Oncogenic osteomalacia, an uncommon condition that orthopedic surgeons may come across, has been described in an editorial by Thomas O. Carpenter (N Engl J Med. 2003; 348:1705-1708). It is seen as decreased mineralization of newly formed bone with the clinical findings of osteomalacia. Patients with oncogenic osteomalacia frequently present with fractures and more severe pain than that in hypophosphatemic osteomalacia, which it resembles. They also may report muscle weakness. In this condition, renal loss of phosphate is excessive. The serum calcium usually is normal but serum alkaline phosphatase is raised.
Tumors are associated with oncogenic osteomalacia. They usually are benign, and in themselves, of little clinical significance, unlike the musculoskeletal disease they cause. The tumors may be small and difficult to detect. They tend to arise in the head and neck and must be searched for with great diligence as their removal can dramatically alter the course of the disease. A substantial number of deaths can be avoided by removing the associated tumor.
A recent diagnostic test for oncogenic osteomalacia has been described by Jonsson et al (N Engl J Med. 2003; 348:1656-1663). It is an assay of fibroblast growth factor-23 (FGF-23). Although FGF-23 is readily detectable in the plasma or serum of healthy individuals, it is markedly elevated in those patients with oncogenic osteomalacia and X-linked hypophosphatemia. This suggests that the growth factor has a role in phosphate homeostasis. Whereas oncogenic osteomalacia is an acquired phenotype, X-linked hypophosphatemia usually becomes evident by the second year of life.