Tumor-induced osteomalacia caused by a phosphaturic mesenchymal tumor of the femur
Tumor-induced osteomalacia (TIO) is an extremely rare paraneoplastic syndrome characterized by hypophosphatemia and hyperphosphaturia. Since its first description in 1947, more than 500 cases have been reported worldwide. TIO is an acquired hypophosphatemic osteomalacia caused by decreased phosphorus reabsorption in the renal tubules and increased renal phosphorus excretion. The clinical manifestations of TIO are diverse and non-specific, with the most common presentations being progressive bone pain, fracture, skeletal deformity, and movement dysfunction. However, it is extremely rare for a patient with TIO to present with adjacent bone destruction caused by a phosphaturic mesenchymal tumor, making prompt diagnosis and treatment challenging for clinicians.
This article presents a case of TIO with substantial bone destruction that was successfully treated surgically. A 15-year-old boy presented with a 3-year history of severe systemic multiple bone pain, hypophosphatemia, and mobility impairment. Physical examination showed decreased muscle strength in the bilateral lower limbs, sternal tenderness, and thoracic crushing pain. Laboratory tests demonstrated hypophosphatemia, elevated b-C-terminal telopeptide of type I collagen, normal parathyroid hormone level, elevated serum alkaline phosphatase level, and normal 1, 25-dihydroxy vitamin D level.
Radiography revealed substantial bone destruction in the right distal femur. Magnetic resonance imaging (MRI) showed a tumor in the right distal femur with irregular hyperintense signals on T1- and T2-weighted imagery. 99m Tc-Octreotide scanning and 68Gallium(Ga)-DOTA-TATE positron emission tomography/computed tomography (PET/CT) identified high-intake space-occupying lesions in the right distal femur, which was highly suggestive of oncogenic osteomalacia.
The patient underwent tumor resection and reconstruction with a knee prosthesis in the right lower extremity. Post-operative radiography showed satisfactory positioning of the prosthesis. Pathological results confirmed TIO with a phosphaturic mesenchymal tumor. The serum phosphate level returned to normal post-operatively, and symptoms improved substantially. The visual analogue scale score for bone pain improved from 6 to 7 pre-operatively to 1 to 2 post-operatively. The patient declined adjuvant treatment but underwent rehabilitation and outpatient monitoring.
TIO is a paraneoplastic syndrome usually associated with mesenchymal tumors, where phosphatonins decrease the renal resorption of phosphate, leading to hypophosphatemia, muscle weakness, and osteomalacia. The methods used to detect TIO include 99m Tc-Octreotide scanning and 68Ga-DOTA-TATE PET/CT. Somatostatin receptor imaging utilizes the characteristics of the somatostatin receptor (SSTR) expression in phosphaturic mesenchymal tumors. The accuracy of 68Ga-DOTA-TATE PET/CT in diagnosing TIO is 98.0%, which is significantly higher than other detection methods.
The final diagnosis of TIO depends on the history, clinical manifestations, laboratory tests, imaging, pathological characteristics, and treatment effect. In this case, the phosphaturic mesenchymal tumor had to be differentiated from neoplasms that commonly involve the femur in this age group, mostly osteosarcoma and other malignant tumors. While plain radiography might reflect the radiologic characteristics of the lesion, CT and MRI more accurately assess the extent of the mass within the bone and surrounding soft tissue. The 99m Tc-Octreotide imaging and 68Ga-DOTA-TATE PET/CT play a complementary role in obtaining an accurate diagnosis. Pathological results remain the “gold standard” to diagnose and definitively confirm the presence of a phosphaturic mesenchymal tumor.
Surgical resection is the optimal treatment option to correct the biochemical abnormalities and remineralize the bone substance in patients with TIO. It involves complete surgical excision to remove the pathogenic tumor, followed by pathological examination to confirm whether the tumor is a phosphaturic mesenchymal tumor. Improper selection of surgical procedures or incomplete resection can lead to the recurrence of the phosphaturic mesenchymal tumor and associated symptoms.
This case highlights the importance of accurate diagnosis and proper treatment for patients with a unique presentation of TIO. The successful outcome in this patient demonstrates the effectiveness of surgical resection in treating TIO caused by a phosphaturic mesenchymal tumor, even in cases with substantial bone destruction. The use of advanced imaging techniques, such as 99m Tc-Octreotide scanning and 68Ga-DOTA-TATE PET/CT, played a crucial role in the accurate diagnosis and localization of the tumor. The pathological confirmation of the phosphaturic mesenchymal tumor and the normalization of serum phosphate levels post-operatively further validate the diagnosis and treatment approach.
In conclusion, TIO caused by a phosphaturic mesenchymal tumor is a rare condition that can present with significant bone destruction. A comprehensive diagnostic approach, including clinical evaluation, laboratory tests, advanced imaging techniques, and pathological examination, is essential for accurate diagnosis. Surgical resection remains the primary treatment option, with the potential for complete resolution of symptoms and biochemical abnormalities. This case serves as an important reminder for clinicians to consider TIO in the differential diagnosis of patients presenting with hypophosphatemia and bone pain, even in the presence of substantial bone destruction.
doi.org/10.1097/CM9.0000000000000458
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