Microscopic findings of the blastomatoid pulmonary carcinosarcoma. On microscopic view, the blastomatoid pulmonary carcinosarcoma displayed a biphasic growth pattern (A, haematoxylin-eosin (HE), x 20). The epithelial elements showed glandular differentiation (B, x 100) and displayed focal cellular atypia and mitotic figures (B, inset) resembling high-grade fetal adenocarcinoma. These structures were surrounded by malignant, blastomatoid spindle cells (C, x 200), which showed partial rhabdomyosarcomatous differentiation with rhabdomyoblasts (C, inset, desmin). Immunohistochemical staining with pan-cytokeratin (D) was positive in the carcinomatous areas. Both components did not express thyroid transcription factor 1 (TTF-1; E). Membranous beta-catenin (F) expression was detected in the epithelial structures; MDM2 (G) and cyclin-dependent kinase 4 (CDK4; H) were expressed in both components (x 200).