Small cell carcinoma of the kidney. A clinicopathologic, immunohistochemical, and ultrastructural study.

Three cases of primary small cell carcinoma of the kidney with light microscopic, immunohistochemical, and electron microscopic findings are reported. Two patients died of disseminated disease 8 months and 1 year, respectively, after the diagnosis and the third was free of tumor after 18 months. Immunohistochemical studies revealed keratin immunostaining of tumor cells in two cases and staining for neuron-specific enolase in the third. The third case also showed a few dense neurosecretory granules at the ultrastructural level. Although no strong conclusions regarding histogenesis can be drawn, this study indicates that small cell carcinoma of the kidney exists and does not necessarily exhibit a neuroendocrine differentiation. Small cell carcinoma of the kidney must be considered in the differential diagnosis of malignant renal tumor, especially in cases in which a large necrotic tumor is present. Based on the few cases presented in this study and on the one previously reported case, small cell carcinoma of the kidney appears to be an aggressive tumor.