Oncogenic osteomalacia: two case reports with surprisingly different outcomes

Oncogenic osteomalacia is a rare paraneoplastic syndrome of acquired hypophosphatemic osteomalacia, resulting from a deficit in renal tubular phosphate reabsorption, in which fibroblast growth factor 23 (FGF23) seems to be implicated. This condition is usually associated with a phosphaturic mesenchymal tumor of mixed connective tissue located in the bone or soft tissue. The clinical and the radiologic findings are the same as those seen in osteomalacia, and the biochemical features include renal phosphate loss, low serum phosphate and 1,25-(OH)(2) vitD(3) levels, increased alkaline phosphatase, and normal calcium, PTH, calcitonin, 25-OH-vitD(3) and 25,25-(OH)(2) vitD(3). We present two cases of oncogenic osteomalacia associated with phosphaturic mesenchymal tumors, which were histologically similar, but presented a completely different evolution. In the first patient, the tumor developed on the sole of the foot. Following removal of the mass, the symptoms resolved and biochemical and radiological parameters returned to normal. However, in the second patient, a liver tumor developed and resection did not resolve the disease. Multiple lesions appeared in several locations during follow-up. This disease usually remits with complete tumor resection. Nevertheless, if this is not possible, oral treatment with phosphate, calcium and calcitriol can improve the symptoms. If scintigraphy of the tumor shows octreotide receptors, patients may respond partially to therapy with somatostatin analogs, with stabilization of the lesion.