Microarchitectural parameters and bone mineral density in patients with tumour-induced osteomalacia by HR-pQCT and DXA

Healing of tumor-induced osteomalacia as assessed by high-resolution peripheral quantitative computed tomography is not similar across the skeleton in the first years following complete tumor excision

Tumor-induced osteomalacia is caused by excessive fibroblast growth factor 23 production mainly from phosphaturic mesenchymal tumors. Surgical excision or tumor ablation are the preferred treatment. Information on bone microarchitecture parameters assessed by high-resolution peripheral quantitative computed tomography is limited. We report a woman with hypophosphatemic osteomalacia with generalized pain, weakness and recurrent fractures, and a large thoracic vertebral mass extending to the posterior mediastinum. Detailed radiologic and histopathologic evaluation revealed a phosphaturic mesenchymal tumor. Two surgeries were necessary for complete removal of the mass. Clinical symptoms improved after attaining normophosphatemia. Four-year post-surgical HR-pQCT parameters, compared to baseline, showed in the left distal radius, stable trabecular and cortical volumetric bone mineral density although below reference range. There was stability of trabecular number and thickness. Both stiffness and failure load decreased. A shift in cortical parameters was noted in year 2. In the left distal tibia, trabecular volumetric bone mineral density decreased whereas cortical volumetric bone mineral density markedly increased, as did cortical area. There was stability in the trabecular number and thickness. Both stiffness and failure load improved. Findings from HR-pQCT measurements in this patient disclosed that the healing of osteomalacia is not similar across the peripheral skeletal sites in the first years following tumor removal. Results contrasted low but stable volumetric bone mineral density in the distal radius with increase in the distal tibia at the expense of cortical bone. Our report helps further delineate the pattern of bone healing after treatment of this rare bone disorder.

Approaching virtual osteoid volume estimation and in-depth tissue characterization in patients with tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO) poses a significant diagnostic challenge, leading to increased disease duration and patient burden also by missing clinical suspicion. Today, diagnosis of osteomalacia relies on invasive iliac crest biopsy, if needed. Therefore, a noninvasive method would be beneficial for patients with severe osteomalacia, such as TIO, to inform their clinical management and address specific needs, like estimating the regeneration capacity at high osteoid volumes (OVs) or the potential of a hungry bone syndrome after tumor removal. Furthermore, given the lack of comprehensive histological characterization of TIO, there is a need for additional tissue characterization. Therefore, our assessment encompassed iliac crest biopsies that were examined using quantitative electron backscattered microscopy, Raman spectroscopy, micro-computed tomography, and histology to analyze the biopsy tissue. Our clinical assessment encompassed DXA and high-resolution peripheral quantitative computed tomography (HR-pQCT) alongside with biochemical analyses and clinical evaluations. Combining imaging and clinical data, we established a model to predict the OV. We compared 9 TIO patients with 10 osteoporosis (OPO) patients and 10 healthy controls. Histological analyses confirmed a pronounced OV in TIO patients (OPO: 1.20% ± 1.23% vs TIO: 23.55% ± 12.23%, P < .0005), and spectroscopy revealed lower phosphate levels in TIO biopsies. By combining HR-pQCT and laboratory diagnostics, we developed a linear regression model to noninvasively predict the OV revealing significantly higher modeled OV/BVmodel values of 24.46% ± 14.22% for TIO compared to the control group (5.952% ± 3.44%, P ≤ .001). By combining laboratory diagnostics, namely, ALP and Tt.BMDRadius measured by HR-pQCT, we achieved the calculation of the virtual osteoid volume to bone volume ratio (OV/BVmodel) with a significant correlation to histology as well as reliable identification of TIO patients compared to OPO and control. This novel approach is potentially helpful for predicting OV by noninvasive techniques in diagnostic procedures and improving the clinical management of TIO.

Shift in Calcium From Peripheral Bone to Axial Bone After Tumor Resection in Patients With Tumor-Induced Osteomalacia

CONTEXT

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by excessive production of fibroblast growth factor 23 (FGF23) by a tumor. After successful tumor resection, patients can recover from hypophosphatemia quicky. However, data on the changes in bone mineral density (BMD) and microstructure in the short term after surgery remained unclear.

OBJECTIVE

This work aimed to investigate the postoperative changes in BMD and microstructure both in peripheral and axial bone in TIO patients.

METHODS

We evaluated BMD and microarchitecture in 22 TIO patients using high-resolution peripheral quantitative computed tomography (HR-pQCT) and dual-energy x-ray absorptiometry (DXA) before and 3 months after surgery in this retrospective study.

RESULTS

In this study, a total of 22 TIO patients who had recovered serum phosphate levels postoperatively were enrolled. After surgery, areal BMD (aBMD) increased by 21.6% in the femoral neck, by 18.9% in the total hip, and by 29.5% in the lumbar spine. Moreover, TBS increased by 14.1% (all P < .001). In contrast, trabecular or cortical volumetric BMD (vBMD), and microstructure of trabecular bone (trabecular number, separation and bone volume ratio) and cortical bone (cortical thickness and porosity) at the distal radius or tibia were further deteriorated. Correlation analyses found that changes in femoral neck and total hip aBMD were both conversely associated with changes in trabecular vBMD and bone volume ratio, while positively correlated with change in trabecular separation at the distal radius.

CONCLUSION

Although aBMD and microstructure in the axial bone were improved, vBMD and microstructure in the peripheral bone were further impaired shortly after surgery. Correlation of improvement of aBMD in the total hip and femoral neck with deterioration of vBMD and microstructure at the distal radius indicated a shift in calcium from the peripheral bone to the axial bone in the short term after tumor resection in TIO patients.

Global guidance for the recognition, diagnosis, and management of tumor-induced osteomalacia

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by mesenchymal tumors that secrete fibroblast growth factor 23 (FGF23). Patients present with progressive bone pain, muscle weakness, and fragility fractures. TIO is characterized by hypophosphatemia, excess renal phosphate excretion, and low/inappropriately normal 1,25-dihydroxyvitamin D (1,25(OH)₂ D) levels. Rarity and enigmatic clinical presentation of TIO contribute to limited awareness among the medical community. Accordingly, appropriate diagnostic tests may not be requested, leading to delayed diagnosis and poorer patient outcomes. We have developed a global guidance document to improve the knowledge of TIO in the medical community, enabling the recognition of patients with TIO and appropriate referral. We provide recommendations aiding diagnosis, referral, and treatment, helping promote a global standard of patient management. We reviewed the literature and conducted a three-round Delphi survey of TIO experts. Statements were drafted based on published evidence and expert opinions (≥70% consensus required for final recommendations). Serum phosphate should be measured in patients presenting with chronic muscle pain or weakness, fragility fractures, or bone pain. Physical examination should establish features of myopathy and identify masses that could be causative tumors. Priority laboratory evaluations should include urine/serum phosphate and creatinine to assess renal tubular reabsorption of phosphate and TmP/GFR, alkaline phosphatase, parathyroid hormone, 25-hydroxyvitamin D, 1,25(OH)₂ D, and FGF23. Patients with the clinical/biochemical suspicion of TIO should be referred to a specialist for diagnosis confirmation, and functional imaging should be used to localize causative tumor(s). Recommended treatment is tumor resection or, with unresectable/unidentifiable tumors, phosphate salts plus active vitamin D, or burosumab.

Bone Volumetric Density, Microarchitecture, and Estimated Bone Strength in Tumor-Induced Rickets/Osteomalacia Versus X-linked Hypophosphatemia in Chinese Adolescents

Tumor-induced rickets/osteomalacia (TIR/O) severely impairs bone microarchitecture and bone strength. However, no study has described the microarchitectural quality of bone in adolescent patients with TIR/O. TIR/O affects bone quality more severely than the inherited causes of hypophosphatemia, the most common form of which is X-linked hypophosphatemia (XLH). Nevertheless, differences of the microarchitectural quality of the bone between TIR/O and XLH have never been clarified. Therefore, in this study, we used high-resolution peripheral quantitative computed tomography to assess bone microarchitecture in five Chinese adolescent TIR/O patients, and these were compared with 15 age- and gender-matched XLH patients as well as 15 age- and gender-matched healthy controls. Compared with the healthy controls, the TIR/O patients presented with significantly lower volumetric bone mineral densities (vBMDs), severely affected bone microarchitecture, and profoundly weaker bone strength. The distal tibia was more severely affected than the distal radius. Compared with the XLH patients, the TIR/O patients showed deteriorated bone quality notably at the distal tibia and in the cancellous compartment, reflected by 45.9% lower trabecular vBMD (p = 0.029), 40.2% lower trabecular fraction (p = 0.020), 40.6% weaker stiffness (p = 0.058), and 42.7% weaker failure load (p = 0.039) at the distal tibia. The correlation analysis showed that a higher level of serum FGF23 and a lower level of serum phosphate were associated with a poorer bone microarchitecture and a weaker estimated bone strength in the hypophosphatemic patients of our study. In conclusion, our study demonstrated significantly lower vBMDs, severely impaired bone microarchitecture, and profoundly weaker bone strength in Chinese adolescent patients with TIR/O, notably at the distal tibia, compared with the same parameters in age- and sex-matched healthy controls and XLH patients, which was possibly caused by excessive FGF23 production and secretion, chronically severe hypophosphatemia, and weak mechanical stimulus at the lower extremities. These findings further our understanding of the impact of different kinds of hypophosphatemic rickets/osteomalacia on bone quality.