Benign effusions

Abstract

Serous effusion is a common finding in clinical practice and may be the sign of a systemic disease or a local disorder. Most of the clinically detected effusions, both in adults and children, are associated with reactive conditions. Several laboratory tests, such as protein content of the fluid, lactate dehydrogenase and cholesterol level, and adenosine deaminase activity, are used in the evaluation of serous effusions, shedding valuable insight into the etiology and risk of malignancy. However, to exclude or to diagnose a malignant effusion, cytological examination is frequently needed.

Effusions contain a variety of cells depending on the underlying pathology. Mesothelial cells, as the local elements, are almost always present in the effusions. The other nonneoplastic cells that are commonly encountered are macrophages and blood-borne cells. Some incidental cellular and noncellular elements may also be observed. The predominant cell type in an effusion may provide a clue for the etiology.

Mesothelial cells are characterized by centrally/paracentrally located round to oval nuclei. Their chromatin is finely granular; the nuclear membrane is usually prominent and smooth. Nucleoli may be distinct, but the presence of macronucleoli is not an expected finding. The nuclear-to-cytoplasmic ratio of mesothelial cells varies. Binucleation is a common feature; multinucleated forms may also occur. Mesothelial cells have a characteristic two-tone stained dense cytoplasm. They may contain multiple or single vacuoles. Large hydropic vacuoles displacing the nuclei toward the edge may mimic signet ring cells. Mesothelial groups typically have knobby contours and show slit-like spaces between the cells called “windows.” Cell-in-cell arrangements are common. Mitosis may be seen. When serous membranes are irritated and injured, mesothelial cells proliferate and may show both cellular and structural atypia. There are several well-known conditions causing atypia in mesothelial cells so that clinical information is important in evaluation.

Differentiating reactive mesothelial proliferations from metastatic carcinomas may be difficult in some cases due to either atypical features in mesothelial cells or bland appearance of some carcinomas. The pattern and the cellular features should be evaluated together in such cases, and morphological findings should be combined with immunohistochemistry if the diagnosis is still in doubt. Ancillary tests are needed also for the differential diagnosis between benign and malignant mesothelial proliferations. Malignant mesotheliomas generally do not show clear-cut malignant nuclear features, and pattern analysis is more important than individual cell characteristics for recognizing malignant mesotheliomas in serous effusions.