DCIS (ductal carcinoma in situ) is a common pre-stage for breast cancer. The goal of this clinical trial is to learn if FAPI-PET/MRI (an imaging technique with a weakly radioactive drug) helps to diagnose hidden invasive breast cancer in participants with DCIS. The main question it aims to answer is:

How good can FAPI-PET/MRI diagnose hidden invasive breast cancer in DCIS?

Researchers will compare FAPI-PET/MRI results to tissue samples obtained from surgery treatment to see if the FAPI-PET/MRI images show invasive breast cancer certainly.

Participants will

• receive the radioactive drug and lie in an imaging device for 45 minutes including a break
• visit the clinic once again for a checkup and test

Ductal carcinoma in situ (DCIS) is a common precursor to breast cancer where abnormal cells are present within the milk ducts without breaking through their walls (in situ). If these cancerous cells have already breached the boundaries of the milk ducts, it is referred to as "invasive carcinoma." About half of these precancerous conditions develop further into invasive carcinomas over time. However, since doctors cannot precisely predict this at an individual level, treating DCIS often involves removing the affected tissue to prevent progression towards invasive cancer. Typically, diagnosis relies on performing a biopsy, during which cells from the altered tissue are extracted and examined under a microscope. Unfortunately, determining whether or not an invasive carcinoma has developed is not always possible with certainty with this method; indeed, about one quarter of cases involving existing invasive carcinomas might remain undetected by conventional biopsy procedures.

Consequently, being able to ascertain before surgery if an invasive carcinoma exists or not would be highly beneficial. This study examines a novel method that could potentially enhance the discovery of hidden (also called 'occult') invasive carcinomas. Specifically, it employs positron emission tomography (PET) utilizing a radiopharmaceutical agent known as [68Ga]Ga-FAPI-46. This compound selectively targets and binds to fibroblast activation protein (FAP), abundant on surfaces of tumor-associated fibroblasts. By tagging this structure with a radiotracer, researchers aim to achieve precise visualization and assessment of tumor extent. For the PET examination, a minimal dose of the radiopharmaceutical is administered intravenously while sensitive cameras capture images of particular body sections externally (PET). Additionally, magnetic resonance imaging (MRI) of the breast is conducted concurrently to ensure anatomical orientation, image enhancement (attenuation correction), and additional information acquisition.

The trial visits will be scheduled during the regular treatment process. These will not extend the participant's overall treatment duration significantly, as all measures within the scope of the study typically occur within 30 days, but no more than 51 days. During this period, effective contraception is necessary.

Researchers will then compare the imaging results to the pathology ground truth to evaluate the feasibility of the method.

Inclusion Criteria:

• Newly diagnosed DCIS with extent in mammography, MRI or ultrasound suspected to range > 4 cm
• Planned DCIS resection (breast conserving or mastectomy) as per guideline recommendation
• Written informed consent
• For women of child bearing potential: confirmed menstrual period (if applicable) and a negative highly sensitive urine or serum pregnancy test
• Women of childbearing potential (WOCBP) and male patients with partners of childbearing/reproductive potential must agree to use highly effective contraception (Pearl index < 1) when sexually active. This applies for the time period between signing of the informed consent form up to the final trial visit.

Exclusion Criteria:

• Contraindications for MRI (specific metallic implants, severe claustrophobia, history of anaphylaxis following MRI contrast agent application)
• GFR < 30 mL/(min∙1.73 m²)
• Current pregnancy or within last 8 weeks before begin of study participation
• Current nursing or within last 8 weeks before begin of study participation
• Inability to understand the nature, risks, and benefits of the study
• History of diagnosis of ipsilateral invasive breast cancer
• Concurrent diagnosis of contralateral invasive cancer, if not curatively treated by surgery > 1 year ago
• Known hypersensitivity to the active substance or to any of the excipients of the Investigational Medicinal Product