Track

Case Reports

Abstract

Advancements in molecular testing are refining our understanding and classification of soft tissue neoplasms in pediatric patients, particularly those categorized as "unclassified spindle cell neoplasms." Chromosomal aberrations involving Pleomorphic Adenoma Gene 1 (PLAG1) are recognized in various tumors, including pediatric soft tissue tumors characterized by PLAG1 rearrangements. These tumors typically exhibit low-grade fibroblastic and myxoid morphology and express specific markers including desmin and CD34. However, differentiation between pediatric fibromyxoid tumors and lipoblastomas remains challenging due to overlapping gene fusions, necessitating further molecular characterization. Here, we present a 4-month-old infant girl with a gradually enlarging mass near her left thumb. MRI revealed a well-defined lesion without intramuscular extension or intralesional fat. Biopsies revealed a spindle cell lesion within a myxoid background and fibrous components. Minimal areas of lipomatous differentiation were noted solely towards the edges of the lesion. The lesional cells stained for desmin and CD34. These findings raised concern for Primitive Myxoid Mesenchymal Tumor of Infancy (PMMTI). However, next generation sequencing (NGS) did not reveal BCOR internal tandem duplication. Additionally, RNA sequencing detected PLAG1 rearrangement resulting in a novel MEG3::PLAG1 fusion. The lesion was subsequently diagnosed as Fibromyxoid tumor with PLAG1 fusion. This case highlights the utility of molecular testing in uncovering a novel MEG3::PLAG1 fusion that allowed differentiation of this unique lesion with no notable lipomatous differentiation from PMMTI and lipoblastomas.