Monday, October 10, 2011

Phosphaturic Mesenchymal Tumor

Phosphaturic mesenchymal tumors are rare mesenchymal tumors that have a tendency to cause osteomalacia or rickets. The mechanism is thought to be related to secretion of fibroblast growth factor 23 (FGF-23), which inhibits renal phosphate reabsorption (causing phosphaturia) and renal production of 1,25-dihydroxyvitamin-D3. Serum concentrations of 1,25-dihydroxyvitamin-D3 are low in almost all cases.

These tumors have a variable histomorphological appearance and can be classified as:
  • Mixed connective tissue variants: Small spindled cells with and admixture of osteoclast-like giant cells, microcystic areas, prominent blood vessels, cartilage-like matrix, and osseous metaplasia.
  • Osteoblastoma-like tumors: Vascular, osteoblastic fibrous tissue with osteoclast-like giant cells and irregularly deposited osteoid.
  • Non-ossifying fibroma-like tumors: Moderately vascular tumor with swirling bundles of spindle cells and scattered osteoclast-like giant cells with areas of osteoid deposition.
  • Chondromyxoid fibroma-like:
Many oncogenic osteomalacia-associated tumors previously reported in the literature as hemangiopericytoma, osteosarcoma, giant cell tumor, and non-ossifying fibroma may have actually represented one of these variants of phosphaturic mesenchymal tumor.

Most cases of mesenchymal tumor-associated osteomalacia are due to the mixed connective tissue variant. The tumors occur in bone or soft tissue with an equal frequency. Of the bone lesions, involvement of the appendicular skeleton is more common. Patients tend to be middle-aged adults (range 3-73) with a long history of osteomalacia.

Mixed connective tissue variant tumors are usually small, inapparent lesions. The tumors may express somatostatin receptors, making octreotide scanning a helpful imaging technique for localizing these small tumors.

The images above are from a 60-year-old man with a 2-year history of progressive osteomalacia, fractures, and a left supraacetabular lesion. The patient was diagnosed at an outside facility with hemangiopericytoma and oncogenic osteomalacia. More careful evaluation of the slides revealed a diagnosis of phosphaturic mesenchymal tumor, mixed connective tissue variant.

Some tumors may express somatostatin receptors and be positive on octreotide scans.


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  • Weidner N, Santa Cruz D. Phosphaturic mesenchymal tumors. A polymorphous group causing osteomalacia or rickets. Cancer. 1987 Apr 15;59(8):1442-54.

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