Human: My doctor says I need to have a mammogram. What is a mammogram and why do I need one? Assistant: 
A mammogram is an X-ray of the breast used to screen for breast cancer. It can detect tumors that are too small to feel during a physical exam, as well as any changes in the tissue of the breast. The goal of a mammogram is to find any signs of breast cancer early enough to treat it before it has spread.