Peptide Receptor Radionuclide Therapy for a Phosphaturic Mesenchymal Tumor

Tumor-induced osteomalacia is a very rare paraneoplastic syndrome. It can be caused by phosphaturic mesenchymal tumor (PMT), a generally benign tumor that produces fibroblast growth factor 23 (FGF-23), which can cause a severe renal phosphate wasting syndrome. Upon complete surgical removal of the tumor, FGF-23 normalizes and the osteomalacia is cured. In cases in which surgery is not feasible, radiofrequency ablation (RFA) is the treatment of choice. We describe a case with a PMT situated in the sacrum, in close proximity to the sacral plexus. Both surgery and RFA were considered potentially nerve damaging. Since the tumor showed expression of somatostatin receptors, we opted for a peptide receptor radionuclide therapy (PRRT) with ¹⁷⁷Lu-DOTATOC. However, the therapy did not show the expected success, since the FGF-23 level had even temporarily increased. The patient was then successfully treated with RFA. A partial remission of the tumor was achieved and FGF-23 levels nearly normalized. Despite some severe neurological side effects, the patient showed a remarkable clinical improvement, with no symptoms of osteomalacia within a few weeks.