Fetal lung interstitial tumor (FLIT): a case report and review of prenatal imaging features.

Fetal lung interstitial tumor (FLIT) is a rare benign mesenchymal lung tumor that is often mistaken for more common congenital lung lesions such as congenital pulmonary airway malformation (CPAM). Prenatal detection is exceedingly uncommon. Here, we report a case of prenatally detected FLIT presenting in the third trimester with rapid interval growth, mediastinal shift, and fetal hydrops, raising concern for a malignancy, including pleuropulmonary blastoma. Prenatal ultrasonography and fetal magnetic resonance imaging (MRI) demonstrated a rapidly enlarging right lung mass first identified late in gestation with associated findings of pleural effusion, ascites, and diffuse skin edema consistent with hydrops. Progressive mass effect prompted expedited delivery. Postnatal imaging confirmed a large pulmonary lesion, and the neonate developed significant respiratory distress requiring lobectomy. Histopathologic evaluation revealed an immature mesenchymal proliferation diagnostic of FLIT. This case highlights the imaging overlap between FLIT, CPAM, and pleuropulmonary blastoma and underscores the importance of considering FLIT in the differential diagnosis of late-presenting fetal lung masses. Although imaging is essential for detection and prenatal planning, definitive diagnosis requires histopathologic confirmation, and complete surgical resection yields an excellent prognosis.