Paranuclear blue inclusions in small cell undifferentiated carcinoma: a diagnostically useful finding demonstrated in fine-needle aspiration biopsy smears.

In a study of bone marrow aspiration smears, Wittchow et al. (Mod Pathol 1992;5:555-558) described a highly characteristic finding, paranuclear blue inclusions (PBIs), found almost exclusively in cases of metastatic small cell undifferentiated carcinoma (SCUC). PBIs are 1-4 microns, light blue, cytoplasmic inclusions best visualized with Romanowsky-type stains. These inclusions are most easily found indenting the nuclei within clusters of closely opposed tumor cells. In the current study air-dried fine-needle aspiration biopsy (FNAB) smears from 146 primary and metastatic small cell and non-small cell adult and childhood malignancies were reviewed. PBIs were found in 28/32 (88%) of SCUC but were observed in only 4/44 (9%) non-small cell carcinomas, 2/21 (9.5%) lymphomas, 1/8 (12.5%) melanomas, 0/14 sarcomas, and 6/27 (22%) small round cell neoplasms. These results suggest that the presence of PBIs in air-dried FNAB smears of adult neoplasms, while not pathognamonic of SCUC, are a diagnostically useful finding. PBIs may be seen in a variety of different childhood small round cell neoplasms which limits their utility in this setting. The recognition of PBIs is most important to the cytologist who may not have access to ancillary studies, such as immunohistochemistry and electron microscopy.