A tumor is an abnormal growth of body tissue. Tumors can be cancerous or noncancerous. The development of the new biological drugs in the treatment of cancer has recently renewed the concept of “tumor heterogeneity”. The effects of the new targeted therapy, such as angiogenesis inhibitors and antivascular therapies, are more complex, compared to the well-known cytotoxic drugs. The intra- and inter-tumor heterogeneity, heterogeneity between metastatic disease, and also heterogeneity that happens over time, profoundly influence the treatment outcome of patients. At present, tailored medicine heavily relies on the pathological characterization of tumor by biopsies but we should have multiple biopsies of the same tumor in the same patients, to study the dedifferentiation of a metastatic disease under treatment.

Considering the invasive limitations and the issue of expression heterogeneity, molecular imaging, represent one of the most important and attractive challenge of Radiology. It allows non-invasive whole body characterization of in vivo tumor tissues by using multimodality imaging techniques (such as FDG-PET, non FDG-PET, DWI-MRI, Ultrasound targeted contrast Agents and Optical Imaging). All together, these techniques, offer a morphological and quantitative/functional imaging characterization, in a real time manner. They can be safely performed and repeated multiple times, providing information about heterogeneity in the primitive or metastatic disease. On the other hand, the future of molecular imaging is limited in terms of sensitivity, specificity and clinical translatability. The sensitivity is dependent on the molecular probes that are used and the specificity is dependent on the imaging signal obtained. Indeed, still few multimodality imaging techniques are applied in the clinical trials. In this view, we expect that the improvement of the technology behind molecular imaging, with a more availability and knowledge of the application of these techniques in the clinical settings, will definitely improve the diagnosis and the monitoring treatment in clinical cancer practice.