Peptide Receptor Radionuclide and Octreotide: A Novel Approach for Metastatic Tumor-Induced Osteomalacia

KEY MESSAGES: Octreotide can be used as an adjunctive therapy to increase phosphorus levels in patients with tumor-induced osteomalacia. Malignant phosphaturic mesenchymal tumor (PMT) may benefit from treatment with peptide receptor radionucleotide therapy. CONTEXT: The success of treatment modalities for malignant PMT is limited. Octreotide has been used to treat hypophosphatemia in patients with tumor-induced osteomalacia with equivocal results. To our knowledge, there are no reports of octreotide or peptide receptor radionuclide therapy use for malignant PMT. CASE DESCRIPTION: We report a 40-year-old man having hypophosphatemia, phosphaturia (tubular maximum of phosphorus corrected for glomerular filtration rate of <2.5 mg/dL), and somatostatin avid lesions in the right foot region with metastasis to both lungs. The patient had been subjected to resection of the primary tumor from the foot with thoracoscopic removal of the lung secondaries. Histology from all three lesions showed a spindle cell soft tissue tumor with a high mitotic index and somatostatin receptor 2 and 5 positivity. A trial of subcutaneous octreotide therapy at a dose of 100 μg thrice daily resulted in an increase in serum phosphorus levels from an average of 1.44 mg/dL to an average of 2.3 mg/dL. Finally, the affected limb was amputated, and the hypophosphatemia persisted postoperatively. In view of persistent hypophosphatemia and transient response to octreotide, the patient was administered four cycles of peptide receptor radionuclide therapy using (177)Lutetium, which showed moderate improvement of serum phosphorus levels. CONCLUSION: Although octreotide use has been reported in four patients with benign PMT, to our knowledge, this is the first case of malignant PMT that has used peptide receptor radionuclide therapy in the treatment of malignant PMT. This moderately beneficial evidence is likely to guide the future use of radionuclide treatments in such tumors.