Polyclonal napsin A expression: a potential diagnostic pitfall in distinguishing primary from metastatic mucinous tumors in the lung.

CONTEXT: Napsin A is a useful marker for distinguishing primary from metastatic lung tumors. Mucinous lung tumors may be difficult to distinguish from metastatic mucinous tumors.

OBJECTIVES: To evaluate napsin A expression in lung and extrapulmonary mucinous tumors on both histology and cytology specimens and to determine napsin A's utility in differentiating primary from metastatic mucinous tumors.

DESIGN: Napsin A immunohistochemistry was performed using a rabbit polyclonal antibody on formalin-fixed, paraffin-embedded surgical and fine-needle aspiration biopsy-derived, paraffin-embedded cell block specimens. Positive expression was defined as coarse, granular, cytoplasmic staining in 10% or more of tumor cells.

RESULTS: Sixteen of 32 mucinous lung tumors (50%) and 16 of 33 extrapulmonary mucinous tumors (48%), including 15 of 18 of gastrointestinal origin (83%), expressed napsin A. Positivity was concordant between surgical and cell block specimens in 5 of 9 cases (56%). In 3 of 4 discordant cases, napsin A expression was detected on the surgical specimen but not the cell block. The cell block material in these cases was paucicellular.

CONCLUSIONS: Napsin A shows decreased sensitivity and specificity for mucinous lung tumors and is unlikely to be reliable as a sole immunohistochemical marker of lung origin for such tumors (52% specificity in this study). The high frequency of napsin A expression in gastrointestinal mucinous tumors makes it particularly unreliable in distinguishing metastatic gastrointestinal from primary lung mucinous tumors. However, napsin A expression analysis may facilitate distinguishing mucinous tumors of pulmonary from those of nongastrointestinal origin. Interpretation of napsin A staining may be problematic in mucinous tumor specimens of low cellularity such as cell blocks.