Spontaneous Partial Regression of Fetal Lung Interstitial Tumor With A2M::ALK Rearrangement in a Neonate

The differential diagnosis for neonatal primary lung masses includes developmental anomalies and congenital lung tumors. Fetal lung interstitial tumor (FLIT) is a rare benign mesenchymal lesion which presents either antenatally or within the first 3 months of age. FLIT is a circumscribed solid-cystic mass which histologically resembles the fetal lung during the canalicular stage at 20-24 weeks of gestation. It is composed of immature mesenchymal cells expanding the interstitium and irregular airspace-like structures. Of all published cases, only 1 identified an α2-macroglobulin (A2M)::anaplastic lymphoma kinase (ALK) fusion and all cases underwent surgical resection in the neonatal or infancy period. We present the second case of FLIT with an A2M::ALK fusion diagnosed postnatally in a neonate which partially regressed spontaneously during conservative management with interim resection at 39 months of age, and provide a review of the literature.