Cytomorphological features of metastatic squamous cell carcinoma in serous effusions.

OBJECTIVE To study the cytomorphological features of squamous cell carcinoma (SCC) in serous effusions. METHODS Conventional smears from 23 SCC were reviewed for the following features: cellularity, single cells, cell clusters, long cords, squamous pearls, cell-in-cell, 'windows', polygonal cells, anucleated cells, tadpole cells, fibre cells, third-type cells, refractile cytoplasmic ring, two-tone cytoplasm, cytoplasmic density, cytoplasmic vacuoles, signet ring cells, nuclear/cytoplasmic ratio, nuclear hyperchromasia, irregular nuclear membrane, nuclear centricity, double or multiple nuclei, nucleolus, anaplastic cells and mitosis. Subsequently, we also reviewed 15 reactive mesothelium, 18 mesotheliomas and 24 adenocarcinomas for comparison. RESULTS Most SCC smears comprised mixed populations of single cells and cell clusters. Keratinized squamous cells and squamous pearls were diagnostic for well-differentiated SCC. Squamous eddies, appreciated on the cell block sections appearing as two-dimensional cell clusters with a swirling pattern, were characteristic for SCC. Poorly differentiated/third-type cells with atypical cytoplasmic keratinization, manifest as two-tone cytoplasm with a well-defined border and occasionally a refractile central ring, were an important clue for the squamous origin of the cells. Features including cell-in-cell, cytoplasmic vacuolation, double or multinucleated cells, anaplastic cells and mitotic figures were present in various degrees, and were not of diagnostic value. CONCLUSIONS Certain cytological features are diagnostic for SCC, whereas other features of poorly differentiated SCC often overlap with those of reactive mesothelial cells, mesothelioma, metastatic adenocarcinoma or urothelial carcinoma in serous effusions. The combination of clinical history, appropriate immunohistochemical panel and cytomorphological features are important to ensure a correct diagnosis.