'Rachitic rosary sign' and 'tie sign' of the sternum in tumour-induced osteomalacia

Phosphaturic Mesenchymal Tumor of the Greater Trochanter: A Case Report

This is the case of a 56-year-old Hispanic male with a history of multiple fractures and electrolyte abnormalities, including hypophosphatemia and phosphaturia. Physical examination, imaging studies, and laboratory workup may have suggested the presence of a phosphaturic mesenchymal tumor (PMT) causing osteomalacia. The patient underwent surgery for en bloc tumor removal, and the histopathological analysis confirmed the presence of neoplastic cells consistent with PMT with minimal immunohistochemical positivity to S100 protein, which is atypical for this type of tumor. This case highlights the challenges in diagnosing PMTs due to their rarity and variable presentation. It emphasizes the importance of considering PMT in the differential diagnosis for unexplained hypophosphatemia and osteomalacia-like symptoms, especially in persistent disease after parathyroidectomy for presumed primary hyperparathyroidism. The atypical immunohistochemical profile observed in this case contributes to the growing body of knowledge about the heterogeneity of PMTs and underscores the need for comprehensive diagnostic approaches in suspected cases.

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.