Benign versus malignant

Benign tumours (lipomas, most moles, many adenomas) are usually harmless unless their size or position causes trouble, though some, such as colon polyps, are steps on the road to malignancy and are removed for that reason. Malignancy is defined by invasion through the tissue boundary and the capacity to metastasise; under the microscope, malignant cells show abnormal nuclei, high division rates and loss of normal architecture. A pathologist's job on a biopsy is first to make this call, and the whole staging system, treatment intent and prognosis flow from it.