Most Osteomalacia-associated Mesenchymal Tumors Are a Single Histopathologic Entity

Abstract

Oncogenic osteomalacia (OO) is a rare paraneoplastic syndrome of osteomalacia due to phosphate wasting. The phosphaturic mesenchymal tumor (mixed connective tissue variant) (PMTMCT) is an extremely rare, distinctive tumor that is frequently associated with OO. Despite its association with OO, many PMTMCTs go unrecognized because they are erroneously diagnosed as other mesenchymal tumors. Expression of fibroblast growth factor-23 (FGF-23), a recently described protein putatively implicated in renal tubular phosphate loss, has been shown in a small number of mesenchymal tumors with known OO. The clinicopathological features of 32 mesenchymal tumors either with known OO (29) or with features suggestive of PMTMCT (3) were studied. Immunohistochemistry for cytokeratin, S-100, actin, desmin, CD34, and FGF-23 was performed. The patients (13 male, 19 female) ranged from 9 to 80 years in age (median 53 years). A long history of OO was common. The cases had been originally diagnosed as PMTMCT (15), hemangiopericytoma (HPC) (3), osteosarcoma (3), giant cell tumor (2), and other (9). The tumors occurred in a variety of soft tissue (21) and bone sites (11) and ranged from 1.7 to 14 cm. Twenty-four cases were classic PMTMCT with low cellularity, myxoid change, bland spindled cells, distinctive "grungy" calcified matrix, fat, HPC-like vessels, microcysts, hemorrhage, osteoclasts, and an incomplete rim of membranous ossification. Four of these benign-appearing PMTMCTs contained osteoid-like matrix. Three other PMTMCTs were hypercellular and cytologically atypical and were considered malignant. The 3 cases without known OO were histologically identical to the typical PMTMCT. Four cases did not resemble PMTMCT: 2 sinonasal HPC, 1 conventional HPC, and 1 sclerosing osteosarcoma. Three cases expressed actin; all other markers were negative. Expression of FGF-23 was seen in 17 of 21 cases by immunohistochemistry and in 2 of 2 cases by RT-PCR. Follow-up (25 cases, 6-348 months) indicated the following: 21 alive with no evidence of disease and with normal serum chemistry, 4 alive with disease (1 malignant PMTMCT with lung metastases). We conclude that most cases of mesenchymal tumor-associated OO, both in the present series and in the reported literature, are due to PMTMCT. Improved recognition of their histologic spectrum, including the presence of bone or osteoid-like matrix in otherwise typical cases and the existence of malignant forms, should allow distinction from other mesenchymal tumors. Recognition of PMTMCT is critical, as complete resection cures intractable OO. Immunohistochemistry and RT-PCR for FGF-23 confirm the role of this protein in PMTMCT-associated OO.

The Phosphaturic Mesenchymal Tumor: Why is Definitive Diagnosis and Curative Surgery Often Delayed?

“…Several tumor types allegedly cause tumor-induced osteomalacia; however, the majority of cases are most likely caused by a single histopathologic entity-phosphaturic mesenchymal tumors [6]. Morphologically, these tumors are characterized by bland, uniform spindled to stellate cells that often are intermixed with an unusual smudgy matrix with a well-developed capillary network and variable calcification.…”

“…Morphologically, these tumors are characterized by bland, uniform spindled to stellate cells that often are intermixed with an unusual smudgy matrix with a well-developed capillary network and variable calcification. In the majority of cases no atypia is present and mitotic activity and necrosis are absent [5,6]. Most tumors express FGF-23 by immunohistochemistry, of which the proliferating cells in the tumor are immunoreactive [3,6].…”

“…The resection of the identified tumor resulted in resolution of the osteomalacia and he postulated that the granuloma was secreting a rachitogenic substance. The majority of cases of tumor-induced osteomalacia reported are benign, but malignant varieties have been described [6,15,[22][23][24]. Although tumor-induced osteomalacia has been reported in association with 140 different tumors [19], most tumorinduced osteomalacia-associated tumors are of a single histopathologic entity: phosphaturic mesenchymal tumors [5,6].…”

“…The majority of cases of tumor-induced osteomalacia reported are benign, but malignant varieties have been described [6,15,[22][23][24]. Although tumor-induced osteomalacia has been reported in association with 140 different tumors [19], most tumorinduced osteomalacia-associated tumors are of a single histopathologic entity: phosphaturic mesenchymal tumors [5,6]. Since Prader et al first recognized the link between neoplasia and osteomalacia, there have been approximately 250 to 300 cases of tumor-induced osteomalacia described in the literature [3,5,6].…”

“…Although tumor-induced osteomalacia has been reported in association with 140 different tumors [19], most tumorinduced osteomalacia-associated tumors are of a single histopathologic entity: phosphaturic mesenchymal tumors [5,6]. Since Prader et al first recognized the link between neoplasia and osteomalacia, there have been approximately 250 to 300 cases of tumor-induced osteomalacia described in the literature [3,5,6]. These tumors are now known to cause renal phosphate wasting, hypophosphatemia, and decreased serum 1,25-dihydroxyvitamin D3 levels resistant to vitamin D supplementation.…”