Fluorodeoxyglucose (FDG) and human epidermal growth factor receptor 2 (HER2) positron emission tomography (PET) scan in breast cancer

Participants will be asked to fast for a few hours before the FDG- and HER2-PET scans. If participants are diabetic, they will be given instructions about their diabetes medications and their blood sugar will be checked upon arrival at the hospital. Participants will be asked to undergo a standard-of-care FDG-PET scan and a novel HER2-PET scan at baseline in the Nuclear Medicine Department at Fiona Stanley Hospital. Each scan will occur on different days within 1 week. We will ask participants to wait a minimum of 24 hours between scans. It is expected that the entire procedure will take 2-3 hours per scan. The entire procedure will be conducted by Radiologists and Nuclear Medicine physician's and their team. The Australian Diagnostic Reference Level (DRL) for standard FDG-PET imaging is 3.5 MBq/kg (for patients weighing between 50 and 120 kg). The 68Gallium-HER tracer will be administered at a dose of 107MBq. As participants will have HER2 positive or HER2 low metastatic breast cancer, baseline will be prior to treatment with the antibody-drug conjugate trastuzumab-deruxtecan (T-Dxd). The HER2-PET scan uses gallium-labelled HER2 binding peptides that have an affinity for the HER2 receptors which are present in many breast cancers. The radio-isotope injection (HER2 or FDG) is administered via a needle (cannula) into a vein in the hand or arm. Following this, participants will undergo positron emission tomography (PET). This involves participants entering a PET scanner for approximately 20 minutes. Upon disease progression based on RECIST criteria, participants will be asked to undergo a second FDG-PET and HER2-PET scan. After these second scans, this will be the end of the study for participants with progressive disease. Treating clinicians will determine further monitoring appropriate for each individual. Participants with stable disease throughout the study will return to standard follow-up imaging as determined by the treating clinician and this will be the end of the study. For those with HER2 avid but FDG-PET non-avid disease with known visceral disease, CT scan will also be performed It is anticipated that the study will take 1 year from the date of recruitment. As this is a PET scan study, there is no requirement for adherence to an intervention. However, we will monitor scan attendance and record results in electronic medical records.