This study focuses on people with HER2-positive breast cancer, which means their cancer cells have a protein called HER2. The study compares two lengths of treatment with HER2-targeted medications: 6 months versus 12 months. These medications, like trastuzumab and pertuzumab, help stop cancer cells from growing and can mark them for destruction by the immune system. People in this study have already had chemotherapy and surgery and have no remaining invasive cancer.

Participants in the study will receive either 6 or 12 months of treatment and will be closely monitored with tests like echocardiograms (ECHO) or MUGA scans, which check heart function, and imaging tests like mammograms or MRIs. There will also be optional blood and tissue sample collections. After completing the treatment, participants will be checked every 6 months for up to 5 years and then annually for a total of 10 years.

• The study will last up to 10 years, with regular follow-ups.
• Participants may undergo up to 17 cycles of treatment, each cycle lasting 21 days.
• Side effects and quality of life will be closely monitored and compared between the two treatment lengths.