Diagnosis: Phosphaturic mesenchymal tumour.

Phosphaturic mesenchymal tumours are very rare neoplasms of soft tissue and bone. Mostly located in the extremities and the head and neck region. The etiology is unknown. Tumour cells are genetically characterized by rearrangements in the Fibronectin 1 gene (FN1). They cause tumour-induced osteomalacia through production of fibroblast growth factor 23 (FGF23). FGF23 is a phosphaturic hormone that inhibits phosphate reuptake from renal proximal tubules and reducing renal vitamin D production. As a compensatory mechanism phosphate and calcium is released from bone, causing systemic demineralization.

The morphology is usually of bland spindle to stellate cells in a hemangiopericytoma-like capillary network. The tumour cells produce a characteristic basophilic smudgy matrix that often calcifies. Adipose tissue, myxoid, chondroid and osteoid areas may also be present. Mitotic activity and necrosis are rare. Malignant morphology is most often found in locally recurring lesions.

Immunohistochemistry: Most often positive for SATB2, CD56, ERG, FGFR1 and most often negative for keratins, CD34, S100 and SMA.

Molecular detection of FN1-FGFR1 or FN1-FGF1 fusion confirms the diagnosis but is not required in routine work.

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