A number of diagnostic tests are required for detection and management of cancer of which imaging plays an important (pivotal) role in cancer management. Imaging has made rapid strides in the past few years with newer modalities and new indications getting added as time progresses. Molecular and metabolic imaging with 18F Flurodeoxyglucose Positron Emission Tomography (FDG PET) and other non-FDG isotopes has revolutionized oncologic imaging and changed our approach towards cancer diagnosis, staging and surveillance. Newer indications get added to the literature. The focus is to understand the basic principles of FDG PET imaging and know few of its clinical applications in oncology In the past few decades oncologists and oncosurgeons have attempted to cure many types of cancers or at least provide patients with a longer disease-free life, there has been a spectacular parallel and supportive revolution in imaging diagnosis technology. Presently, imaging plays a central role in the management of all cancers. It is used for screening, for preliminary diagnosis, to establish the extent and distribution of disease, for biopsy guidance, staging, prognostication, therapeutic planning, judging response to therapy and restaging. In oncology, the role of imaging has shifted from merely providing anatomical information to providing insight into tumor biology using techniques such as Computerized Tomography (CT) and Magnetic Resonance Imaging (MRI) perfusion imaging, MR spectroscopy and Positron Emission Tomography (PET) imaging. These techniques can demonstrate metabolic response to therapy which precedes anatomical changes. If there is less than optimal response to treatment then an earlier assessment guides a change in the management regime, thus avoiding the potential morbidities. PET-CT using18 F-Flurodeoxyglucose (FDG) PET has in the last few years has become an established modality in the management of several cancers. It is an imaging technique that provides information about the functional and metabolic changes associated with cancer. PET scanning requires the use of molecules that are labeled with radio nuclides. Numerous such positron-emitting radio-isotopes exist and are used in clinical experiments and research. However, in clinical practice the principal radio-isotope used is the positron-emitting 18 F-FDG which is a glucose analogue labeled with 18 F. The positron emitters including 18 F FDG have short half lives and are produced in cyclotrons. FDG is injected intravenously and is transported from the plasma to the cells by glucose transporters (GLUT 1 and GLUT 4). It then undergoes phosphorylation within the cell by the enzyme hexokinase and is converted to FDG-6-phosphate. FDG-6-phosphate is not metabolized and gets trapped in the cell. Cancer cells demonstrate increased glycolysis which is believed to be due to upregulation of glucose transporters and hexokinase and reduced levels of glucose-6-phosphatase thus limiting further metabolism of the tracer in cancer cells. FDG PET and PET-CT play an important role in the 1.Initial diagnosis/evaluation, especially in malignancies/metastatic secondaries most importantly Lymphoma, Carcinoma Breast, Carcinoma Lung, Colorectal Cancer; Malignant Melanoma An FDG avid tissue visualized on the PET-CT scan helps us to localize metabolically active lesion and act as guide for FNAC/biopsy 2.Staging of the disease, 3.Assess response to treatment /Monitor response to therapy by semiquantitative SUV value e.g patient responding to treatment and those who do not respond Non Responder Responder 4.Assist in restaging of diseases 5.Radiation treatment planning 6.In the future help in development of newer molecules for therapeutic purpose e.g (newer anti cancer drugs) As with any other non invasive diagnostic modality PET-CT scan performed with 18F-FDG too has its limitations as enumerated below. 1.Altered biodistribution of FDG related to hyperglycemia and accompanying insulin increase in the body causes normal glucose to competes with FDG for uptake in tumoral tissue thus causes decrease uptake in tumor and increase background. Increase insulin levels in the body following meals or insulin injection as in diabetics cause insulin to push glucose and FDG towards skeletal muscles and myocardium causing decrease uptake in the tumor and increase background. Hence it is very important that patient remains fasting for 6 †8 hours prior to the scan, as even a small quantity of food intake prior to the scan can alter biodistribution of the FDG and cause greater degree of false negative scan. For diabetic patients on medication require stringent protocol for an optimal scan results, Bone marrow activation encountered in patients receiving bone marrow stimulating drugs cause FDG distribution in bone marrow significantly with little uptake in the tumoral tissue. 2.Brain typically shows intense FDG uptake as it utilizes glucose exclusively, hence lesions can be missed, thus PET-CT scans are usually performed from base of skull to mid thigh unless indicated otherwise. 3.FDG is excreted through the kidneys into ureters and into the bladder,with a higher tracer concentration at these sites, lesions cannot be assessed with confidence at these anatomical sites. Not all malignancies and tumors are FDG avid .Poorly avid FDG tumors are Prostate, Bronchoalveolar carcinoma, neuroendocrine tumors, low grade sarcomas and NHL (Non Hodgkins Lymphoma). Also not all FDG avid tissue are malignant e.g. 1.Infection 2.Granulamatous lesions 3.Arthritis 4.Post surgery/radiotherapy
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