fetal lung interstitial tumor

Definition: FLIT is the proposed designation for a tumefactive, lobar-based lesion presenting either prenatally or within 3 months of age in all cases.

Fetal lung interstitial tumor (FLIT) pathogenetic relationship to type 1 pleuropulmonary blastoma (cystic pleuropulmonary blastoma) remains uncertain to date.

Most infants present with respiratory difficulties of variable severity within the first 5 days of life.

On account of the solid appearance of these lesions, there is considerable uncertainty about the nature of the mass, but a bronchogenic cyst and congenital cystic adenomatoid or pulmonary airway malformation (CCAM-CPAM) were the more frequent preoperative clinical impressions.

Synopsis

 unique solid or mixed solid/cystic lung mass
 immature interstitial mesenchyme
 irregular airspace-like structures mimicking abnormal incompletely developed lung
 The lining epithelium is composed of regularly distributed cuboidal cells.
 columnar cells lining the aerial space.
 The interstitial cells are monotonous and uniformly distributed in the interstitial compartment.
 The individual cells have clear cytoplasm that surrounds a central round to ovoid-shaped nucleus.
 The periodic acid-Schiff (PAS) stain shows intense granular cytoplasmic positivity in the interstitial cells. Diastase digestion of the PAS stain eliminates the granular positivity to confirm its nature as glycogen.
 The interstitial spindle cells may extensively replace the polygonal interstitial cells with overgrowth and alteration of the basic microcystic pattern.
 The interstitial spindle cell component has a resemblance to the congenital peribronchial myofibroblastic tumor.
 presence of smooth muscle beneath the columnar epithelium with features of a terminal bronchiole-alveolar duct.
 small airway-like cartilage
 detected in the prenatal period to 3 months of age (median, 1-day old).

Immunochemistry

 The cuboidal cells lining the spaces show uniform nuclear reactivity for TTF-1.
 Virtually all of the interstitial cells are diffusely positive for vimentin.
 Desmin reactivity is a more focal finding in the interstitial cells and the positivity was not restricted to a population of subepithelial cells as in the case of cystic pleuropulmonary blastoma (PPB).
 These interstitial cells are myogenin-negative.
 A minority of epithelial lining cells and scattered interstitial cells display nuclear positivity for Ki-67 (MIB-1).

Electron microscopy

Electron microscopy shows that both epithelial lining and interstitial cells contained abundant glycogen in the cytoplasm.

Differential diagnosis

 pleuropulmonary blastoma type 1 (cystic PPB)
 congenital peribronchial myofibroblastic tumor (CPMT)

The differential diagnosis of congenital lung lesions includes a variety of pulmonary malformations, and uncommon or rare neoplasms such as the pleuropulmonary blastoma (PPB) and congenital peribronchial myofibroblastic tumor (CPMT).

Although most of the congenital lesions have a predominantly cystic appearance, the exceptions of a more solid process are the type 3 congenital cystic adenomatoid or pulmonary airway malformation (CCAM-CPAM) and the CPMT.

The interstitial spindle cell component has a resemblance to the congenital peribronchial myofibroblastic tumor.

References

 Diagnosis of "Fetal Lung Interstitial Tumor" Requires a FISH Negative for Trisomies 8 and 2. de Chadarévian JP, Liu J, Pezanowski D, Stefanovici C, Guzman M, Katz DA, Pascassio JM. Am J Surg Pathol. 2011 Jul;35(7):1085. PMID: 21677544

 Fetal lung interstitial tumor (FLIT): A proposed newly recognized lung tumor of infancy to be differentiated from cystic pleuropulmonary blastoma and other developmental pulmonary lesions. Dishop MK, McKay EM, Kreiger PA, Priest JR, Williams GM, Langston C, Jarzembowski J, Suchi M, Dehner LP, Hill DA. Am J Surg Pathol. 2010 Dec;34(12):1762-72. PMID: 21107081