Lung

Contributed by Rita Alaggio, M.D.

A neonate presents with a solid lung mass on imaging and symptoms of respiratory difficulty. Which of the following is the most likely diagnosis?

1. Bronchogenic cyst
2. Congenital pulmonary airway malformation (CPAM)
3. Fetal lung interstitial tumor (FLIT)
4. Inflammatory myofibroblastic tumor (IMT)
5. Pulmonary hamartoma

C. Fetal lung interstitial tumor (FLIT) is a benign neoplasm of interstitial mesenchymal cells in the fetal lung, typically presenting as a well circumscribed solid or spongy mass in neonates, often leading to respiratory distress. Imaging may reveal a solid or partially cystic lesion but definitive diagnosis relies on characteristic histopathological features. Although rare cases harboring A2M::ALK fusions have been reported, it remains uncertain whether these represent true FLITs or a distinct diagnostic entity.

Answer D is incorrect because, while IMT can occur in the lungs, it is more common in older children or adults rather than in neonates. IMT often exhibits more aggressive behavior with symptoms, such as fever or weight loss. Histologically, IMT contains spindle shaped myofibroblastic cells with inflammatory infiltrates, unlike the bland, uniform mesenchymal cells seen in FLIT. Answer A is incorrect because bronchogenic cysts are congenital cystic lesions that typically present as well defined, fluid filled masses rather than solid or spongy masses. They are usually asymptomatic unless they become large enough to cause compression, which makes respiratory distress less common in neonates compared to FLIT. Answer B is incorrect because CPAM is a congenital lung anomaly characterized by abnormal cystic airspaces lined by respiratory epithelium. Unlike FLIT, CPAM lesions are more microcystic or multicystic and lack the mesenchymal proliferation and histological features of FLIT. It also has a distinct developmental pathogenesis compared to FLIT. Answer E is incorrect because pulmonary hamartomas are benign lung lesions that typically occur in adults rather than in neonates. They are composed of a mixture of cartilage, fat and connective tissue, unlike the uniform mesenchymal cells with glycogen content seen in FLIT. Pulmonary hamartomas also lack the clinical presentation of respiratory distress seen in neonates with FLIT.

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Reference: Fetal lung interstitial tumor

Which of the following macroscopic features best describes fetal lung interstitial tumor (FLIT)?

1. Diffuse, bilateral nodules with pleural involvement
2. Masses containing central calcification
3. Necrotic masses with irregular cystic components
4. Poorly circumscribed masses infiltrating the pleura
5. Well circumscribed, intraparenchymal, solid or microcystic masses

E. Well circumscribed, intraparenchymal, solid or microcystic masses. FLIT typically presents as well circumscribed, solid or spongy microcystic intraparenchymal masses. Answer D is incorrect because FLIT is well demarcated and does not infiltrate. Answer C is incorrect because necrosis and irregular cystic components are not characteristic of FLIT. Answer B is incorrect because central calcification is not a described feature. Answer A is incorrect because FLIT is localized, not diffuse or bilateral.

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Reference: Fetal lung interstitial tumor