Antibody-based PET Imaging and Treatment Response in Breast Cancer Treated With an Antibody-drug Conjugate.

Patient must have signed a written informed consent prior to any trial specific procedures. When the patient is physically unable to give their written consent, a trusted person of their choice, independent from the investigator or the sponsor, can confirm in writing the patient's consent;

Patients enrolled in the prospective cohort of the OASIS study;

Patient with locally advanced or metastatic breast cancer eligible to receive T-DXd as part of their standard care;

At baseline imaging at least two "target" lesions fulfilling the following criteria: (1) anatomically transaxial diameter ≥ 1.5 cm and measurable per RECIST1.1. and (2) metabolically assessable with a maximum standard uptake value corrected for lean body mass (SUVmax) ≥ 1.5 x SUVmean + 2 standard deviations (SD) of the liver measured in a 3-cm-diameter spherical volume of interest (VOI) in normal liver parenchyma;