Malignant Phosphaturic Mesenchymal Tumor With Lung Metastasis

Malignant phosphaturic mesenchymal tumors (MPMT) are rarely seen soft tissue tumors. They can result in tumor-induced osteomalacia with hypophosphatemia. These tumors show FN1::FGFR1/FGF1 gene fusions. We present a 59-year-old male patient with a swelling in his right knee. Magnetic resonance imaging examination revealed a soft tissue mass with a maximum diameter of 2 cm in his distal right thigh. Histopathologically, the tumor was composed of atypical spindle cells. Coagulative tumor cell necrosis, extensive osteoid-like matrix, calcifications, and aneurysmal bone cyst-like areas were present. Mitotic index was 16/mm². The patient had a low blood phosphorus level, a high alkaline phosphatase level, and a normal calcium level. FN1::FGFR1 fusion was detected by next-generation sequencing (NGS) method. A diagnosis of MPMT was made. Twenty months after the initial diagnosis, newly developed nodules in the lungs were detected by the PET-CT scan. Temozolomide administration was initiated. We report a patient with an MPMT who presented to the clinic with soft tissue swelling as the first complaint. The NGS method contributed to the diagnostic processes. After 35 months of follow-up, he is alive with lung metastasis.