Phosphaturic mesenchymal tumor in the temporalis muscle causing tumor-induced osteomalacia: a case report and literature review.

Tumor-induced osteomalacia (TIO) is a rare paraneoplastic syndrome characterized by renal phosphate wasting, hypophosphatemia, and impaired bone mineralization, primarily caused by phosphaturic mesenchymal tumors (PMTs). Due to the small size and non-specific clinical presentation, PMTs are often difficult to localize, resulting in delayed diagnosis. This study presents a case of a 54-year-old man who experienced a 2-year history of progressive polyarthralgia affecting the knees, hips, and lower back, which exacerbated over the past 3 months. Laboratory tests revealed hypophosphatemia, elevated alkaline phosphatase (ALP) levels, and increased parathyroid hormone (PTH) levels. Moreover, a dual-energy X-ray absorptiometry (DXA) scan indicated low bone mineral density (BMD), while a whole-body bone scintigraphy revealed multiple rib pathological fractures, indicative of osteomalacia. Although serum fibroblast growth factor 23 (FGF23) was not measured, the clinical and biochemical profiles were highly consistent with tumor-induced osteomalacia (TIO). Subsequent Gallium-68 1,4,7,10-tetraazacyclododecane 1,4,7,10-tetraacetic acid tyrosine-3-octreotate positron emission tomography/computed tomography (68Ga-DOTA-TATE) positron emission tomography/computed tomography (PET/CT) confirmed a hypermetabolic lesion in the left temporalis muscle. The lesion was completely resected, and histopathological examination confirmed the diagnosis of PMT, with positive immunostaining for special AT-rich sequence-binding protein 2 (SATB2), neural cell adhesion molecule (CD56), and somatostatin receptor type 2 (SSTR2). No tumor recurrence was observed during follow-up at 6 months after surgery. Clinicians should consider occult PMTs in patients with unexplained hypophosphatemia, bone pain, or pathological fractures, and employ functional imaging modalities such as 68Ga-DOTA-TATE PET/CT for early tumor localization and potential cure through resection.