Fibrosarcomatous Dermatofibrosarcoma Protuberans With COL1A1-PDGFB Fusion in a 2-Year-Old Child: A Rare Occurrence With Spectrum of Histopathological Findings and Review of Literature

Dermatofibrosarcoma protuberans (DFSP) is an intermediate-grade fibroblastic neoplasm commonly seen in young and middle-aged patients and rarely in pediatric patients. Fibrosarcomatous transformation is common in adults but extremely uncommon in children. Here, we present a case of a 2-year-old child who presented with a progressively enlarging subcutaneous mass in the knee. Histopathological examination revealed a spindle cell tumor with a storiform and fascicular pattern. Immunohistochemistry showed variable cluster of differentiation 34 (CD34) expression, with positivity in storiform areas and negativity in fascicular regions. Next-generation sequencing confirmed the diagnosis by detecting a collagen type I alpha 1 (COL1A1)-platelet-derived growth factor subunit B (PDGFB) fusion, with the PDGFB breakpoint in exon 2 (chromosome 22) and COL1A1 in intron 47 (chromosome 17). This case represents only the fifth reported instance of fibrosarcomatous DFSP in a child under 10 years old. While wide local excision remains the standard treatment for DFSP, targeted therapy with imatinib may be considered for unresectable, recurrent, or metastatic cases, though guidelines for pediatric patients are not yet established. This case highlights the importance of molecular testing in confirming the diagnosis of rare pediatric soft tissue tumors and contributes to the limited literature on fibrosarcomatous DFSP in very young children.