Semiquantitative muscle parameters derived from FAPI and FDG PET/CT in evaluating sarcopenia among patients with malignant tumors.

Background: The objective of this study is to explore and compare the potential utility of fibroblast activation protein inhibitor (FAPI) and fluorodeoxyglucose PET/computed tomography (CT) in assessing sarcopenia among patients with malignant tumors.

Methods: A retrospective analysis was conducted on 127 patients with histologically confirmed malignant tumors who underwent both 18F/68Ga-FAPI and fluorine-18-fluorodeoxyglucose (18F-FDG) PET/CT scans. Clinical characteristics and PET/CT parameters of maximum and mean standard uptake value (SUVmax and SUVmean) of muscle at the 3rd lumbar (L3) level were reviewed. Skeletal muscle area at the L3 level was measured, and skeletal muscle index was calculated to determine sarcopenia. The association between sarcopenia and PET/CT parameters was analyzed.

Results: The incidence of sarcopenia was 41.7% among these 127 patients. Higher age, male, lower BMI, lower SUVmax and SUVmean of muscle from 18F/68Ga-FAPI PET/CT, and lower SUVmax of muscle from 18F-FDG PET/CT were correlated with a higher prevalence of sarcopenia (P < 0.05). Besides, no significant differences in SUVmax and SUVmean of muscle were noted between 18F-FAPI and 68Ga-FAPI groups. The best cutoff value of SUVmax of muscle from 18F/68Ga-FAPI PET/CT was 1.17, yielding the area under the curve (AUC) of 0.764 and sensitivity and specificity of 74.3% and 71.7%, while the optimal cutoff value of SUVmax of muscle from 18F-FDG PET/CT was 0.76, with an AUC of 0.642 and sensitivity and specificity of 36.5% and 86.8%, respectively.

Conclusion: Patients with sarcopenia exhibit decreased muscle uptake of FAPI and fluorodeoxyglucose. FAPI PET/CT emerges as a more valuable tool for sarcopenia assessment in patients with malignant tumors compared to fluorodeoxyglucose PET/CT.