Monitoring Tumor Response to Therapy

Imaging tumor cell proliferation is of high interest for both research and clinical practice in oncology. High proliferation rates are characteristic of most malignant tumors, whereas in benign tumors the fraction of prolifer-ating cells is comparatively small. Therefore, imaging of proliferation may provide a sensitive and specific tool for differentiation of benign and malignant tumors. Furthermore, prolif-eration imaging may be used for de-tection of metastatic disease in patients with known cancer. Because many benign lesions that mimic metastases See page 845 on morphologic imaging demonstrate low proliferation rates or absence of proliferation, imaging of proliferation has the potential to significantly im-prove the accuracy of cancer staging. Finally, radiotherapy and chemo-therapy rapidly decrease proliferation rates in responding tumors. This effect usually precedes a reduction in tumor size. Imaging of cellular proliferation may therefore provide an earlier read-out for therapeutic effects than do size measurements by CT or MRI. Many novel targeted agents, such as protein kinase inhibitors, have a predomi-nantly cytostatic effect and do not cause rapid tumor shrinkage. The beneficial effects of these therapies may therefore be underestimated by conventional response criteria such as the Response Evaluation Criteria in Solid Tumors, which are based on a significant reduction of tumor size within a few weeks of therapy. Prolifer-ation imaging is therefore of enormous interest for the further clinical develop-ment of these novel therapeutic agents