FAPI PET-guided tumor volume delineation for radiotherapy planning: A comprehensive review across solid tumors

Accurate gross tumor volume (GTV) delineation remains a critical yet error-prone step in radiotherapy (RT) planning. Conventional imaging and [¹⁸F]FDG PET/CT are limited by poor tumor-to-background contrast and non-specific inflammatory uptake. ¹⁸F/⁶⁸Ga-labeled fibroblast activation protein inhibitor (FAPI) PET, which targets cancer-associated fibroblasts (CAFs) within the tumor microenvironment, offers a biologically distinct and potentially superior alternative. This review summarizes the current evidence across multiple solid tumors, highlighting its potential to improve gross tumor volume delineation, refine staging, and reduce interobserver variability compared with conventional imaging and [¹⁸F]FDG PET. Although FAPI PET frequently altered target volumes and RT planning, the available evidence is limited by small, predominantly retrospective studies and the absence of histopathological validation or long-term clinical outcomes. At present, FAPI PET should be considered a complementary imaging tool with particular value in selected clinical settings. Prospective multicenter studies are needed to establish standardized segmentation methods and determine whether improved target delineation translates into better patient outcomes.