All but one of these patients had a history of tumor progression before enrollment, suggesting that changes in tumors during the study may represent the natural course of unresectable or incompletely resected phosphaturic mesenchymal tumors.(2, 26, 27) Burosumab's mechanism of action targets excess FGF23, addressing renal phosphate wasting and impaired vitamin D synthesis with consequent osteomalacia. Here, FGF23 is linked to osteomalacia.