Fiducial markers are bits of metal the size of a grain of rice that are often placed around a gastrointestinal tumor before radiation therapy. The markers show up on images (such as X-rays) to better pinpoint a tumor. Because organs in the body shift around when we move, fiducial markers permanently mark the tumor and help make sure radiation is aimed precisely. That way, radiation can be as exact as possible, affecting just the cancerous cells and minimizing harm to healthy tissue. Using imaging to help guide radiation is called image-guided radiation therapy (IGRT).