Reactive histiocytic proliferation in the pleural fluid mimicking metastatic signet ring adenocarcinoma.

Reactive nodular and diffuse histiocytic proliferations of mesothelial and non-mesothelial lined sites have been sporadically reported in the literature. However, there is no cytologic literature describing this process. We report a case of reactive histiocytic proliferation mimicking a metastatic signet ring adenocarcinoma in pleural fluid from a 33-year-old white male. Ancillary studies such as immunohistochemistry should be used to elucidate the cell of origin and avoid diagnostic errors.