Multimodal Functional Imaging in Tumor-induced Osteomalacia: ⁶⁸Ga-DOTATATE, ¹⁸F-FDG, and ⁶⁸Ga-FAPI PET/CT Findings

Abstract

Tumor-induced osteomalacia, resulting from tumoral overproduction of fibroblast growth factor 23, is a rare paraneoplastic syndrome clinically characterized by muscle weakness, bone pain, and pathological fractures. Fewer than 1,000 cases have been reported in the literature. Laboratory findings typically include hypophosphatemia, normal or decreased levels of 1.25-dihydroxy vitamin D, and normal or elevated levels of FGF23. Functional imaging methods, particularly somatostatin receptor imaging, play an important role in the localization of the tumor. In this case of tumor-induced osteomalacia, we presented the findings from gallium-68 (⁶⁸Ga)-DOTATATE, ⁶⁸Ga-fibroblast, and ¹⁸F-fluorodeoxyglucose positron emission tomography imaging modalities used for tumor localization.

Ethics

Informed Consent: Written informed consent was obtained from all patients before imaging.

Figure 1.

A 65-year-old man with a history of bilateral hip fractures treated with prosthetic hip replacements presented with diffuse bodily pain and muscle weakness. The man, who was ambulatory with the aid of a cane, had chronic hypophosphatemia. Laboratory tests revealed a serum phosphate level of 0.8 mg/dL (reference range: 2.5-4.5 mg/dL), a parathyroid hormone level of 70 pg/mL (reference range: 15-65 pg/mL), and a calcitriol (1.25-dihydroxy vitamin D) level of 23 pg/mL (reference range: 20-80 pg/mL). Serum FGF23 levels were elevated. Urinary phosphate excretion was also elevated (1960 mg/day; reference: 400-1300 mg/day). Assessment of bone mineral density by dual-energy X-ray absorptiometry showed T-scores of -1.4 in the lumbar spine and -1.6 in the hip region. Genetic analysis revealed a heterozygous mutation in the FGF23 gene. With the patient’s consent, ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) and ⁶⁸Ga-fibroblast (⁶⁸Ga-FAPI) PET/CT imaging were performed to determine lesion localization; however, no abnormal tracer uptake was observed (A, D). Subsequently, a ⁶⁸Ga-DOTATATE PET/CT scan was conducted. Axial PET and fused PET/CT images (I, K) revealed a focal tracer-avid lesion measuring approximately 2.5×1.2 cm at the proximal end of the right radius (maximum standard uptake value: 39.9). Corresponding CT images (J) revealed a mild increase in density within the medullary bone marrow area. The maximum intensity projection image (H) confirmed the localization of the primary lesion in the proximal right radius. Contrast-enhanced magnetic resonance imaging of the right forearm showed a lesion corresponding to the area of ⁶⁸Ga-DOTATATE uptake that appeared hyperintense on T2-weighted images, hypointense on T1-weighted images, and exhibited homogeneous contrast enhancement. The patient with additional comorbidities did not consent to histopathological examination. The patient is currently under follow-up and receiving vitamin D and oral phosphate supplementation.

Tumor-induced osteomalacia (TIO) typically originates from phosphaturic mesenchymal tumors, which are usually small, slow-growing, and benign, and can be located in soft tissue or bone throughout the body from the skull to the feet (1,2). These tumors are most commonly localized in the lower extremities and the head and neck region, whereas the upper extremities are the least frequently affected sites (3). Due to their typically small and solitary nature, identifying the exact location of these tumors is often challenging (4). Localization generally begins with functional imaging techniques, including somatostatin receptor (SSTR) imaging (⁶⁸Ga-DOTATATE or octreotide scan) and ¹⁸F-FDG PET/CT (5). In a recent meta-analysis, SSTR-PET/CT demonstrated a higher detection rate for culprit tumors in TIO patients compared to ¹⁸F-FDG PET/CT (6). Fibroblast activation protein inhibitor (FAPI) is known to accumulate in various benign and malignant conditions (7,8). Although only a limited number of TIO cases have been evaluated with FAPI imaging in the literature, some of these have shown uptake at the tumor site (9). We evaluated the contribution of ⁶⁸Ga-FAPI PET/CT, alongside ⁶⁸Ga-DOTATATE PET/CT and ¹⁸F-FDG PET/CT, to tumor localization in a man with TIO. Our findings demonstrate the superiority of ⁶⁸Ga-DOTATATE PET/CT over ¹⁸F-FDG and ⁶⁸Ga-FAPI PET/CT for lesion localization.