Cellular mesoblastic nephroma in an infant: report of the cytologic diagnosis of a rare paediatric renal tumor.

Congenital mesoblastic nephroma is a rare pediatric tumor with a favorable clinical outcome. Cytological features of this uncommon tumor and diagnostic difficulties with other commoner pediatric renal neoplasms have been inadequately discussed in the available literature. We describe the case of a 1-year-old girl who presented with a right renal mass. Fine-needle aspiration smears consisted of a few cellular clusters of spindle cells with mitotic activity and mild nuclear pleomorphism. No blastema was identified. A cytologic impression of mesoblastic nephroma was rendered, which was confirmed on histopathological examination of the right nephrectomy specimen as a cellular mesoblastic nephroma. Cytologic diagnosis of mesoblastic nephroma has important prognostic and therapeutic implications. The cytopathologist should carefully evaluate smears from such patients and attempt to differentiate mesoblastic nephroma from Wilms' tumor and clear cell sarcoma. (c) 2009 Wiley-Liss, Inc.