Radioactive seed localization is a technique used to locate and remove small breast cancers that are detected on a mammogram or ultrasound but cannot be felt by your surgeon during a breast exam. The procedure can take place up to a week before your lumpectomy. A radiologist places a tiny radioactive seed, about the size of a grain of rice, in your breast to mark the exact location of the cancer. The implanted seed contains a small amount of radiation, which helps your surgeon find the cancer at the time of your surgery using a handheld radiation-detection device. The seed is removed by your surgeon during your lumpectomy.