcinoma (LCNEC) of the lung has been proposed as a new disease entity. To establish diagnostic features, bronchial brush cytologic findings were evaluated. Bronchial brush cytology material of 20 LCNECs was evaluated by light microscopy and stained immunocytochemically with protein gene product 9.5 (PGP9.5), neuron-specific enolase, and neural cell adhesion molecule antibodies. The findings were compared with those for 19 poorly differentiated adenocarcinomas (ACs), 18 poorly differentiated squamous cell carcinomas (SCCs), and 20 small cell lung carcinomas (SCLCs). Frequently observed characteristic cytologic findings of LCNECs were necrotic background (90.0%), large tumor cell size (90.0%), naked nuclei (90.0%), and nuclear streaking (90.0%). Nuclei in all LCNECs showed a fine granular chromatin pattern and possessed one or a few nucleoli. Indian-filing and rosette arrangements were observed in less than one-half of the LCNECs. In poorly differentiated ACs and SCCs, these features were less frequent, whereas thick nuclear membranes were observed more often. In SCLCs, tumor cell adhesions and Indian-filing or nuclear molding were observed more frequently than in LCNECs, whereas a necrotic background, tumor cell clusters, large tumor cells, and nucleoli were less prominent. The majority of LCNECs (80.0%) had a positive immunocytochemical reaction for PGP9.5, in contrast to the low positive reactions for ACs (42.1%) and SCCs (30.8%). Large cell neuroendocrine carcinomas can be diagnosed preoperatively by bronchial brush cytology using reliable parameters, including tumor cell size, naked nuclei, thin nuclear membranes, nuclear streaking, high PGP9.5 positivity, and a necrotic background. Copyright 2003 American Cancer Society.
