473 Pulmonary Blastoma in Pregnancy, a Rare Diagnostic Challenge

Introduction/Objective Pulmonary blastoma (PB) is a rare, highly aggressive biphasic tumor of sarcomatoid carcinoma subtype. These neoplasms frequently harbor CTNNB1 mutations leading to aberrant nuclear expression of the beta-catenin protein. Additionally, some adult cases harbor DICER1 mutations. Herein, we describe a unique case of PB diagnosed in pregnancy. Methods/Case Report A 30-year-old female at 35-weeks gestation presented with complaints of shortness of breath and chest pain. Imaging revealed a 7.9 cm mediastinal mass involving the pericardium and left upper lobe. A biopsy and subsequent resection showed a biphasic neoplasm composed of fetal adenocarcinoma with a lesser component of undifferentiated carcinoma and focal high-grade carcinoma with neuroendocrine differentiation. The mesenchymal component consisted of a primitive blastema-like stroma with foci of immature cartilage. The glandular component was positive for CAM5.2, TTF-1, and SALL-4. Both the glandular and mesenchymal components had aberrant cytoplasmic and nuclear staining of beta-catenin. Synaptophysin was focally positive in the fetal adenocarcinoma and poorly differentiated adenocarcinoma. Stains for desmin, glypican 3, PLAP, AFP, CDX2, CD30, GATA3, MyoD1, myogenin, Napsin, OCT3/4, PAX-8, and SOX10 were negative. NGS studies detected mutations in CTNBB1 and DICER1 genes. Results NA Conclusion Pulmonary blastoma is a rare, aggressive biphasic neoplasm with distinctive morphologic, immunophenotypic, and molecular features. Awareness of this entity is critical to accurately render this challenging diagnosis.