Oncogenic vitamin D resistant hypophosphatemic osteomalacia (benign ossifying mesenchymal tumor of bone): case report

A 53-year-old Japanese woman suffered hypophosphatemic osteomalacia secondary to bone tumor. Her clinical symptoms did not improve for a long time following the oral administration of Alfacalcidol (vitamin D3). A bone survey using radiographs revealed a small tumor located in the proximal head of the left fibula. Hypophosphatemia and low levels of 1-alpha,25-dihydroxycholecalciferol (1-alpha,25(OH)2D3) returned towards normal levels soon after an en-bloc tumor resection, and bone pain and muscle weakness gradually disappeared. The histology, especially of the trabecular bone of the left fibula, showed typical features of osteomalacia with demineralization. The tumor comprised numerous small blood vessels, spindle shaped or oval neoplastic cells, a few multinucleated giant cells, osteoid tissue with or without calcification, chondroid tissue and hemangioma. The spindle shaped or oval cells contained a round nucleus with no mitotic figures. These clinical and histological findings coincide well with a diagnosis of benign ossifying mesenchymal tumor of the bone with vitamin D resistant hypophosphatemic osteomalacia.