This study looks at the effectiveness and safety of a drug called **Ado-trastuzumab emtansine (T-DM1)**, combined with radiation and chemotherapy for treating **HER2-positive salivary gland cancer**. HER2 is a protein found in some cancer cells. T-DM1 is a special medicine that targets these HER2 cancer cells and delivers chemotherapy directly to them. This study is for patients who have had surgery to remove cancer and will receive treatment for up to one year, with follow-up for three years. The **FDA** (Food and Drug Administration) has not approved T-DM1 for salivary gland cancer yet, but it is used for other cancers.

• **Duration:** Treatment up to 1 year, follow-up for 3 years.
• **Participant Size:** About 55 people will join.
• **Eligibility:** Must have HER2-positive salivary gland cancer, be 18 or older, and meet other health conditions.