To help protect healthy tissues from the radioactive drugs that target tumors, oncologists laboriously map the tumors by hand from sections of three-dimensional (3-D) images. But now researchers have developed a faster way. Based on computed tomography (CT) imaging data from a patient, 3-D graphic design software creates a model of the tumor in just an hour or two. When combined with a test that simulates the fate of therapeutic radioactivity in the body, the tumor model also lets oncologists personalize the radiotherapy dose. A study of several patients showed that this fast, new method maps organs and radiotherapy doses as accurately as conventional methods. The new software is expected to be used in a clinical trial of a tumor-targeted radiotherapy; a commercial version is scheduled for release to other cancer centers next year.