Phosphaturic Mesenchymal Tumour of the Soft Tissue in the Finger Presenting without Tumour-Induced Osteomalacia: A Case Report

Phosphaturic mesenchymal tumour (PMT) is a rare neoplasm that can occur with or without tumour-induced osteomalacia (TIO). While PMTs commonly arise in the lower extremities or head and neck, digital involvement is rare. A 70-year-old man presented with a 12-year history of an enlarging subcutaneous mass on the dorsal aspect of his left ring finger. Imaging studies suggested an organised thrombus within a venous malformation. Marginal excision was performed, and histopathological examination revealed a lesion composed of capillaries and small vessels surrounded by spindle cells with small ovoid nuclei, showing immunopositivity for somatostatin receptor 2, CD56, ERG and focally fibroblast growth factor-23 (FGF23). These findings were consistent with a diagnosis of PMT, mixed connective tissue and non-phosphaturic variant. No recurrence has been observed at 4 years and 6 months postoperatively. This report emphasises the diagnostic difficulty of PMTs in atypical locations and the utility of immunohistochemical profiling in confirming the diagnosis.