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Abstract Context Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome usually caused by oversecretion of fibroblast growth factor 23 (FGF23) from a phosphaturic mesenchymal tumor (PMT). PMTs are usually benign neoplasms but some of them show malignant characteristics. Objective The aim of this study was to compare the clinical characteristics of benign and malignant PMTs inducing TIO. Methods On March 31, 2023, we performed a systematic review of individual patient data analysis in Medline, Google Scholar, Google book, and Cochrane Library using the terms “tumor induced osteomalacia,” “oncogenic osteomalacia,” “hypophosphatemia,” with no language restrictions and according to Preferred Reporting Items for Systematic reviews and Meta-Analyses criteria. Results Overall, we collected data from 837 patients with TIO in which the diagnosis of benign and malignant PMT was specified. Of them, 89 were affected by malignant PMT and 748 by benign PMT. Patients with malignant PMTs were younger and presented bone pain, functional impairment, and bone deformities more frequently. Malignant PMTs showed higher values of intact FGF23 and a higher mortality rate. Conclusion The study results identify the clinical characteristics of patients with malignant TIO, permitting the early identification of patients with PMT at increased risk of malignancy. This may significantly improve the diagnostic approach to disease. Further experimental studies are mandatory to clarify the role of FGF23 in the pathogenesis of malignancy in PMTs.

Abstract Context Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by excessive production of fibroblast growth factor 23 (FGF23) by a tumor. Hyperparathyroidism (HPT) including secondary HPT (SHPT) and tertiary HPT (THPT) in TIO patients, which is believed to be associated with phosphate supplementation, has not been well documented. Objectives To clarify the prevalence, clinical characteristics, and risk factors for HPT in a large cohort of Chinese patients with TIO in our hospital. Design, setting, and participants This retrospective study enrolled 202 patients with TIO. Main outcome measurements Occurrence of HPT in patients with TIO. Results HPT was observed in 91 patients (91/202, 45.1%): 84 patients (41.6%) with SHPT and 7 patients (3.5%) with THPT. All patients with THPT underwent parathyroidectomy and only 1 patient experienced recurrence. Compared with patients without HPT, patients with SHPT had longer disease duration, higher rate of phosphate and calcitriol supplementation, lower serum calcium, lower urine calcium excretion, and higher urine phosphate excretion. Compared with patients with SHPT, patients with THPT had even longer disease duration and a higher rate of phosphate and calcitriol supplementation. PTH levels showed positive correlation with intact FGF23 and 1,25-dihydroxyvitamin D levels, but not 25-hydroxy vitamin D level in patients with TIO. Multivariate logistic regression analysis showed that long disease duration and phosphate supplementation were independently associated with occurrence of HPT in patients with TIO. Further logistic regression analysis and restricted cubic spline model revealed dose-response relationship between cumulative dose of phosphate supplementation and PTH levels. Conclusions HPT is common in patients with TIO. To avoid the occurrence of HPT in patients with TIO, timely diagnosis and tumor resection is necessary and an excessive dose of phosphate supplementation is not suggested before surgery.

Tumor-induced osteomalacia (TIO) is a rare acquired paraneoplastic syndrome caused by a mesenchymal tumor secreting a phosphaturic hormone called FGF23. Patients present with bone pain, fragility fractures and muscle weakness. Biochemical results show hypophosphatemia, raised serum alkaline phosphatase and reduced calcitriol. We report the case of a 44-year-old man who presented to the Emergency Departement with acute low back pain revealing extensive subchondral fractures between D2 and L5. Investigations showed partial Fanconi syndrome; nevertheless, he had profound hypophosphatemia, low 1,25-OH vitamin D and raised FGF23 levels suggesting a diagnosis of tumor-induced osteomalacia. A subcutaneous lesion was identified in the left leg on a PET-CT initially performed to rule out malignancy in the context of Fanconi syndrome. Tumorectomy enabled complete resolution of the electrolyte disturbances within days of surgery. This case shows that TIO may present as partial Fanconi syndrome, highlighting the importance of testing other electrolytes in cases of hypophosphatemia and the need to look for TIO in cases of partial Fanconi with severe hypophosphatemia.

Summary Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome caused by tumoral overproduction of FGF-23. Due to local recurrence, we describe the long-term efficacy and safety profile of burosumab, an anti-FGF-23 monoclonal antibody, in a TIO patient after three unsuccessfully surgical attempts. Introduction TIO is a rare paraneoplastic syndrome caused by tumoral overproduction of fibroblast growth factor 23 (FGF23), resulting in hyperphospaturia, hypophosphatemia, and osteomalacia. Surgery is the only definitive treatment, but tumor can locally recur, even after years from primary surgery. Furthermore, some tumors cannot be removed by surgery due to their location. Methods We describe the case of a 54-year-old woman affected by recurrent TIO who, after three unsuccessful surgical attempts of tumor removal, was treated with burosumab, an anti-FGF-23 monoclonal antibody. Results The patient was referred to our Bone Unit after experiencing several fractures in different sites, both traumatic and non-traumatic. At the time of first evaluation, at the age of 46, serum-phosphate (SP) was 1.2 mg/dL (reference range (RR) 2.5–4.5), 24-h urinary phosphate was 842 mg (RR 400–1000), and intact-FGF-23 was 117 pg/mL (RR 25–45). Imaging showed a metabolic pre-sacral lesion that firstly underwent to exploratory laparotomy. Then, patient underwent to surgical excision of tumor. After 18 months of well-being, tumor relapsed and even the subsequent surgery was not able to completely remove it. Since 2015, patient was maintained in phosphorus supplements and 1,25(OH) 2 vitamin D 3 , but SP levels never normalized. In September 2019, she was started on burosumab, initially at the dose of 0.3 mg/kg/month, progressively increased to the current 0.8 mg/kg/month, with great improvement of pain, physical performance, and normalization of SP levels. Burosumab was temporary and cautionary discontinued for COVID-19 pneumonia, with a worsening of SP. After restart of burosumab, biochemistry returned to normal. Conclusions To our knowledge, this is the first European patient affected by TIO treated with burosumab for more than 2 years. Burosumab is a promising therapy in the medical treatment of TIO refractory or not eligible for definitive surgery, with good efficacy and safety profile.

Among bone-material qualities, mineralization is pivotal in conferring stiffness and toughness to the bone. Osteomalacia, a disease ensuing from inadequate mineralization of the skeleton, is caused by different processes leading to decreased available mineral (calcium and/or phosphate) or enzymatic alterations. Vitamin D deficiency, which remains the major cause of altered mineralization leading to inadequate intestinal calcium and phosphate absorption, may be also associated with other conditions primarily responsible for abnormal mineralization. Given the reality of widespread vitamin D inadequacy, a full biochemical assessment of mineral metabolism is always necessary to rule out or confirm other conditions. Both too-high or too-low serum alkaline phosphatase (ALP) levels are important for diagnosis. Osteomalacic syndrome is reversible, at least in part, by specific treatment. Osteomalacia and bone mineralization themselves constitute largely unexplored fields of research. The true prevalence of the different forms of osteomalacia and the recovery after proper therapy have yet to be determined in the real world. Although non-invasive techniques to assess bone mineralization are not available in clinical practice, the systematic assessment of bone quality could help in refining the diagnosis and guiding the treatment. This review summarizes what is known of osteomalacia recent therapeutic developments and highlights the future issues of research in this field.

Key Points Paraneoplasias are caused by soluble factors produced by tumour cells or are the consequence of an immune reaction against these cells An underlying malignancy should be sought if musculoskeletal symptoms do not show an adequate response to therapy Arthritis, myositis, periostitis, fasciitis and osteomalacia can all be musculoskeletal manifestations of a paraneoplastic disease Due to the properties of soluble factors produced by tumour cells, some paraneoplasias show a characteristic involvement of specific musculoskeletal tissues (e.g. VEGF inducing HOA or FGF23 inducing TIO) Despite their rarity, knowledge of distinct clinical patterns of paraneoplasias is essential for the rheumatologist, because their recognition allows for a timely diagnosis and potentially life-saving therapy The dissection of underlying molecular mechanisms can provide insights in the pathogenesis for rheumatic as well as neoplastic diseases Although paraneoplastic syndromes are rare, it is important that rheumatologists can recognise the typical clinical patterns of these syndromes in their rheumatic disease patients to allow for timely diagnosis and potentially life-saving therapy. Here, the authors focus on well-defined paraneoplastic musculoskeletal syndromes for which the evidence of a causal relationship to an underlying tumour is not only based on a temporal relationship but also on underlying pathomechanisms. For patients that present with musculoskeletal symptoms, diagnostic procedures carried out by physicians and rheumatologists are primarily aimed at confirming or excluding the occurrence of primary rheumatic diseases. Another important trigger for musculoskeletal disease, however, is the presence of a tumour. Careful clinical investigation and knowledge of the gestalt of musculoskeletal syndromes related to respective tumour entities is of utmost importance for the diagnosis of paraneoplastic rheumatic diseases such as hypertrophic osteoarthropathy, paraneoplastic polyarthritis, RS3PE syndrome, palmar fasciitis and polyarthritis, cancer-associated myositis and tumour-induced osteomalacia. This places great responsibility on rheumatologists in diagnosing malignancies and referring the patient for effective treatment. The selective influence of tumours on musculoskeletal tissue is surprising and indicates that tumours alter tissues such as the periosteum, synovial membrane, subcutaneous connective tissue, fascia, muscles and bones by specific molecular processes. Some of the underlying mechanisms have been unravelled, providing valuable information on the physiologic and pathophysiologic roles of mediators such as vascular endothelial growth factor and fibroblast growth factor 23.

Tumor-induced osteomalacia (TIO) is an ultrarare paraneoplastic syndrome due to the overproduction of Fibroblast Growth Factor 23 (FGF23). Although other phosphate-regulating substances (i.e., \"phosphatonins\") have been demonstrated to be expressed in tumors associated with TIO, very limited information is available about their circulating levels and clinical significance. Here, we report a patient with a long-standing history of low serum phosphate values due to a sino-nasal phosphaturic mesenchymal tumor (PMT), in which the co-expression of FGF23 and FGF7 was detected in the neoplastic tissue and their serum levels determined before and after surgical tumor excision. The serum level of FGF23 was increased (312.0 pg/mL) before surgery and promptly normalized 24 h after tumor excision. In contrast, the value of FGF7 was high (202.6 pg/mL) before surgery, fell at 24 h after surgery (108.6 pg/mL), increased again after one week (252.0 pg/mL), and finally normalized (33.4 pg/mL) one year later, when also serum phosphate was in the normal range. This is the first sino-nasal PMT in which expression of FGF7 was observed and the first case of TIO in which the serum levels of FGF7 were monitored during the course of the disease (i.e., before and after tumor excision). Even though \"phosphatonins\" in addition to FGF23 are thought to contribute to the phosphate wasting leading to TIO, the observed increase in FGF7 levels one week after surgery (+ 31.66% compared to the preoperative value), when phosphate levels tended to normalize, may suggest a mild, if ever, phosphaturic effect of FGF7. Further studies are needed to define the mechanisms underlying the different post-surgical time-dependent decrease in the serum levels of FGF23 and FGF7 and the role of FGF7 in patients with TIO and more in general in phosphate homeostasis.

Abstract Context Tumor-induced osteomalacia (TIO) is an ultra-rare, paraneoplastic syndrome caused by tumors that secrete fibroblast growth factor 23 (FGF23). Initial signs and musculoskeletal symptoms can be nonspecific and unrecognized, leading to long delays in diagnosis and treatment, and resulting in severe and progressive disability in patients with TIO. Objective This review aimed to identify published evidence on healthcare resource use in TIO to better understand the burden of the disease. Evidence acquisition A targeted literature review was conducted to identify publications reporting on disease characteristics and healthcare resource use associated with TIO. Evidence synthesis In total, 414 publications were included in the review, of which 376 were case reports. From the case reports, data on 621 patients were extracted. These patients had a mean (SD) age of 46.3 (15.8) years; 57.6% were male. Mean time from first symptoms to diagnosis of TIO was 4.6 (4.7) years and, in cases where imaging tests were reported, patients underwent a mean of 4.1 (2.7) procedures. Tumor resection was attempted in 81.0% of patients and successful in 67.0%. Fracture was reported in 49.3% of patients. Results from association analyses demonstrated that longer time to diagnosis was associated with poorer tumor resection outcomes and a higher probability of tumor recurrence. Unfavorable tumor resection outcomes were associated with greater use of pharmacologic treatment and a greater likelihood of orthopedic surgery. Conclusion TIO is associated with a substantial healthcare resource burden. Improvements in the diagnostic process could lead to better management of TIO, thereby benefiting patients and reducing that burden.

A cause of hypophosphatemia is phosphate wasting disorders. Knowledge concerning mechanisms involved in phosphate wasting disorders has greatly increased in the last decade by the identification of phosphatonins, among them FGF-23. FGF-23 is a primarily bone derived factor decreasing renal tubular reabsorption of phosphate and the synthesis of calcitriol. Currently, pharmacological treatment of these disorders offers limited efficacy and is potentially associated to gastrointestinal, renal, and parathyroid complications; therefore, efforts have been directed toward newer pharmacological strategies that target the FGF-23 pathway. This review focuses on phosphate metabolism, its main regulators, and phosphate wasting disorders in adults, highlighting the main issues related to diagnosis and current and new potential treatments.