This article is intended to provide an overview of various aspects of clinical PET/CT. These include discussions of:(i) Important areas of clinical application;(ii) Opportunities in clinical research;(iii) Scanner and operating-mode considerations (e.g. BGO vs. LSO, LYSO or GSO scanners, 2D vs. 3D imaging).(iv) Study-specific considerations (e.g. patient preparation and positioning issues, injected dose, use of CT contrast agents).

Radiation therapy has a major role in treatment of malignant tumor not only as an adjuvant treatment for surgery but also as a primary modality in treatment of inoperable tumors or as an alternative to surgery in order to preserve organ function and/or avoiding surgical complications.Treatment planning has evolved from simple conventional planning to conformal techniques such as Three-Dimentional conformal radiation therapy and intensity Modulated Radiation Therapy (IMRT). These techniques which are based on anatomical information from CT scan or MRI allow us to confirm radiation dose distribution to the target volume with a minimum possible dose to normal tissues. A major step in planning process is delineating tumor bearing tissues (target volume). The most important limitation is that the extent of target volume is not always discernible based on CT scan or MRI information.Positron emission tomography (PET) usingradioactive fluorine-18 labelled FDG as a tracer enables biological imaging of tumors and highlights its proliferating areas. This imaging technique which has an important role in diagnosis, staging and restaging of tumors has been introduced in radiation planning to facilitate tumor delineation. By using the same table and positioning for a PET / CT scan it is possible to fuse both images and combine the physiologic information from PET and superior images of anatomy and localization from CT scan. Using PET / CT scan in radiotherapy planning has the following potential advantages: 1- Differentiating between tumor and normal tissue. Take for an example; in lung cancer a distal atelectasis could be mixed up with a central tumor in CT scan.2- Including the tumor spread such as metastatic lymph nodes into the target volume which could be missed in CT information.3- Assessing the tumor response after terminating the planned treatment and prescribing additional boost dose to the remaining functional area.4- In some organs such as lung which lesions move considerably during respiration and radiotherapy, acquiring gated 4-D PET scan and hybrid PET / CT make it possible to assess the tumor motion precisely and allowing tighten tumor margins.In non-small cell lung cancer using FDG-PET has resulted in safe decrease in radiotherapy volume in a considerable number of patients and allowed dose escalation in tumor. In esophageal and Head and neck carcinoma it is possible to detect unrecognized lymph node metastasis and to delineate the tumor volume more precisely. In Hodgkin’s lymphoma, PET may be essential in involved field radiotherapy after chemotherapy to safely decrease the treatment volume while avoiding geographic miss. In cervical carcinoma PET can help a clinician to decide whether to encompass the para-aortic lymph nodes in treatment volume or not. PET has also an emerging role in radiotherapy of primary brain tumors.Multiple studies suggest an increasingly important role for PET in radiotherapy planning. However, using PET should follow strict standardized protocols. Further investigations are needed to reveal its exact role in radiation planning for different malignancies and to define safe recommendations.

Radiation therapy (RT), staging, treatment planning, monitoring and evaluation of response are traditionally based on Computed Tomography (CT) and Magnetic Resonance Imaging (MRI). These radiological investigations have the significant advantage to show the anatomy with a high resolution, being also called anatomical imaging. In recent years, so called biological imaging methods which visualize metabolic pathways have been developed. To date, the most prominent biological imaging system in use is Positron Emission Tomography (PET), whose diagnostic properties have clinically been evaluated for years. The combination of PET and CT in a single system (PET/CT) to form an inherently fused anatomical and functional dataset has provided an imaging modality which could be used as the prime tool in the delineation of tumor volumes and the preparation of patient treatment plans, especially when integrated with virtual simulation. This powerful method offer complementary imaging of various aspects of tumor biology. The aim of this review is to discuss the valences and implications of PET/CT in RT. The focus will be on evaluation of the following topics: the role of biological imaging for tumor tissue detection/delineation of the gross tumor volume (GTV) and for the visualization of heterogeneous tumor biology. The role of fluorodeoxyglucose-PET in lung will be discussed. There was also evidence for utility of PET in head and neck cancers, lymphoma and in esophageal cancers, with promising preliminary data in many other cancers. The best available approach employs integrated PET/CT images, acquired on a dual scanner in the radiotherapy treatment position after administration of tracer according to a standardized protocol, with careful optimization of images within the RT planning system and carefully considered rules for contouring tumor volumes. The impact of amino acids (AA) -PET in target volume delineation of brain gliomas was discussed.Furthermore, it is summarized the data of the literature about tumor hypoxia and proliferation visualized by PET. It concluded that regarding treatment planning in radiotherapy, PET offers advantages in terms of tumor delineation and the description of biological processes. PET/CT will play an increasing valuable role in RT planning for a wide range of cancers.

Background: There are many challenges in oncologic imaging including: more accurate staging, determine site with highest probability of tumor existence for biopsy, especially for brain tumors because of the sensitive anatomical position, determine the exact location of the tumor for radiotherapy treatment planning and etc. Positron emission tomography (PET) is imaging modality which provides some special functional information. The basis of the PET system is detection of tow photons which are generated by positron/electron encounter which occurs directly after positron emission by radionuclide. However, it is powerless in providing anatomical information. Computed tomography (CT) generates three dimensional reconstructions of patients anatomy based on differences in X-ray attenuation. Combination of PET and CT is able to provide anatomical and physiological information of patients synchronizes. Objectives: In this paper, we evaluate application of PET/CT for improve oncologic imaging. Methods: The papers were searched in PubMed and Scopus databases with the relevant key words including, PET/CT, Oncologic Imaging and Cancer staging. Results: The most relevant biomarker for PET is 8F-FDG (F-18 Fluorodeoxyglucose), which is incorporated into malignant cells by glucose transporters which are over expressed by cancer cells. CT is thecommonimaging modality in diagnosis the cancers. It is due to its speed, high special resolution and relative cost-effectiveness. The combination of these two modalities in one system produced PET/CT system which is more accurate than these two imaging modalities separately and enhances the sensitivity and specificity of diagnosis. For example in a study which used PET/CT for differentiation between the malignant and benign disease Presacral, sensitivity, specificity, the positive predictive value and the negative predictive value are 100%, 96%, 88% and 100% respectively. In another study, which was done for preradiotherapy staging of head-and-neck SCCs, it was shown that FDG-PET/CT detected 17 of 17 heminecks and 26 of 27 nodal zones histologically positive by dissection (100% and 96% sensitivity, respectively). Conclusions: PET/CT can provide diagnosis and more accurate staging, location of tumor with its capability in providing the anatomical and physiological information synchronizes. Therefore PET/CT plays an important role in oncology.

Introduction: Nowadays with the advent of 64-slice CT modules associated with PET scanners, full clinical cardiac assessment is possible. But, the use of CT for AC of PET image can introduce misalignment artifacts due to time discrepancy between CT and PET data acquisition and produce the artifactual distribution uptake values in all segments of the myocardial wall. The aim of this study is to evaluate the misalignment effect induced by spurious patient motion in-between the two modalities on regional uptake value in the myocardial wall.Methods: The study was performed using 3 patients including one NH3 perfusion and two 18Ffluorodeoxyglucose viability examinations acquired on the Biograph TP 64 PET/CT scanners using routine cardiac PET/CT protocol. For accurate quantitative analysis, attenuation and emission data were also obtained using RSD thorax phantom. Manual shifts between the CT and PET images ranging from 0 to 20 mm in six different directions were applied. Thereafter, attenuation correction was applied to the emission data using the manually shifted CT images. The reconstructed PET images using shifted CT images for attenuation correction were compared with the reference PET image (PET image corrected with misalignment free original CT image). VOI-based analysis, regression and Box and Whisker plots were determined using 500 VOIs located within the myocardial wall in each PET dataset. For accurate assessment of the activity distribution in myocardium wall, 17-segment bull’s eye view analysis was evaluated for PET images.Results: The absolute percentage relative difference in uptake value increased in all simulated movements with increasing misalignments for both phantom and clinical studies. In patient studies, in reference to the bull’s eye view models, VOI-based analysis and visual analysis of PET images in standard axes as evaluated by expert clinicians, the significant variation in uptake value in comparion with the reference PET images were observed in the anteroseptal (23.55±9.92) and lateroinferior (32.96±9.74) segments in backward and forward directions respectively.Conclusion: although misalignment can introduce artifactual nonuniformities in myocardial wall but the anterior, lateral and septal regions were more vulnerable by misalignment. The variation were more significant for right, backward and forward directions.so that were caused erroneous clinical interpretation even in little misalignment (5 mm).but the significant errors can observe in medium (10mm) mismatch for PET image corrected using shifted CT in the left, down and up directions.