ACTIVE is a prospective, single-arm, phase I/IIa study examining an active surveillance strategy for small, screen-detected, luminal breast cancer. Patients aged 70 or older with clinical stage I ER+/HER2- breast cancer are eligible. The goal of this clinical trial is to learn if an active surveillance strategy (serial imaging rather than therapeutic intervention) is a safe approach to monitor small breast cancers. The main question to answer is the proportion of participants who experience tumor progression by 12 months.