Properties of [18F]FAPI monitoring of acute radiation pneumonia versus [18F]FDG in mouse models

Objective

In this study, the uptake characteristics of [¹⁸F]fibroblast activation protein inhibitor (FAPI) molecular imaging probe were investigated in acute radiation pneumonia and lung cancer xenografted mice before and after radiation to assess the future applicability of [¹⁸F]FAPI positron emission tomography/computed tomography (PET/CT) imaging in early radiotherapy response.

Methods

Initially, the biodistribution of [¹⁸F]FAPI tracer in vivo were studied in healthy mice at each time-point. A comparison of [¹⁸F]FAPI and [¹⁸F]fluorodeoxyglucose (FDG) PET/CT imaging efficacy in normal ICR, LLC tumor-bearing mice was evaluated. A radiation pneumonia model was then investigated using a gamma counter, small animal PET/CT, and autoradiography. The uptake properties of [¹⁸F]FAPI in lung cancer and acute radiation pneumonia were investigated using autoradiography and PET/CT imaging in mice.

Results

The tumor area was visible in [¹⁸F]FAPI imaging and the tracer was swiftly eliminated from normal tissues and organs. There was a significant increase of [¹⁸F]FDG absorption in lung tissue after radiotherapy compared to before radiotherapy, but no significant difference of [¹⁸F]FAPI uptake under the same condition. Furthermore, both the LLC tumor volume and the expression of FAP-ɑ decreased after thorax irradiation. Correspondingly, there was no notable [¹⁸F]FAPI uptake after irradiation, but there was an increase of [¹⁸F]FDG uptake in malignancies and lungs.

Conclusions

The background uptake of [¹⁸F]FAPI is negligible. Moreover, the uptake of [¹⁸F]FAPI may not be affected by acute radiation pneumonitis compared to [¹⁸F]FDG, which may be used to more accurately evaluate early radiotherapy response of lung cancer with acute radiation pneumonia.