This study is testing a new way to give a cancer medicine called *Trastuzumab Deruxtecan* (T-DXd). T-DXd is normally given through an IV, but this study is testing it as a shot under the skin. The study is for adults with advanced cancers that have spread to other parts of the body. They must have certain types of cancer, like breast or stomach cancer, that are positive for HER2, a protein that makes cancer grow faster. Participants need to have tried other treatments before joining. **Key Points:** - **Study Length and Visits:** The study involves several visits to the doctor to monitor safety and how well the treatment works. - **Compensation:** Participants might receive compensation for their time and travel. - **Risks:** Potential risks include side effects from the medication or injection site reactions. Before joining, participants need to sign a form agreeing to the study. Some people can't join, like those who had severe allergies to similar drugs or certain heart problems. Always discuss with your doctor to see if this study is right for you.