summary Oncogenic Osteomalacia is a paraneoplastic syndrome of renal phosphate wasting caused by a bone or soft tissue tumor. Patients may present with osteopenia and pathologic fractures. Diagnosis is made with PET/SPECT scans to identify the location of the tumor. Treatment involves identifying and resecting the tumor. Epidemiology Demographics age bracket age of onset is late childhood to early adulthood Etiology Pathophysiology the tumor secretes a humoral factor ("phosphatonin") that affects the proximal renal tubules phosphatonins include FGF-23 (most important), sFRP-4, and MEPE reduces calcitriol production in the kidney and inhibits phosphate transport leads to increased renal phosphate excretion, hypophosphatemia and osteomalacia types of tumors that cause oncologic osteomalacia (known as phosphaturic mesenchymal tumor) benign tumors (more common) phosphaturic mesenchymal tumors (mixed connective tissue variant) e.g. hemangiopericytoma (commonest cause) osteoblastoma-like tumors ossifying fibrous tumors nonossifying fibrous tumors malignant causes (rare) osteosarcoma fibrosarcoma Genetics mutations the key phosphatonin is FGF23, and this condition is linked to a mutation in the gene coding for FGF23 Associated conditions orthopaedic manifestations pathological fractures of long bones and vertebrae Presentation Symptoms generalized bone and muscle pain fractures of long bones, ribs and vertebrae proximal muscle weakness fatigue Imaging Radiographs findings diffuse osteopenia Looser's zones (pseudofractures) Octrotide scans (radiolabeled somatostatin analog) gallium-68 DOTA-octreotate PET scan indium-111 pentetreotide SPECT/CT indications to identify primary tumors when TIO is suspected will only identify tumors expressing somatostatin receptors Studies Lab Values Serum Ca Serum P Alk phos PTH 25-(OH)vit D 1,25-(OH)vit D Urinary Ca Osteomalacia Low Low High High Low Low Low Osteoporosis Normal Normal Variable Normal Normal Normal Normal Tumor induced osteomalacia Low Very low Low Low Low Low Low Osteopetrosis Normal Normal High Normal Normal Normal Normal Treatment Nonoperative phosphate supplementation with 1,25-dihydroxyvitamin D Operative tumor removal outcomes resolution of hypophosphatemia and low vitamin D levels within hours of resection