Phosphaturic Mesenchymal Tumor

Abstract

Phosphaturic mesenchymal tumors (PMTs) are neoplasms associated with tumor-induced osteomalacia. Patients typically present with pathologic fractures in the setting of chronic hypophosphatemic hyperphosphaturic osteomalacia, as well as gradual muscle weakness, bone pain, and difficulty walking. Because of their rarity and nonspecific symptomatology, phosphaturic mesenchymal tumors often go undiagnosed for years. Even when discovered on imaging, the tumors can be diagnostically challenging for radiologists. Phosphaturic mesenchymal tumors often tend to be small and can be located nearly anywhere in the body, and, therefore, can mimic many other tumors. This case highlights the imaging and pathologic markers of a phosphaturic mesenchymal tumor, often found in a patient with tumor-induced osteomalacia.

FIG 1.

⁶⁸Ga DOTATATE PET/CT demonstrates marked radiotracer uptake within the petrous aspect of the left temporal bone.

FIG 2.

From superior to inferior, axial (A–C) MR images show an avidly enhancing mass (solid arrows) extending from the middle ear along the floor of the middle cranial fossa, with substantial intraosseous involvement. A small portion of the mass (dashed straight arrow) closely approximates the geniculate ganglion of the facial nerve (solid curved arrow) (circles in B and C denote the external auditory canal). The mastoid air cells are completely opacified (asterisks). Coronal image (D) shows the mass extending intracranially, with associated dural thickening (dashed curved arrow).

FIG 3.

Noncontrast temporal bone CT performed 2 days after MR imaging again shows the mass (solid straight arrows) extending from the mesotympanum (A) into the anterior hypotympanum (B), which is widened. The mass is locally destructive (C), causing erosion of the skull base with involvement of the foramen spinosum (dashed straight arrow) and left temporomandibular joint (curved arrow). The margins of the mass are indistinguishable from fluid density within the middle ear and mastoid air cells (asterisks), which remained opacified.

FIG 4.

Preoperative otoscopy of the left tympanic membrane reveals a reddish mass (arrowheads) within the anterior mesotympanum. An amber effusion (star) is seen posterior to the mass, surrounding the incudostapedial joint (asterisk).

FIG 5.

Histologic examination of this tumor reveals bland, spindled-to-stellate neoplastic cells (A, H&E, original magnification ×400) situated in a hyalinized matrix with a well-developed capillary network including ectatic, staghorn vessels (asterisk) as well as characteristic deposition of coarse calcification (B, H&E, original magnification ×200; arrow). The tumor is reactive for SSTR2A (C, immunohistochemistry, original magnification ×200) and shows evidence of FGF23 mRNA expression (D, chromogenic in situ hybridization, original magnification ×200).