Invasive squamous cell carcinoma (ISCC) represents 5 to 15% of breast cancers. Despite the rarity of this pathology, the number of patients with Invasive squamous cell carcinoma treated at HCL is significant.

Patients at risk of metastasis are usually referred to the HCL Nuclear Medicine departments to perform positron emission tomography - computed tomography (PET/CT) with [18]F-FDG as an extension assessment. The investigators know that [18]F-FDG PET/CT has limited diagnostic performance for assessing the extent of breast cancer with a sensitivity of 66-96% for all histologies combined. For the ISCC, these performances are even lower with average Standard Uptake Value (SUV) values of 3.4 [2.8-3.9] versus 6.6 [4.8-9.7] for the others histological types of breast cancer. False negatives in [18]F-FDG PET/CT are due to an insufficient osteoblastic and immune response in the tumor stroma. Avril & al. showed 65.2% false negatives with [18]F-FDG PET/CT for ISCC. This is why the search for new imaging techniques in this indication is particularly relevant.

Targeting fibroblast activation protein (FAP), a type II membrane glycoprotein belonging to the dipeptidyl peptidase-4 family, is a promising strategy for imaging tumor stroma, particularly in epithelial carcinomas .

The investigators would like to compare the [18]F-FDG PET/CT technique currently used to this new emerging modality.

The investigators hypothesize superior diagnostic performance of [68]Ga-FAPI PET/CT compared to [18]F-FDG PET/CT for the assessment of ISCC extension, with a gold standard histological.

The investigators translate this into the hypothesis of finding 30% of positive FAPI PET when the [18F]FDG PET/CT is negative or doubtful.

The advantage of this project and this new imaging modality is to not undertreat patients wrongly classified as non-metastatic. The investigators therefore wish to offer [68]Ga-FAPI PET/CT to patients with negative [18]F-FDG PET/CT.

The FAPICL project constitutes a seed project before a larger structuring study.

Inclusion Criteria:

• Adult patient (≥18 years old),
• Beneficiary or entitled to a social security system;
• Patient having agreed to participate in the study and signed written informed consent;
• 18F-FDG PET/CT performed as part of the assessment of extension of an infiltrating lobular carcinoma, the result of which is negative or doubtful on at least one lesion.

Exclusion Criteria:

• Pregnant, parturient or breastfeeding women. A pregnancy test before inclusion will be carried out for women of childbearing age.
• Persons deprived of liberty by a judicial or administrative decision
• People receiving psychiatric care
• People admitted to a health or social establishment for purposes other than research
• Adults subject to a legal protection measure (guardianship, curatorship)
• Subjects participating in another interventional research including an exclusion period still ongoing at inclusion.