Spindle Cell/Sclerosing Rhabdomyosarcoma With PAX8::PPARG Fusion.

The spindle cell/sclerosing subtype of rhabdomyosarcoma is classified based on genetic features into the three categories of MYOD1 -mutated, gene fusion-driven, and a subset without a currently identified genetic driver event. The gene fusion-driven spindle cell/sclerosing rhabdomyosarcomas are heterogenous and characterized by increasing numbers of gene fusions, the most common gene partners being VGLL2 , NCOA2 , and TFCP2 . Here we report a spindle cell/sclerosing rhabdomyosarcoma that arose in the orbit of a 4-year-old male. This tumor harbored a unique PAX8::PPARG fusion. PAX8::PPARG fusions have previously only been described in follicular thyroid carcinoma and follicular variant of papillary thyroid carcinoma. Our report adds to the growing number of gene fusions in spindle cell/sclerosing rhabdomyosarcomas.