Ga-FAPI-PET/CT improves diagnostic staging and radiotherapy planning of adenoid cystic carcinomas - Imaging analysis and histological validation.

BACKGROUND

Adenoid cystic carcinomas (ACCs) are rare epithelial tumors mostly situated in the head and neck region and characterized by infiltrative growth. The tumor stroma of ACCs includes cancer-associated fibroblasts (CAFs) expressing Fibroblast Activation Protein (FAP), a new target for positron emission tomography (PET) imaging. Here we describe the value of PET/ computed tomography (PET/CT) imaging using Ga-labelled FAP-Inhibitors (Ga-FAPI-PET/CT) and their clinical potential for staging and radiotherapy planning in 12 ACC patients (7 primary, 5 recurrent).

PATIENTS AND METHODS

Patients underwent contrast enhanced staging CT (ceCT) and magnetic resonance imaging (ceMRI) before Ga-FAPI - PET/CT. PET-scans were acquired 10, 60 and 180 minutes after administration of 150-250 MBq of Ga-labelled FAPI tracers. SUV and SUV values of ACCs and healthy organs were obtained using a 60% of maximum iso-contour. FAP and alpha smooth muscle actin (α-SMA) immunohistochemistry was performed in 13 cases (3 with and 10 without Ga FAPI-PET/CT). Staging and radiotherapy planning based on Ga-FAPI-PET/CT versus ceCT/MRI alone were compared.

RESULTS

We observed elevated tracer uptake in all ACCs. Immunohistochemistry showed FAP-expressing CAFs in the tumor. Compared to conventional staging, Ga-FAPI-PET/CT led to upstaging in 2/12 patients and to detection of additional metastases in 3 patients, thus in total 42% of patients had their staging altered. Moreover, Ga-FAPI PET improved the accuracy of target volume delineation for radiotherapy, as compared to CT and MRI.

CONCLUSION

Ga-FAPI-PET/CT is a promising imaging modality for ACC, increasing the accuracy of staging exams and radiotherapy planning volumes, as compared conventional to CT and MRI.