Retraction halos: nature's dotting pen in effusion cytology processed by ultrafast Papanicolaou stain

Effusion cytology is challenging when solitary adenocarcinoma cells resemble histiocytes or mesothelial cells, and when adenocarcinomas look like histiocytic hyperplasia. It is also challenging to locate rare tumor cells in markedly inflammatory background. Over a period of 9 years, 9,474 cases of body cavity fluids including 3,642 cases of pleural, 1,900 cases of peritoneal and 345 cases of pericardial effusions and 3,587 cases of pelvic washes were processed with Ultrafast Papanicolaou stain. Retraction halos around spherical solitary cells and tissue fragments were observed whenever the background was inflammatory, whereas absent around histiocytes or histiocytic aggregates, and inconspicuous around solitary mesothelial cells. Retraction halos were helpful when the tumor fragments are obscured by marked inflammation, the quantity of tumor fragments is sparse, or the tumor cells resemble histiocytes or mesothelial cells. In short, retraction halos are nature's dotting pen for effusion cytology and are particularly useful in challenging cases.