The levels of somatostatin receptors in causative tumors of oncogenic osteomalacia are insufficient for their agonist to normalize serum phosphate levels

Tumor-induced osteomalacia characterized by "painful knee joint with difficulty in moving": a case report

BACKGROUND

Tumor-related osteomalacia (TIO) is a rare paraneoplastic syndrome characterized by severe hypophosphatemia and osteomalacia. The diagnosis of TIO can be very difficult because of its nonspecific nature of clinical manifestations. Here we reported a case of young TIO patient with "painful knee joint with difficulty in moving" to improve the clinical diagnosis and treatment levels.

CASE PRESENTATION

The patient's clinical features were consistent with TIO. A tumor was successfully located in left tibial by ⁶⁸Ga-DOTATATE PET/CT, and then was surgically resected. Upon pathologic assessment, the tumor was diagnosed as phosphaturia stromal tumor (PMT) with positive Vim staining. After the surgery, serum phosphate level rapidly recovered and symptoms significantly improved.

CONCLUSION

TIO should be considered in patients with chronically hypophosphorus osteomalacia in the setting of no family history. Early removal of the responsible tumors is clinically essential for the treatment, and imaging examination is of great significance for tumor localization.

Diagnostic Utility of Somatostatin Receptor 2A Immunohistochemistry for Tumor-induced Osteomalacia

CONTEXT

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic disorder caused by excessive fibroblast growth factor 23 (FGF23) secretion. FGF23 immunohistochemistry (IHC) is proposed as a useful adjunctive marker to confirm TIO diagnosis. However, it often stains focally, limiting its diagnostic utility.

OBJECTIVE

This work aimed to compare the diagnostic performance between somatostatin receptor 2A (SSTR2A) and FGF23 IHC for TIO.

METHODS

We retrospectively reviewed TIO-diagnosed patients in Severance Hospital between July 2006 and May 2020. Histologic evaluation was performed using histoscore (H score) (expression area proportion score [0-2] × intensity score [1-3], [total, 0-6]). FGF23 and SSTR2A IHC were performed using unstained slides from 18 localized TIO patients and 9 and 15 non-TIO controls with bone and soft-tissue tumors, respectively. SSTR2A positivity was defined as cytoplasmic, membranous, or Golgi staining in more than 1% of tumor cells, and negativity as nonspecific nuclear staining. FGF23 positivity was defined as cytoplasmic expression in more than 1% of the tumor area and negativity as nonspecific nuclear staining.

RESULTS

Suspicious lesions were successfully detected in 14 of 15 patients who underwent 68Ga-DOTATOC scans. Diffuse cytoplasmic SSTR2A expression was identified in all TIO patients and focal weak nuclear staining in 12 of 15 controls. FGF23 cytoplasmic expression was identified in 11 of 18 TIO patients and diffuse nuclear staining in 9 of 9 controls. The H score was higher in SSTR2A than in FGF23 IHC (median [interquartile range]: 6 [6-6] vs 1 [0-2], P < .001).

CONCLUSION

SSTR2A IHC with H-score quantification might be a more sensitive, adjunctive diagnostic tool than FGF23 IHC for TIO diagnosis.

Tumor-induced osteomalacia: a case report

Background

Tumor-induced osteomalacia is a rare, acquired paraneoplastic syndrome, including hypophosphatemia, high serum alkaline phosphatase, reduced active vitamin D, suboptimal bone mineral density, bone pain, fragility fractures, and muscle weakness.

Case presentation

We report a case of 74–year–old male of mixed ancestry with hypophosphatemia resistant to treatment despite optimal compliance, associated with profound reduction of bone mineral density and multiple nontraumatic fractures, including bilateral rib fractures, lower-thoracic (T11, T12) vertebrae, and two fractures involving the surgical and anatomical neck of the right humerus. We discuss an approach to identifying the underlying cause of hypophosphatemia associated with fragility fractures, and options for management of this rare condition.

Conclusion

Although rare, tumor-induced osteomalacia can be diagnosed if a logical stepwise approach is implemented. Surgery could be curative if the tumor is properly located and is resectable.

Diagnostic performance of 68Ga-DOTATOC PET/CT in tumor-induced osteomalacia

OBJECTIVE

Tumor-induced osteomalacia (TIO) is caused by typically small tumors that secrete fibroblast growth factor 23 (FGF23). As tumor resection is the only effective treatment for TIO, it is important to detect the culprit tumor. We aimed to assess the utility of ⁶⁸Gallium-DOTA-D-Phe(1)-Tyr(3)-octreotide (⁶⁸Ga-DOTATOC) PET/CT in TIO and the correlation between biochemical parameters and the PET/CT results.

METHODS

Thirty-five patients with clinically suspected TIO who had undergone ⁶⁸Ga-DOTATOC PET/CT were retrospectively analyzed. ⁶⁸Ga-DOTATOC PET/CT results were compared with biochemical parameters and the final diagnosis, including histopathology.

RESULTS

⁶⁸Ga-DOTATOC PET/CT detected focal uptake consistent with TIO in 21/35 patients, one of which was considered false positive. In 16 patients, the cause of osteomalacia was confirmed histologically as phosphaturic mesenchymal tumor (n = 15) or fibrous dysplasia (n = 1). The other four patients were judged clinically as true positive by subsequent MRI and the clinical course. Overall, the detection rate of ⁶⁸Ga-DOTATOC PET/CT was 57% (20/35). Median tumor maximum standardized uptake value (SUVmax) was 6.9 (range 1.5-37.7). There was no significant difference in serum intact FGF23 level between DOTATOC-positive and DOTATOC-negative cases, and no significant correlation was observed between intact FGF23 level and tumor SUVmax.

CONCLUSIONS

⁶⁸Ga-DOTATOC PET/CT was clinically useful in detecting culprit tumors and subsequent patient management in TIO.

Tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is a rare paraneoplastic syndrome characterized by hypophosphatemia resulting from decreased tubular phosphate reabsorption, with a low or inappropriately normal level of active vitamin D. The culprit tumors of TIO could produce fibroblast growth factor 23 which plays a role in regulating renal Pi handling and 25-hydroxyvitamin D 1α-hydroxylase activity. Chronic hypophosphatemia could eventually lead to inadequate bone mineralization, presenting as osteomalacia. The diagnosis should be considered when patients manifest as hypophosphatemia and osteomalacia, or rickets and needs to be differentiated from other disorders of phosphate metabolism, such as the inhereditary diseases like X-linked hypophosphataemic rickets, autosomal dominant hypophosphataemic rickets, autosomal recessive hypophosphataemic rickets and acquired diseases like vitamin D deficiency. Localization of responsible tumors could be rather difficult since the vast majority are very small and could be everywhere in the body. A combination of thorough physical examination, laboratory tests and imaging techniques should be applied and sometimes a venous sampling may come into handy. The technology of somatostatin-receptor functional scintigraphy markedly facilitates the localization of TIO tumor. Patients undergoing complete removal of the causative neoplasm generally have favorable prognoses while a few have been reported to suffer from recurrence and metastasis. For those undetectable or unresectable cases, phosphate supplements and active vitamin D should be administrated and curative intended radiotherapy or ablation is optional.

A Challenging Case of Tumor-Induced Osteomalacia: Pathophysiological and Clinical Implications

We investigated the usefulness of fibroblast growth factor 23 (FGF23) intraoperative assay to monitor tumor resection in patients with oncogenic osteomalacia. A 33-year-old man with 5 years' history of lumbar and pelvis pain together with multiple vertebral fractures was admitted to our hospital. He was diagnosed with ankylosing spondylitis 1 year before. Laboratory investigation showed low tubular reabsorption of phosphate (0.41 mmol/L) despite chronic hypophosphatemia (0.39/L). Increased plasma values of FGF23 (673 pg/mL; n.v. < 95 pg/mL) were also observed. A full-body CT scan showed two suspicious areas in the head of the right femur and in the right tibia; however, the Octreoscan™ showed an increased uptake of the tracer only in the femur. We decided to remove first the head femur lesion and perform intraoperative FGF23 assay to confirm tumor resection; if this had been unsuccessful, we would have extended the operation to excise the second bone lesion. FGF23 basal values and 10, 60, and 225 min after excision of the femoral head were 423, 127, 56, and 30 pg/mL, respectively. The brisk fall of FGF23 values suggested that the head femur lesion was responsible for the syndrome. Histological examination revealed a mesenchymal highly vascular tumor. This is the first report showing the possibility of intraoperative FGF23 assay to monitor tumor resection in patients with tumor-induced osteomalacia.

Tumour-induced osteomalacia

Tumour-induced osteomalacia (TIO), also known as oncogenic osteomalacia, is a rare paraneoplastic disorder caused by tumours that secrete fibroblast growth factor 23 (FGF23). Owing to the role of FGF23 in renal phosphate handling and vitamin D synthesis, TIO is characterized by decreased renal tubular reabsorption of phosphate, by hypophosphataemia and by low levels of active vitamin D. Chronic hypophosphataemia ultimately results in osteomalacia (that is, inadequate bone mineralization). The diagnosis of TIO is usually suspected when serum phosphate levels are chronically low in the setting of bone pain, fragility fractures and muscle weakness. Locating the offending tumour can be very difficult, as the tumour is often very small and can be anywhere in the body. Surgical removal of the tumour is the only definitive treatment. When the tumour cannot be located or when complete resection is not possible, medical treatment with phosphate salts or active vitamin D is necessary. One of the most promising emerging treatments for unresectable tumours that cause TIO is the anti-FGF23 monoclonal antibody KRN23. The recent identification of a fusion of fibronectin and fibroblast growth factor receptor 1 (FGFR1) as a molecular driver in some tumours not only sheds light on the pathophysiology of TIO but also opens the door to a better understanding of the transcription, translocation, post-translational modification and secretion of FGF23, as well as suggesting approaches to targeted therapy. Further study will reveal if the FGFR1 pathway is also involved in tumours that do not harbour the translocation.

Diagnostic Modalities for FGF23-Producing Tumors in Patients with Tumor-Induced Osteomalacia

Fibroblast growth factor 23 (FGF23) is a hormone that is produced by osteocytes and regulates phosphate and vitamin D metabolism through binding to the Klotho-FGF receptor complex. Excessive actions of FGF23 cause several kinds of hypophosphatemic rickets/osteomalacia. Tumor-induced rickets/osteomalacia (TIO) is a paraneoplastic syndrome caused by overproduction of FGF23 from the responsible tumors. Because TIO is cured by complete resection of the causative tumors, it is of great clinical importance to locate these tumors. Several imaging methods including skeletal survey by magnetic resonance imaging and octreotide scintigraphy have been used to identify the tumors that cause TIO. However, none of these imaging studies indicate that the detected tumors are actually producing FGF23. Recently, systemic venous sampling was conducted for locating FGF23-producing tumor in suspected patients with TIO and demonstrated that this test might be beneficial to a subset of patient. Further studies with more patients are necessary to establish the clinical utility of venous sampling in patients with TIO.

FGF23 Producing Mesenchymal Tumor

A 40-year-old patient was referred to Clinic of Endocrinology due to hypophosphatemia causing pain, cramps, and weakness of muscles. Moreover, his bone mineral density was very low. The previous treatment with phosphorus and active vitamin D metabolites was ineffective. In lab tests the hypophosphatemia, hyperphosphaturia, and elevated FGF23 levels were found. Somatostatin receptor scintigraphy (SRS) showed increased radiotracer uptake in the right maxillary sinus and CT scans confirmed presence of tumor in this localization. Biopsy and cytological examination created suspicion of mesenchymal tumor—glomangiopericytoma. Waiting for surgery the patient was treated with long acting Somatostatine analogue, and directly before operation short acting Octreotide and intravenous phosphorus were used. Histology confirmed the cytological diagnosis and the phosphatemia return to normal values in 10 days after the tumor removal.

Tumor-induced osteomalacia: an important cause of adult-onset hypophosphatemic osteomalacia in China: Report of 39 cases and review of the literature

Tumor-induced osteomalacia (TIO) is an acquired form of hypophosphatemia. Tumor resection leads to cure. We investigated the clinical characteristics of TIO, diagnostic methods, and course after tumor resection in Beijing, China, and compared them with 269 previous published reports of TIO. A total of 94 patients with adult-onset hypophosphatemic osteomalacia were seen over a 6-year period (January, 2004 to May, 2010) in Peking Union Medical College Hospital. After physical examination (PE), all patients underwent technetium-99m octreotide scintigraphy ((99) Tc(m) -OCT). Tumors were removed after localization. The results demonstrated that 46 of 94 hypophosphatemic osteomalacia patients had high uptake in (99) Tc(m) -OCT imaging. Forty of them underwent tumor resection with the TIO diagnosis established in 37 patients. In 2 patients, the tumor was discovered on PE but not by (99) Tc(m) -OCT. The gender distribution was equal (M/F = 19/20). Average age was 42 ± 14 years. In 35 patients (90%), the serum phosphorus concentration returned to normal in 5.5 ± 3.0 days after tumor resection. Most of the tumors (85%) were classified as phosphaturic mesenchymal tumor (PMT) or mixed connective tissue variant (PMTMCT). Recurrence of disease was suggested in 3 patients (9%). When combined with the 269 cases reported in the literature, the mean age and sex distribution were similar. The tumors were of bone (40%) and soft tissue (55%) origins, with 42% of the tumors being found in the lower extremities. In summary, TIO is an important cause of adult-onset hypophosphatemia in China. (99) Tc(m) -OCT imaging successfully localized the tumor in the overwhelming majority of patients. Successful removal of tumors leads to cure in most cases, but recurrence should be sought by long-term follow-up.

Matrix extracellular phosphoglycoprotein is expressed in causative tumors of oncogenic osteomalacia

Oncogenic osteomalacia (OOM), or tumor-induced osteomalacia, is a rare disease characterized by renal phosphate wasting and osteomalacia. It arises due to the secretion of fibroblast growth factor 23 (FGF-23) from causative tumors. Matrix extracellular phosphoglycoprotein (MEPE) is predominantly expressed in odontoblasts, osteoblasts, and osteocytes. Although the presence of MEPE mRNA has been reported in some OOM tumors, little is known about the prevalence of MEPE expression in OOM tumors. In this study, the expression of MEPE and FGF-23 in OOM tumors was investigated at the transcriptional and translational levels. Eleven causative OOM tumors were analyzed by quantitative real-time reverse transcription-polymerase chain reaction and immunohistochemistry for MEPE and FGF-23 expression. Hemangiopericytomas and giant cell tumors, pathological diagnoses that are common in cases of OOM, were obtained from non-osteomalacic patients and analyzed as controls. The gene expression level of FGF23 and MEPE in OOM tumors was 10(4)- and 10(5)-times higher, respectively, than in non-OOM tumors. Immunohistochemical staining revealed that FGF-23 protein was expressed in all OOM tumors, and MEPE was expressed in 10 out of 11 OOM tumors. Thus, MEPE expression was common in OOM tumors, similar to FGF-23. These results indicate that, in addition to the hypophosphatemic effects of FGF-23, MEPE or the MEPE-derived acidic serine aspartate-rich MEPE-associated motif peptide may contribute to decreased bone mineralization in OOM patients.

Tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO) is a rare and fascinating paraneoplastic syndrome in which patients present with bone pain, fractures, and muscle weakness. The cause is high blood levels of the recently identified phosphate and vitamin D-regulating hormone, fibroblast growth factor 23 (FGF23). In TIO, FGF23 is secreted by mesenchymal tumors that are usually benign, but are typically very small and difficult to locate. FGF23 acts primarily at the renal tubule and impairs phosphate reabsorption and 1α-hydroxylation of 25-hydroxyvitamin D, leading to hypophosphatemia and low levels of 1,25-dihydroxy vitamin D. A step-wise approach utilizing functional imaging (F-18 fluorodeoxyglucose positron emission tomography and octreotide scintigraphy) followed by anatomical imaging (computed tomography and/or magnetic resonance imaging), and, if needed, selective venous sampling with measurement of FGF23 is usually successful in locating the tumors. For tumors that cannot be located, medical treatment with phosphate supplements and active vitamin D (calcitriol or alphacalcidiol) is usually successful; however, the medical regimen can be cumbersome and associated with complications. This review summarizes the current understanding of the pathophysiology of the disease and provides guidance in evaluating and treating these patients. Novel imaging modalities and medical treatments, which hold promise for the future, are also reviewed.