This clinical trial is focused on studying the long-term safety of a treatment for people with advanced cancers known as HER2-expressing or HER2-mutated solid tumors. These are types of cancers that have specific changes in a protein called HER2, which can affect how the cancer grows. The treatment being studied is a medication called trastuzumab deruxtecan, also known by its code name DS-8201a. This medication is given as a solution through an infusion into the veins.

The purpose of the study is to provide continued access to this treatment for patients who are benefiting from it and to monitor its long-term safety. Participants in this study will continue to receive the medication as long as they are experiencing positive effects from it. The study will track any side effects or health changes that occur during this time.

Throughout the study, participants will receive regular infusions of the medication and will be monitored by healthcare professionals to ensure their safety. The study aims to gather information on any adverse events, which are unexpected health issues that might arise from the treatment. This information will help in understanding the long-term effects of the medication and ensuring it is safe for continued use in treating these types of cancers.