ABSTRACT Oncogenic osteomalacia (OOM) is an uncommon metabolic and bone disease caused by fibroblast growth factor 23 (FGF23), a phosphaturic factor produced by phosphaturic mesenchymal tumors (mixed connective tissue variant, PMTMCTV) characterized by phosphate leakage from kidneys and subsequent hypophosphatemia. In this paper, we present the case of a patient, 42-year-old woman affected by left side limp and pain involving lumbar spine, pelvis and hip joints, referred to the Rheumatology Department of our Hospital for the treatment of a suspected sero-negative spondilo-arthritis. During hospitalization patient began an immuno-suppressive therapy with TNF-alpha inhibitors associated with Pamidornate, Indometacin, Esomeprazole and vitamin D3. Nevertheless pain did not decrease and a new examination found a worst hypophosphatemia (1 mg/dl) with normal Ca and PTH’s plasma values. During the same check-up a painful bulge on the anterior part of the right knee was observed and the Magnetic Resonance Imaging scan revealed an ovular solid lesion in the soft tissue closed to the upper part of the patella. Histological analysis identified the lesion as a PMTMCTV. After surgical removal patient got complete recovery. We will discuss about diagnostic evaluation, differential diagnosis and treatment.
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