The use of radio-labeled antitumor drugs in the treatment of solid tumors by the method of administering a radio-labeled anticancer drug to a patient and imaging at least a part of the patient using Positron Emission Tomography imaging is described. The method can be used to monitor delivery of antitumor drugs to tumors, to predict the effectiveness of therapy with a particular antitumor drug or combination of antitumor drugs, to assess the effectiveness of modulators of cellular accumulation, to individualize therapy and to evaluate the effectiveness of antitumor drugs with respect to particular cancers. Particularly preferred drugs are labeled taxanes, e.g., .sup.11C-paclitaxel and .sup.11C-docetaxel, labeled anthracyclines, e.g., .sup.11C-doxorubicin and .sup.11C-epirubicin, and other radiolabeled drugs, e.g. .sup.11C-topotecan, .sup.11C-SN-38, and .sup.11C-imatinib. The invention further describes antitumor drugs labeled with the radioactive label .sup.11C and methods of preparing radio-labeled drugs.