Clinical History:

The patient is a 55-year-old man with chest pain and dyspnea after a ground-level fall. He also reports persistent generalized body pain over the past year. His past medical history includes hypertension, hyperlipidemia, diabetes, and coronary artery disease.

Imaging findings:

A CT scan of the chest showed numerous bilateral rib fractures and a mass in the sternum (Figure 1). Additional imaging showed femoral, scapula, and pubic rami fractures. A PET scan showed areas of FDG avidity in the maxillary sinus as well as in the sternal mass (Figure 2).

Clinically, the constellation of findings is concerning for tumor-induced osteomalacia. Laboratory studies show elevated serum FGF-23 and low serum phosphorus, supporting this clinical impression. The maxillary sinus lesion was biopsied first and reported as an EBV-positive non-keratinizing undifferentiated nasopharyngeal carcinoma. As this tumor type is not known to be associated with tumor-induced osteomalacia, a biopsy of the sternal lesion was pursued.

Microscopic Description:

The lesion is composed of sheets of cytologically bland round to spindle-shaped cells involving bone (Figure 3). There is abundant vasculature throughout the lesion (primarily small capillaries). Focal calcifying matrix with a “grungy” appearance is observed (Figure 4). The lesional cells are positive for FGF23 mRNA via chromogenic in-situ hybridization (Figure 5).

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

What is the most likely diagnosis?

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Mesenchymal chondrosarcoma

The correct answer is:

C) Phosphaturic mesenchymal tumor (PMT)

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