The role of PET in monitoring therapeutic response

Full text: PET and PET/CT are the fastest growing imaging modalities worldwide. PET technology has already become an important diagnostic tool for staging disease, evaluating the treatment effects and the long-term follow-up of cancer patients in developed nations. In summary, PET/CT studies can affect the patient management in about 25-30% of the oncological patients. The most useful indication of FDG-PET in cancer patients is the differentiation between recurrence and post-therapy changes. Responses to therapy can be identified earlier and with greater accuracy than is possible with anatomic imaging modalities. Prognostic information available through 18F-FDG PET is superior to that of conventional imaging for many cancers. The sensitivity and specificity of FDG-PET in detecting recurrence amounted to 94% and 92% respectively. On the contrary, the sensitivity and specificity of conventional images were only 78% and 68%. FDG-PET is increasingly used to monitor tumor response in patients undergoing chemotherapy, chemoradiotherapy and radiopharmaceutical therapy. 18F-FDG PET is an accurate study for differentiating residual viable tumor tissue from therapy-induced fibrosis. It may allow the prediction of tumor response and patient outcome very early in the course of therapy. Overall survival and progression-free survival are correlated highly significantly with metabolic responders in PET. In one paper, the median progression-free survival of metabolic non-responders was only 1.8 months but that of metabolic responders was 5.9 months. In patients without a significant change in tumor 18F-FDG uptake, the treatment change should be considered early in the course of therapy. In patients with solid tumors treated by preoperative chemotherapy, a change in 18F-FDG uptake of 35%-50% within the initial weeks of chemotherapy has been found to provide the highest accuracy for the prediction of histopathologically complete response. Early identification of non-responding patients is of great clinical importance. Effective chemotherapy causes a marked decrease in tumor 18F-FDG uptake (15-40%) within 1-3 wk after the initiation of therapy. This has been extensively investigated for malignant lymphomas, but also for several solid tumors, such as breast cancer. Visual interpretation of PET scans is sufficient for assessment of tumor response after completion of therapy. However, quantitative analysis is generally required, if PET imaging is used to predict tumor response early in the course of therapy. FDG PET may become the method of choice for the early assessment of treatment response to primary chemotherapy in patients with breast cancer. Although about 70-80% of patients show clinical response to primary chemotherapy, the pathological response rate is only in the range of 20-30%. The sensitivity and specificity to predict complete pathologic response after the first course of chemotherapy were 90% and 74%, respectively. Therefore, as early as after the first course of therapy responding and non-responding tumors can be differentiated by PET. One baseline FDG PET and one scan after the first course of primary chemotherapy seems to be sufficient to identify non-responders at an early stage of therapy. By a threshold defined as a decrease below 55%, compared to the baseline scan, all responders were correctly identified after the first course (sensitivity 100%, specificity 85%). FDG PET may be helpful for improving patient management (30-40%) by avoiding ineffective chemotherapy and unnecessary side effects FDG PET can also be useful for prediction of response to hormonal therapy. In one study which investigated biological tumor characteristics like glucose metabolism and perfusion, determined with dynamic FDG PET and 15O-water PET before any therapy concluded that PET could predict the response to neoadjuvant chemotherapy. The high pretreatment metabolic rate for FDG predicted a poor response to therapy and a low metabolic rate relative to blood flow was found to be a predictor of complete response. (author)