Low-grade fibromyxoid sarcoma with nuclear pleomorphism arising in the subcutis of a child.

Low-grade fibromyxoid sarcoma (LGFMS) represents a rare soft tissue tumor that was first characterized in 1987. LGFMS usually presents as a large, deeply situated mass in adults and is characterized by deceptively bland histopathologic features. LFGMS is less common in superficial soft tissue and in children. It is distinctly uncommon for LGFMS to exhibit nuclear pleomorphism. Herein, we present a case of a 10-year-old male who presented with a subcutaneous back mass that displayed features typical for LGFMS as well as scattered large, hyperchromatic and pleomorphic nuclei. The constellation of clinicopathologic features, including the young age of the patient, the small size and superficial location of the tumor and the presence of scattered nuclear pleomorphism are all unusual features for LGFMS. Fluorescent in situ hybridization (FISH) with a break-apart probe for FUS revealed the presence of a FUS gene rearrangement confirming the diagnosis of LGFMS. This case highlights the importance of maintaining a high index of suspicion for LGFMS even in the context of small, superficially-located tumors, pediatric patients or tumors with scattered nuclear pleomorphism.