Atypical nodular fasciitis with a novel PAFAH1B1-USP6 fusion in a 22-month-old boy.

Nodular fasciitis (NF) rarely occurs in infants aged < 2 years although cranial fasciitis develops predominantly in this age group. Histologically, NF may present high cellularity and brisk mitoses, but atypical forms are generally absent. Here, we report a NF in a 22-month-old Chinese boy. Microscopically, the lesion was manifested as cellular variant of NF. Notably, atypical mitotic figures including multipolar form were identified. Immunohistochemically, the neoplastic cells showed strong positivity for smooth muscle actin. Fluorescence in situ hybridization analysis revealed an unbalanced rearrangement of USP6, along with the USP6 increased copies. Subsequent next-generation sequencing-based technology revealed a novel PAFAH1B1-USP6 fusion gene as well as unusual fusion point on USP6 (exon 9). To the best of our knowledge, this is the only reported case with overt atypical mitosis. This case is also the first published example of genetically confirmed infant NF. Additionally, PAFAH1B1-USP6 fusion has never been described in NF.