THU0421 TUMOR-INDUCED OSTEOMALACIA: DATA FROM A MONOCENTRIC EXPERIENCE ON 16 PATIENTS
Authors: C. Crotti, F. Bartoli, M. Manara, P. A. Daolio, F. Zucchi, R. Caporali, L. Sinigaglia, M. Varenna
Journal: Annals of the Rheumatic Diseases (2020)

Background: Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome due to a phosphaturic tumor, which overproduces fibroblast growth factor-23 (FGF-23), causing hyperphosphaturia, hypophosphoremia, low 1,25(OH) 2 VitD 3 and osteomalacia. Locating the tumor is critical, because lesions are typically small, benign mesenchymal tumors, anywhere in the body; the delay between onset of symptoms and diagnosis ranges from 2.5–28 years. Surgical removal is the only effective therapeutic approach. Objectives: To retrospectively evaluate patients affected by TIO, investigating clinical management and disease outcome. Methods: We retrospectively collected data of patients affected by TIO referred to a tertiary Rheumatology Center between Sep 2000 and Jan 2020. Results: We included 16 patients with a definite diagnosis of TIO, mean age±standard deviation 62.4±14.6 yrs, 56.2% females, mean age at symptoms onset 48.0±14.3 yrs (53.8±13.1 at diagnosis). Mean diagnostic delay between symptoms onset and tumor detection was 6.8±6.4 yrs. All patients complained bone pain, muscle weakness, and fractures before diagnosis of TIO. Biochemical findings were: mean serum Phosphorus (PS) 1.4±0.4 mg/dL (reference range (RR) 2.5-4.6), mean serum Calcium 9.4±0.7 mg/dL (RR 8.4-10.2), mean serum 1,25(OH) 2 VitD 3 30.5±23.4 ng/L (RR 25-86). Intact-FGF-23 was dosed in 9 patients, always resulting elevated: mean 396.6±707.3 pg/mL (RR 25-45). PTH was increased in 30% of cases, while serum alkaline phosphatase was increased in 87.5%. 24h-Urine Phosphorus (PU) was increased in only 13% of patients, but, when renal phosphate wasting by tubular reabsorption of phosphate (TRP) was calculated, PU resulted increased in all. Tumor was localized in all cases (Fig.1) and were localized in bone and soft tissue, by using functional imaging, followed by anatomical techniques. Before the introduction in routinely practice of 68 Ga-DOTATATE-PET-CT in 2013, Octreoscan-SPECT/CT and 18 F FDG-PET were used as imaging modalities. Since 2013, diagnostic delay consistently reduced, from 8.6±8.3 yrs (7 patients) to 4.5±2.6 yrs (9 patients), confirming higher diagnostic accuracy of 68 Ga-DOTATATE-PET-CT. Figure 1. 13 patients underwent surgery; in two cases surgery was not possible due to tumor location, so pharmacological support with phosphate supplements and calcitriol was started; a patient underwent to TC-guided radiofrequency ablation. After surgery, 7 patients experienced a complete remission, 3 had a persistence of the disease, and 3 an overtime relapse, even after a longstanding normalization of PS (6 years). After surgical tumor removal, PS significantly increased in few days (from 1.36±0.39 to 2.9±1.1, p=0.0001), while iFGF-23 levels tended to rapidly decreased (from 396.6±707.3 to 62.8±78.4). Before the introduction of 68 Ga-DOTATATE-PET-CT, 6 patients underwent to imaging-guided closed biopsy to confirm tumor localization; by using 68 Ga-DOTATATE-PET-CT only 2 subjects had closed biopsy. Furthermore, in our population only patients who had biopsy to detect the lesion (7 patients) had relapses compared to patients who did not. Conclusion: To our knowledge, this is the widest European cohort of patients affected by TIO reported in the last two decades. We confirm an important delay between symptoms onset and diagnosis. To locate tumor, a stepwise approach is recommended, starting with a thorough medical history and physical examination, followed by functional imaging, preferring 68 Ga-DOTATATE-PET-CT. Tumor biopsy is not recommended due to the potential cell spilling. Surgery is considered the only definitive treatment, aiming to a wider excision. Active surveillance is always needed, due to the possible relapses, even after a long period of complete clinical and biochemical remission. Disclosure of Interests: : Chiara Crotti: None declared, Francesca Bartoli: None declared, Maria Manara Consultant of: Consultant and/or speaker for Eli-Lilly, MSD, Sanofi-Genzyme, Novartis, Alfa Wasserman and Cellgene, Speakers bureau: Consultant and/or speaker for Eli-Lilly, MSD, Sanofi-Genzyme, Novartis, Alfa Wasserman and Cellgene, Primo Andrea Daolio: None declared, Francesca Zucchi: None declared, Roberto Caporali Consultant of: AbbVie; Gilead Sciences, Inc.; Lilly; Merck Sharp &amp; Dohme; Celgene; Bristol-Myers Squibb; Pfizer; UCB, Speakers bureau: Abbvie; Bristol-Myers Squibb; Celgene; Lilly; Gilead Sciences, Inc; MSD; Pfizer; Roche; UCB, Luigi Sinigaglia: None declared, Massimo Varenna: None declared