Sjögren's syndrome (SS) is a chronic autoimmune disease primarily affecting the salivary and lacrimal glands, leading to symptoms of dryness. Assessing the inflammatory activity is crucial for guiding treatment. Fibroblast activation protein (FAPI) PET is an emerging imaging technique increasingly used for evaluating various inflammations and tumors. Thus this prospective study is going to investigate the uptake characteristics of FAPI PET in Sjögren's syndrome and evaluate its potential in assessing inflammatory activity to guide clinical treatment. A control group of tumor patients undergoing FAPI PET imaging will be included to compare FAPI uptake patterns between autoimmune inflammation and neoplasia.