August 2023 – Bone & Soft Tissue

A 58-year-old man underwent a chest X-ray after a minor elective surgery, and was found to have a 4 cm left pleural-based mass. His past medical history was significant for hypertension, hyperlipidemia, osteomalacia, and obesity. Surgical resection of the mass demonstrated loose arrays of spindled and stellate cells without nuclear atypia. The signing pathologist ordered in-situ hybridization of FGF23 mRNA, which showed overexpression.

Figure 1: H&E
Figure 2: FGF23 ISH

Which of the patient’s comorbidities are likely to resolve with the complete resection of this mass?

  • Obesity
  • Hypertension
  • Osteomalacia
  • Hyperlipidemia

The correct answer is ...


This patient has a rare soft tissue neoplasm called a phosphaturic mesenchymal tumor. The neoplastic cells overexpress and secrete FGF23, a hormone that inhibits phosphate reabsorption by proximal renal tubules. Laboratory workup prior to resection may have shown hypophosphatemia, hyperphosphaturia, and normocalcemia. Many patients present with osteomalacia with recurrent bone fractures and vitamin D deficiency. The tumor-induced osteomalacia often resolves with resection of the tumor. Roughly half of these tumors possess an FN1-FGF1 gene fusion.


  1. Folpe AL. Phosphaturic mesenchymal tumors: A review and update. Semin Diagn Pathol. 2019 Jul;36(4):260-268. doi:10.1053/j.semdp.2019.07.002. Epub 2019 Jul 5. PMID: 31301876. 
  2. Lee JC, Jeng YM, Su SY, Wu CT, Tsai KS, Lee CH, Lin CY, Carter JM, Huang JW, Chen SH, Shih SR, Mariño-Enríquez A, Chen CC, Folpe AL, Chang YL, Liang CW. Identification of a novel FN1-FGFR1 genetic fusion as a frequent event in phosphaturic mesenchymal tumour. J Pathol. 2015 Mar;235(4):539-45. doi:10.1002/path.4465. Epub 2015 Jan 7. Erratum in: J Pathol. 2015 May;236(1):131. PMID: 25319834.

Peter Kundert, M.D., Ph.D.

Resident, Anatomic and Clinical Pathology
Mayo Clinic

Jorge Torres-Mora, M.D.

Consultant, Anatomic Pathology
Mayo Clinic
Assistant Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

MCL Education

This post was developed by our Education and Technical Publications Team.