EGFR Exon 20 Insertion/Duplication Mutation in Fibrous Hamartoma of Infancy With Predominantly Pseudoangiomatous Pattern Mimicking Giant Cell Fibroblastoma.

We describe a case of fibrous hamartoma of infancy (FHI) with predominant pseudoangiomatous histologic pattern that harbors a recently described characteristic molecular feature of FHI: EGFR exon 20 insertion/duplication mutation. A left axillary mass of an 8-month-old male was biopsied. Microscopic findings revealed a CD34-positive spindle cell proliferation accompanied by slit like spaces within collagen, mimicking giant cell fibroblastoma. The mass was excised. The vast majority of the mass showed histologic findings similar to the biopsy. However, the peripheral rim of the mass demonstrated focal triphasic histology suggestive of FHI. Giant cell fibroblastoma was excluded by a negative COL1A1/ PDGFB fusion by reverse transcriptase-polymerase chain reaction. Targeted Sanger sequencing revealed the presence of an EGFR exon 20 insertion/duplication mutation. We posit that identification of the characteristic EGFR exon 20 molecular aberration may be of diagnostic value in FHI with variant histology and help exclude more aggressive diagnoses.