Hypophosphatemia mesenchymal tumors (HMT) are rare, of uncertain origin, and may cause osteomalacia derived from paraneoplastic syndrome. The clinical manifestations are caused mainly by phosphatase secretion promoted by the tumor cells, leading to excessive kidney excretion of phosphate, fractures, bone pain, hypophosphatemia, and low calcitriol levels. HMTs are challenging to locate through anatomic imaging because they are relatively small and occult.

We describe a case that illustrates the potential of nuclear medicine to identify HMT. A 52-year-old female patient complained of diffuse pain, especially in the hips, rib cage, and feet, for over one year. Magnetic resonance imaging (MRI) of the hip demonstrated bilateral avascular necrosis of the femoral heads. To evaluate a possible osteometabolic alteration, such as hyperparathyroidism, she underwent bone scintigraphy, which did not reveal signs of hyperparathyroidism but identified (in addition to the avascular necrosis) signs of hypertrophic osteoarthropathy, suggesting paraneoplastic syndrome. Laboratory tests showed normal PTH, hypophosphatemia, and phosphaturia. The patient initiated oral phosphorus replacement, which only partially reduced bone pain. She was submitted to an FDG-18F PET/CT to search for an occult tumor, which was negative. Considering the patient´s phosphaturia and reports of hypophosphatemia and osteomalacia caused by occult non-mesenchymal tumors of the soft tissues, the hypothesis of phosphate-producing HMT was considered. These tumors arise mainly in the lower limbs, generate pain and a predisposition to small fractures in subchondral bones (more common in the femoral heads), and express somatostatin receptors. Thus, a DOTATATE-68Ga PET/CT was performed to locate this somatostatin-expressing occult tumor. The images showed a 0.5 cm intramuscular nodule with hyperexpression of somatostatin receptors deep within the left thigh muscle. To prepare for surgery, an MRI of the left thigh was performed to locate the nodule with DOTA-68Ga uptake. MRI showed the nodule was quite deep within the muscle and close to the left femoral vascular-nerve bundle. Due to the lesion's small size, deep location, and proximity to the neurovascular bundle, radioguided surgery with DOTA-68Ga was performed to remove the nodule. Histopathology concluded the nodule was consistent with a hypophosphatemia mesenchymal tumor. After the removal of the tumor, the phosphate level normalized, the pain disappeared, and the patient reported improved physical and mental health.

In conclusion, a bone scan was essential to identify the possibility of an occult tumor due to the imaging characteristics of paraneoplastic syndrome, and the DOTATATE-68Ga PET/CT was vital to locate the tumor.