T-DM1 chemotherapy refers to treatment with Kadcyla, which is a targeted treatment that may be used to treat HER2-positive breast cancer in women who meet certain criteria. The chemical name for Kadcyla is T-DM1 (also called ado-trastuzumab emtansine) and it is a combination of two medicines: trastuzumab which is a targeted treatment and DM1 which is a cytotoxic agent (also known as mertansine). Trastuzumab targets HER2-expressing breast cancer cells and delivers DM1 directly to them.