Track

Case Reports

Abstract

Background: Phosphaturic Mesenchymal Tumor (PMT) is a rare neoplasm known for its association with tumor-induced osteomalacia. The presentation of PMT in the thumb is extremely rare. We report the first identification of COL6A3::FGFR1 gene fusion in PMT.

Case Presentation: A 59-year-old male with a past medical history of severe degenerative changes of the left knee and Fuchs' dystrophy of the left eye presented with a left thumb volar mass. Histopathological examination of the soft tissue mass revealed a cellular neoplasm with an infiltrative growth pattern, characterized by spindle cells with small nuclei and inconspicuous nucleoli. Osteoclast-like giant cells and a distinctive calcified matrix with a grungy appearance were noted. No increased mitotic figures activity or nuclear atypia was present. Immunohistochemical staining demonstrated focal and weak expression of CD56 and Epithelial Membrane Antigen (EMA) and negative staining for S100 and SOX10. A novel COL6A3::FGFR1 gene fusion was identified through a gene fusion panel, with no pathogenic mutations detected by solid tumor NGS panel analysis. Although this fusion has not been previously described, its resemblance to the reported FN1:FGFR1 gene fusions associated with PMT supports the overall diagnosis.

Conclusion: This case underscores the importance of considering PMT in atypical locations. The identification of COL6A3:FGFR1 gene fusion contributes to the expanding landscape of molecular insights into phosphaturic mesenchymal tumors.