FAP-Imaging in Rare Cancer Entities - First Clinical Experience in a Broad Spectrum of Malignancies

Purpose: 68Ga-FAPI (fibroblast activation protein inhibitor) is a rapidly evolving and highly promising radiotracer for PET/CT imaging, presenting excellent results in a variety of tumor entities, particularly in epithelial carcinomas. This retrospective analysis sought to evaluate the potential and impact of FAPI-PET/CT in rare cancer diseases with respect to improvement in staging and therapy, based on tracer uptake in normal organs and tumors.Material and Methods: 55 patients with rare tumor entities, defined by a prevalence of 1 person out of 2000 or less, received a 68Ga-FAPI-PET/CT scan. 14 women and 41 men (median age 60) were included within the following subgroups: cancer of unknown primary (n=10), head-and-neck-cancer (n=13), gastro-intestinal and biliary-pancreatic cancer (n=17), urinary tract cancer (n=4), neuroendocrine cancer (n=4) and others (n=7). Tracer uptake was quantified by standardized uptake values SUVmax and SUVmean and the tumor-to-background ratio (TBR) was determined (SUVmax tumor/ SUVmean organ). Results: In 20 out of 55 patients the primary tumor was identified and 31 patients presented metastases (n=88), characterized by a high mean SUVmax in primary (10.1) and metastatic lesions (7.6). Highest uptake was observed by liver metastases (n=6) with a mean SUVmax of 9.8 and a high TBR of 8.7, closely followed by peritoneal carcinomatosis (n=16) presenting a mean SUVmax of 9.8 and an excellent TBR of 29.6. In terms of the included subgroups the highest uptake regarding mean SUVmax was determined in gastro-intestinal and biliary-pancreatic cancer with 9.8 followed closely by urinary tract cancer with 9.5 and head-and-neck cancer (9.1). Conclusion Due to excellent tumor visualization and, thereby, sharp contrasts in terms of high TBRs in primary and metastatic lesions in different rare malignancies, 68Ga-FAPI-PET/CT crystallizes as a powerful and valuable imaging tool, particularly with respect to epithelial carcinomas, and therefore an enhancement to standard diagnostics imaging methodologies. The realization of further and prospective studies is of large importance to confirm the potential of FAP-imaging in oncology.