Primary thoracic myxoid variant of extrarenal rhabdoid tumor in childhood.

Primary extrarenal rhabdoid tumors (RT) are now recognized as a specific entity in pediatric oncological pathology practice. We present an unusual case of a small cell myxoid variant of a thoracic RT in an infant and highlight the importance of recent molecular developments in the diagnosis of these tumors. An 8-month-old child presented with a short history of cough and shortness of breath. Imaging demonstrated a large mass occupying the majority of the thoracic cavity on the right side. A percutaneous needle biopsy of the mass showed fragments of tissue composed of malignant tumor with a predominant "small ovoid cell" phenotype and extensive myxoid change, with small nests and islands of tumor cells; occasional cells demonstrated open vesicular nuclei, prominent nucleoli, and eosinophilic cytoplasmic inclusions. Immunohistochemical staining revealed focal strong cytoplasmic positivity for cytokeratin, focal strong paranuclear cytoplasmic vimentin positivity, and INI1 staining showed normal nuclear positivity in control tissues but was negative in tumor cell nuclei. Electron microscopy demonstrated characteristic paranuclear whorls of intermediate filaments confirming the diagnosis of extrarenal malignant RT. The diagnosis of malignant rhabdoid tumor may be difficult, particularly in cases, such as the present, with a predominant small-cell myxoid phenotype. The characteristic expression patterns of cytokeratin and vimentin provide strong clues to the diagnosis, and the use of INI1 antibody now makes definitive diagnosis possible even on needle core biopsies.