metastatic neoplasm micrometastasis A tumor formed by cells that have spread is referred to as a "metastatic tumor" or a "metastasis", but, strictly speaking, “metastasis” refers to the process of cancer cell migration, not to the tumor mass itself. Small number of cancer cells that have spread from a primary tumor to other parts of the body and are too few to be picked up in a screening or diagnostic test, and are identifiable only under a microscope. [ NCI ] neoplasm micrometastasis NCIt:C38486