Abstract

Stefan Alexander Koerber^1,2,3, Katharina Dendl⁴, Rebecca Finck⁴, Clemens Kratochwil⁴, Jonas Ristau^1,2,5, Sebastian Adeberg^6,7,8, Hendrik Rathke⁴, Manuel Röhrich⁴, Thomas Lindner⁴, Klaus Herfarth^1,7,8,9, Uwe Haberkorn^4,10,11, Juergen Debus^{1,2,5,9,12,13}, Frederik L. Giesel^4,10,14,15

¹Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany; ²National Center for Tumor diseases, (NCT), Heidelberg, Germany; ³Heidelberg Institute of Radiation Oncology , (HIRO), Heidelberg, Germany; ⁴Heidelberg University Hospital, Department of Nuclear Medicine, Heidelberg, Germany; ⁵Heidelberg Institute of Radiation Oncology, (HIRO), Heidelberg, Germany; ⁶Heidelberg University Hospital, Department of Radiation Oncology , Heidelberg, Germany; ⁷National Center for Tumor diseases, (NCT) , Heidelberg, Germany; ⁸Heidelberg Institute of Radiation Oncology, (HIRO) , Heidelberg, Germany; ⁹Heidelberg Ion-Beam Therapy Center (HIT), Department of Radiation Oncology, Heidelberg, Germany; ¹⁰Clinical Cooperation Unit Nuclear Medicine, German Cancer Research Center (DKFZ), Heidelberg, Germany; ¹¹Translational Lung Research Center Heidelberg (TLRC), German Center for Lung Research (DZL), Heidelberg, Germany; ¹²Clinical Cooperation Unit Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany; ¹³German Cancer Consortium (DKTK), partner site Heidelberg, partner site Heidelberg, Heidelberg, Germany; ¹⁴German Cancer Consortium (DKTK), partner site Heidelberg, Heidelberg, Germany; ¹⁵Department of Nuclear Medicine, University Hospital Duesseldorf, Duesseldorf, Germany

For local therapies like radiotherapy reliable staging tools are essential. ⁶⁸Ga-FAPI (fibroblast activation protein inhibitor) is a rapidly evolving and highly promising radiotracer for PET/CT imaging showing excellent results in a variety of tumor entities. This retrospective study aimed to evaluate the potential of FAPI-PET/CT in rare diseases based on biodistribution and tumor uptake and thus facilitate therapeutic approaches such as (radio-)oncological management.

55 patients with rare tumor entities defined by an incidence of 1 person out of 2000 or less underwent ⁶⁸Ga-FAPI-PET/CT. We included 14 women and 41 men (median age: 60 years) within the following subgroups: cancer of unknown primary (n=10), head-and-neck-cancer (n=13), gastrointestinal 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 (SUV)max and (SUV)mean and the tumor-to-background ratio (TBR) was determined. Moreover, the impact of ⁶⁸Ga-FAPI-PET/CT on (radio-)oncological management was evaluated.

⁶⁸Ga-FAPI-PET/CT identified the primary tumor in 20 out of 55 patients and 31 patients with metastases (n=88) demonstrating a high mean SUVmax (primary tumors: 10.1; metastatic lesions: 7.6). Highest uptake was obtained by liver metastases with a mean SUVmax of 9.8 and a rather high TBR of 8.7, tightly followed by peritoneal carcinomatosis 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 observed in gastrointestinal and biliary-pancreatic cancer with 9.8 followed closely by urinary tract cancer with 9.5 and head-and-neck cancer (9.1). ⁶⁸Ga-FAPI-PET/CT enabled early treatment response evaluation and improved radiotherapy planning leading to a high impact on (radio-)oncological management.

Due to strong tracer uptake and sharp contrasts in terms of high TBRs in primary and metastatic lesions, ⁶⁸Ga-FAPI-PET/CT emerges as an enhancement to standard diagnostic imaging with regard to (radio-)oncological management. However, prospective studies are required to confirm the high potential of the new hybrid imaging probe for rare diseases.