About this Research Topic
Pre-treatment PET/CT imaging provides a good clinical overview of the disease status and it is an ideal initial staging technique, especially for carcinomas with a high potential for metastasis. PET/CT, in contrast to PET, is the standard for imaging of cancer patients for treatment planning and follow-up, because the integrated CT scan enhances the sensitivity and accuracy of PET-signals by providing better imaging quality and there is simultaneous anatomic correlation. For radiation oncologist evaluating curative treatment options, PET/CT changes therapy in 10 – 30 % of cases, depending on the type of tumor. For the purpose of target volume definition, a variety of algorithms have been evaluated, depending on the tracers used and the clinical setting. In general, a method based on background subtraction is recommended. The size and the signal intensity, especially the ratio of intensity and volume may influence the volume calculation. Tracer uptake heterogeneity can be an obstacle to volume rendering for threshold-based volume methods, and anatomical correlation on CT images and target volume adaptation by the physician usually are necessary. Positive PET signals can improve the definition of the gross, clinical and planning target volume especially in case of regional metastasis, especially in case of regional metastasis, the target volumes might be increased. Anatomically more focused investigations may be required to decide what is the best local treatment. In combination with high resolution imaging techniques, PET/CT can increase the confidence of physicians and reduce the inter-observer variability in treatment planning treating while planning treatment with high radiation doses of PET-positive lesions. The tracer selection may vary with disease site; (18)F-FDG is used in most clinical situations. Tracers such as (18)F-tyrosine or (18)F-DOPA have been applied for functional imaging studies of brain tumors, especially for high grade glioma, where the assessment of post-radiotherapy tumor response is greatly improved by PET/CT. Brain lesions may remain Gadolinium contrast-avid but may become PET-negative, which is indicative of good local tumor regression. Defining viable tumor by metabolic rather than anatomic imaging adds a new dimension to post-treatment evaluation. For the imaging of prostate cancer, the use of PET remains controversial. Tracers such as (18)F- or (11)C-choline or (11)C-acetate have been studied, and the detection of locally recurrent prostate cancer after external beam radiotherapy may be facilitated. Bone metastasis from any source, can readily be detected with (18)F-NaF. A potential new indication for PET/CT after initiating radiotherapy is the monitoring of the cell proliferation rate of tumors to determine responsiveness with nucleotide traces such as 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT). PET/CT helps assess the biological aggressiveness of the disease, because signal intensity appears to correlate with the histological grade and the proliferation rate of tumor cells. In several settings, such as uterine or esophageal carcinoma, a high tracer uptake is correlated with prognosis of the underlying disease. Assessment of the response with PET/CT also allows tailored and adaptive therapy based on the changes of tracer uptake during or after the treatment. In the case of lymphomas, for example, post-induction complete metabolic response may lead to omission of additional therapy, such as consolidative radiotherapy in Hodgkin’s disease. PET/CT represents the ideal one-stop technique, which is convenient for patients and cost-effective. In many patients the timely use of PET/CT improves the staging, treatment planning and follow-up evaluation. In the present series of papers, PET/CT is established as state-of-the-art oncological imaging tool in the field for diagnosis, treatment response assessment, and follow-up of cancer patients.
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