Persistent tumor-induced osteomalacia confirmed by elevated postoperative levels of serum fibroblast growth factor-23 and 5-year follow-up of bone density changes.
OBJECTIVE
To describe a case of persistent tumor-induced osteomalacia, determine whether serum fibroblast growth factor-23 (FGF-23) levels postoperatively indicate incomplete tumor resection, and report lumbar spine and forearm bone mineral density (BMD) changes during 5 years of follow-up.


METHODS
We present clinical, radiologic, histologic, and bone densitometry data as well as serum FGF-23 levels (determined with use of a novel C-terminal enzyme-linked immunosorbent assay) from the study patient and discuss these findings in the context of previous literature.


RESULTS
A 52-year-old man, who presented with muscle weakness and multiple fractures, was found to have low values for serum phosphorus, serum 1,25-dihydroxyvitamin D, and maximal tubular reabsorption of phosphate per glomerular filtration rate, a high level of serum alkaline phosphatase, and a normal serum concentration of parathyroid hormone, characteristic of tumor-induced osteomalacia. Magnetic resonance imaging to evaluate an abnormality of the left foot revealed a soft tissue mass, biopsy of which confirmed the presence of a benign, phosphaturic, mesenchymal tumor. The baseline serum FGF-23 level (2,050 RU/mL) was more than 17 times the upper limit of normal for adults (23 to 118 RU/mL) and decreased substantially within 1 day after partial resection of the tumor but remained above normal postoperatively. BMD changes indicated rapid substantial recovery of vertebral BMD but ongoing loss of forearm bone density.


CONCLUSIONS
The serum FGF-23 level is high in a substantial proportion of patients with tumor-induced osteomalacia. The postoperative above normal levels of serum FGF-23 correlated with known persistence of tumor in our study patient. In a patient with normal renal function, such as our study patient, levels of serum FGF-23 studied with use of the C-terminal enzyme-linked immunosorbent assay reached their nadir within 24 hours postoperatively. This result suggests that this assay can provide clinicians with rapid prognostic information in patients with known or suspected residual tumor. BMD should be assessed at both appendicular and axial sites in patients with persistent tumor-induced osteomalacia.
Zimering
Mark B
MB
Medical Service, Veterans Affairs New Jersey Health Care System, Lyons, New Jersey 07939, USA.
Caldarella
Felice A
FA
White
Kenneth E
KE
Econs
Michael J
MJ
D015415:Biomarkers / Q000378:metabolism; D015519:Bone Density; D016723:Bone Remodeling; D000089703:Fibroblast Growth Factor-23; D005346:Fibroblast Growth Factors / Q000097:blood*; D005500:Follow-Up Studies; D005528:Foot; D005542:Forearm; D006801:Humans; D008297:Male; D008637:Mesenchymoma / Q000097:blood / Q000150:complications* / Q000473:pathology / Q000601:surgery; D008875:Middle Aged; D010018:Osteomalacia / Q000209:etiology*; D011184:Postoperative Period; D012983:Soft Tissue Neoplasms / Q000097:blood / Q000150:complications* / Q000473:pathology / Q000601:surgery; D013131:Spine / Q000378:metabolism
D015415:Biomarkers; C000717427:FGF23 protein, human; D005346:Fibroblast Growth Factors; D000089703:Fibroblast Growth Factor-23