A 71-year-old woman presented with progressive lumbar discomfort for a 6-month. MRI showed bone destruction of L5 vertebral body with high and low signal intensities on T2- and T1-weighted images, respectively. Bone metastatic lesion was highly suspected. The patient underwent 18F-FDG PET/CT for primary tumor detection. 18F-FDG PET/CT showed intense radiotracer uptake in the L5 vertebral bodywith osteoclastic and expensive bony destruction. In order to further evaluate the situation and investigate the possible occult primary lesions of abdomen and pelvis, the patient was subsequently included in a clinical trial of 68Ga-FAPI. Written informed consent was obtained from the patient. Mild 68Ga-FAPI-04 uptake was observed in the lesion of bone destruction. The SUVmax of the vertebral lesion was significantly lower for FAPI compared to FDG PET/CT imaging. The patient subsequently underwent a excision biopsy of lumbar lesions, which was pathologically confirmed to be a bone metastases of thyroid cancer. At this time, the serum thyroglobulin level was more than 486 ng/mL, TSH was 3.32 µIU/ml and antithyroglobulin antibody level was 9 IU/mL. One month later, the patient received total thyroidectomy and central neck dissection. Histological examination revealed a foci follicular variants of papillary thyroid microcarcinomas(PTCMCa-FV).
It has been reported that FDG preferential uptake by malignant cells with a high turnover rate due to the activation of osteoclasts through the RANK-RANK-L-OPG pathway[1–2]. 68Ga FAPI serves as a promising radiotracer for the assessment of solid tumors[3–4]. Multiple studies reported that 68Ga-FAPI PET/CT showed better signal-to-background ratio than 18F-FDG for detecting the bone, liver and lymph node metastatic lesions in the restaging of PTC, and even the case of radioiodine- refractory differentiated thyroid cancer[5–7]. However, this present case is the first study to reveal that 68Ga-FAPI had lower tumour-to-background ratio in detecting bone metastasis from PTCMCa- FV. This suggested that the expression of FAP in the bone metastasis from PTCMCa was heterogeneous. Therefore, the value of 68Ga-FAPI in bone metastases from PTC requires further investigation.