Abstract
Purpose: Fibroblast activation protein (FAP) is overexpressed in cancer associated fibroblasts of several tumor entities. Recent development of quinoline based positron-emission-tomography (PET)-tracers that act as FAP-Inhibitors (FAPI) demonstrated promising results preclinically and already also in few clinical cases. Consecutively this novel tracer is now applied in our hospital to amend the diagnostics of cancer patients facing limitations of standard exams. Here we analyze the tissue biodistribution and preliminary dosimetry of two members of this new class of PET-radiopharmaceuticals. Methods: A preliminary dosimetry estimate for FAPI-02 and FAPI-04 was based on two patients examined at 0.2h, 1h and 3h after tracer injection using the QDOSE dosimetry software suit. Further PET/CT scans of tumor patients were acquired 1 h after injection of either FAPI-02 (n = 25) or FAPI-04 (n = 25); for 6 patients an intra-individual related FDG-scan (also acquired 1h p.i.) was available. For the normal tissue of 16 organs, a 2 cm Spheric-VOI was placed in the parenchyma, for tumor lesions a threshold segmented VOI was used to quantify SUVmean/max. Results: Very similar to literature values for 18F-FDG, 68Ga-DOTATATE or 68Ga-PSMA-11, an exam with 200 MBq 68Ga-FAPI-2/4 corresponds to an equivalent dose of approx. 3-4 mSv. After a fast clearance via the kidneys the normal organs showed a low tracer uptake with only minimal changes between 10 min and 3 h p.i.. In FAPI-02 the tumor uptake from 1h to 3h p.i. decreased by 75%, whereas the tumor retention was prolonged with FAPI-04 (25% washout). Regarding tumor-to-background ratios, at 1h p.i. both FAPI-tracers performed equally. In comparison to FDG the tumor uptake was almost equal (average SUVmax-FDG 7.41; SUVmax-FAPI-2 7.37; n.s.); the background uptake in brain (11.01 vs 0.32), liver (2.77 vs 1.69) and oral/pharyngeal mucosa (4.88 vs 2.57) was significantly lower with FAPI; other organs were not relevantly different between FDG and FAPI. Conclusion: FAPI-PET/CT is a new diagnostic method in imaging cancer patients. In contrast to FDG no diet/fasting in preparation of the exam is necessary and image acquisition can potentially be started few minutes after tracer application. Tumor-to-background contrast ratios were equal or even improved in comparison to FDG.
- Molecular Imaging
- PET/CT
- Radiotracer Tissue Kinetics
- Cancer associated fibroblasts
- PET/CT
- biodistribution
- dosimetry
- fibroblast activation protein
Footnotes
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- Copyright © 2018 by the Society of Nuclear Medicine and Molecular Imaging, Inc.