**Study Overview:** This study looks at how a special radioactive substance, called IR199, travels and acts in the body of patients with stomach (gastric) or pancreas cancer. This study will help doctors understand if IR199 is safe and how it moves in the body, especially in tumors that have a protein called Claudin 18.2. Patients need to be at least 18 years old and have certain types of cancer that show the Claudin 18.2 protein. They must have at least one tumor with this protein. This study is not for people who recently had treatments that might affect their bone marrow, have certain kidney or urinary problems, or are allergic to radioactive medicines. **Key Points:** - **Participation Involves:** Checking how IR199 moves and behaves in the body. - **Eligibility:** Must have specific types of cancer with Claudin 18.2. - **Risks:** Cannot participate if you've had recent treatments affecting bone marrow or are allergic to radioactive drugs. **Important:** If interested, written consent is needed from you or your guardian.