Despite revolutionizing the field of oncological imaging, Positron Emission Tomography (PET) with [¹⁸F]Fluorodeoxyglucose (FDG) as its workhorse is limited by a lack of specificity and low sensitivity in certain tumor subtypes. Fibroblast activation protein (FAP), a type II transmembrane glycoprotein, is expressed by cancer-associated fibroblasts (CAFs) that form a major component of the tumor stroma. FAP holds the promise to be a pan-cancer target, owing to its selective over-expression in a vast majority of neoplasms, particularly epithelial cancers. Several radiolabeled FAP inhibitors (FAPI) have been developed for molecular imaging and potential theranostic applications. Preliminary data on FAPI PET/CT remains encouraging, with extensive multi-disciplinary clinical research currently underway. This review summarizes the existing literature on FAPI PET/CT imaging with an emphasis on diagnostic applications, comparison with FDG, pitfalls, and future directions.

We evaluated the role of 18F-FDG PET/CT for the assessment of response after two cycles of neo-adjuvant chemotherapy (NACT) for breast cancer. Twenty-three women with locally advanced breast cancer were included in this study. Early response to NACT was evaluated after two cycles using clinical examination, CT, and 18F-FDG PET/CT. Final histopathology following surgery after six cycles of NACT served as reference. Baseline PET/CT demonstrated a total of 26 lesions in 23 patients. The size of the primary tumor ranged from 1.90 cm to 11.60 cm, and the maximum value of the standardized uptake value of FDG (SUVmax) ranged from 3.6 to 38.6 (mean, 11.7). Post-chemotherapy PET/CT examinations were done after two cycles of NACT. The size of the primary tumor on follow-up PET/CT examinations ranged from 0.0 cm to 7.6 cm, and SUVmax ranged from 0.0 to 12.0 (mean, 3.96). On clinical, CT, and PET/CT examinations, 50% reduction in the parameters was taken as the cutoff value to differentiate between responders and non-responders. Post-NACT PET/CT demonstrated that 16 patients were responders and 7 non-responders. Among 16 responders on PET/CT scan, 14 were true positive and 2 were false positive when compared with histopathology. Among seven non-responder patients, six were true negative, and one was false negative. The sensitivity, specificity, and accuracy of PET/CT in detecting responders were 93%, 75%, and 87%, respectively. In conclusion, 18F-FDG PET/CT can differentiate responders from non-responders with high accuracy after two cycles of NACT in patients with LABC.