Successful treatment of tumor-induced osteomalacia due to an intracranial tumor by fractionated stereotactic radiotherapy

Abstract

Context: Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome, characterized by tumor secretion of fibroblast growth factor-23 (FGF23) causing hypophosphatemia due to renal phosphate wasting. TIO is usually caused by small, benign, difficult-to-localize, mesenchymal tumors. Although surgery with wide excision of tumor borders is considered the "gold standard" for definitive therapy, it can be associated with considerable morbidity depending on the location. To date, radiation therapy has not been considered as an effective treatment modality in TIO.

Objective: A 67-year-old female presented with multiple nontraumatic fractures, progressive bone pain, and muscle weakness for 4 years. She was found to have biochemical evidence of urinary phosphate wasting with low serum phosphorus, low-normal serum calcium, normal 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D, and high serum FGF23 levels. TIO was diagnosed. Selective venous sampling for FGF23 confirmed that a 1.7-cm left frontal mass, radiographically similar to a meningioma, was the causative tumor. She declined surgery due to fear of complications and instead underwent fractionated stereotactic radiotherapy for 6 weeks.

Results: In less than 4 years after radiation therapy, she was successfully weaned off phosphorus and calcitriol, starting from 2 g of oral phosphorus daily and 1 μg of calcitriol daily. Her symptoms have resolved, and she has not had any new fractures.

Conclusions: Stereotactic radiotherapy was an effective treatment modality for TIO in our patient. Fractionated stereotactic radiation therapy represents an alternative to surgery for patients with TIO who are not surgical candidates or who decline surgery.

Figure 1.

Imaging studies. A, CT scan of the head revealed a 17 × 13 × 15-mm medial left frontal mass suggestive of meningioma. B, MRI of the head confirmed the lesion. C, Octreoscan was positive for an intracranial lesion corresponding to the left frontal mass. D, FDG-PET demonstrated generalized increased uptake by the brain. E, MRI of the head in 2009 before fractionated stereotactic radiation therapy. F, MRI of the head 1 year after fractionated stereotactic radiation therapy in 2010 revealed stable in size tumor with multiple small hemorrhages within it.

Figure 2.

Selective venous sampling for FGF23. There was a significant step-up in FGF23 concentration as the suspected tumor was approached anatomically.