[¹⁸F]FAPI- 04 PET/CT for pathologic response assessment in pancreatic cancer patients with systemic treatment

To study the association between [¹⁸F]FAPI- 04 uptake on positron emission tomography/computed tomography (PET/CT) and pathologic treatment response (PTR) in patients with pancreatic cancer (PC).

We enrolled 59 patients from August 2021, of whom 28 underwent surgical treatment after systemic therapy. The patients were investigated for a correlation between baseline fibroblast activation protein inhibitor (FAPI) uptake and PTR using College of American Pathologists (CAP) scores. The FAPI PET variables include standardised uptake value (SUV)max, SUVmean, metabolic tumour volume (MTV), and total lesion FAP expression (TLF). A PET/CT scan obtained before surgery in 14 patients facilitated assessing changes in FAPI uptake through treatment, and their association with PTR. Multiplex immunohistochemistry (mIHC) analysis identified the FAPI biodistribution in the PC tumours.

The SUVmax correlated positively with FAP expression in PC tissues. However, there was no correlation between baseline variables and the CAP scores. Treatment resulted in remarkably reduced MTV and TLF in all patients. The baseline SUVmax and SUVmean of patients with a good PTR (CAP score ≤ 2) differed from those after treatment (p = 0.001). An FITC-FAPI probe intuitively showed that cancer-associated fibroblasts (CAFs) and tumour cells had a similar FITC-FAPI fluorescence intensity, indicating a negative association between tumour regression and [¹⁸F]FAPI- 04 uptake.

Greater changes in FAPI uptake through treatment were associated with a better PTR in patients with PC and might be valuable in predicting prolonged survival. These results are clinically meaningful when selecting candidates for conversion surgery during systemic treatment.