"Cannonballs" and psammoma bodies: unusual cytologic features of metastatic pulmonary small-cell carcinoma in a pleural effusion.

Large three-dimensional cell aggregates and psammoma bodies are usually encountered in benign serous effusions (mesothelial hyperplasia and endosalpingiosis), mesotheliomas, and metastatic papillary carcinomas. We report a case of pulmonary small-cell carcinoma occurring in an 88-year-old woman that initially presented with a malignant pleural effusion characterized cytologically by a predominance of large three-dimensional neoplastic cell aggregates ("cannonballs"), associated with rare psammoma bodies. Although the crowded three-dimensional tumor-cell aggregates did not allow detailed cytologic examination, the diagnosis of metastatic small-cell carcinoma could be established noting the characteristic chromatin features of the occasional single neoplastic cells and the characteristic "cell-in-cell" and "stack-of-coins" arrangements of rare small clusters of neoplastic cells. Immunoperoxidase stains showing positivity of the tumor cells for CD56, synaptophysin, and TTF1 further supported this diagnosis. Endobronchial ultrasound-guided fine-needle aspiration of a mediastinal lymph node subsequently confirmed the diagnosis of pulmonary small-cell carcinoma. Metastatic pulmonary small-cell carcinoma should be considered in the differential diagnosis of serous effusions showing large three-dimensional neoplastic cell aggregates and psammoma bodies to prevent a potential diagnostic pitfall.