Tumor-induced osteomalacia (TIO; osteomalacia = soft bones), also known as oncogenic osteomalacia, is a rare form of low blood phosphorus (hypophosphatemia) that can present anytime in life. Individuals with TIO have bone pain, stress fractures, and muscle weakness. Stress fractures are caused by poor bone mineralization (osteomalacia), from low blood phosphorus. Multiple fractures can cause other deformities and even bowing of the legs. Muscle weakness and bone deformities can make movement difficult and painful.
TIO is caused by a small tumor that produces extra amounts of FGF23, a hormone that decreases phosphorus levels in the blood. Extra-high FGF23 levels cause phosphorus loss in the urine and low blood phosphorus. The severity of the hypophosphatemia and the problems it creates depends upon how much FGF23 is produced by the tumor and can vary greatly. These tumors are benign in most cases and can be found anywhere in the body, and even within bones.
TIO is diagnosed based on the clinical findings and biochemical tests (low phosphorus, high FGF23, and other tests). The major challenge is finding the tumor that produced the excess FGF23 since they can be tiny and hard to find. Newer imaging methods using PET/CT scanning have greatly improved the ability to locate the tumor that causes the disease.
Removal of the tumor producing the excess FGF-23 essentially cures the condition; phosphorus returns to normal within hours, although the bone disease (osteomalacia) may take some time to completely heal. Eventually, though, patients fully recover, unless permanent deformities have occurred. When the tumor cannot be located, or when surgical removal is not possible, oral phosphorus and vitamin D in different forms has been used to increase blood phosphorus. However, this approach was not satisfactory in most cases, as the vitamin D doses required may cause high calcium in the urine and kidney damage; and oral phosphorus tastes bad and can cause diarrhea. Fortunately, a new medication, burosumab-twza (Crysvita®) became available in 2018. Burosumab reduces FGF23 in the bloodstream, leading to improvement of the serum phosphorus levels, often to the normal range. The related bone disease heals over time. The doses of burosumab, which is given by under the skin injections every 2 – 4 weeks, must be individually tailored for each patient. Doses are based on weight, serum phosphorus, urine calcium, and urine phosphorus levels. Therefore, treatment requires close supervision by a doctor with experience with this new medication.
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