Application of PET initially focussed on brain and heart. Now it is being primarily used in oncological indications. This development has resulted from successful application of Flourine-18-Fluro- deoxyglucose (FOG) to a growing number of clinical indications at varying stages of diagnosis. staging and follow up. Using FOG in vivo cancer imaging is based on the observation of enhanced glycolysis in tumour cells. A high rate of aerobic glycolysis (degradation of glucose to lactic acid in the presence of oxygen) in several types of cancer cells was first described by Warburg. This phenomenon has been linked to both increase in the amount of glucoase membrane transporters and an increase in the activity of the principal enzyms controlling the glycolytic pathways. It is important to stress that FOG uptake by neoplastic tumours in vivo remains under the dependance of other physiological factors, such as, tissue oxygenation, regional blood flow and peritumoural inflammatory reactions.

Solitary pulmonary nodule:- Predictive accuracy for both benign and malignant nodule is 94% in addition to reducing the complications encountered by other methods of investigations like transthoracic needle aspiration.

Several studies from Germany and Japan have evaluated the role of FDG PET in differentiations of pancreatic adeno carcinoma from benign chronic prancreatitis and mass forming pancreatitis. Sensivity for carcinoma has been reported to be 94% with a specificity ranging from 78% to 90%.

Initial staging by FOG PET has been useful in lung cancer, melaoma, sarcoma and Lymphoma. It is probabaly indicated in other tumour types like ovarian, head & neck and pancreatic carcinoma, especially when the tumour is in an advanced stage or when metastatic lesions are suspected by conventional Imaging or by raised tumour markers. Indeed, in these cases, FDG PET can provide sensitive whole body screening.

Differentiation of scar and residual or recurrent disease is a frequent indication of PET and one of the first to be documented. It is used for lung, head and neck, colorectal carcinomas. FDG has also proven useful in the evaluation of residual masses after therapy for lymphoma.

In a suspected case of recurrence (by raised marker or other clinical signs) where conventional imaging fails to detect because of small size of recurrence PET may have immense value for its high sensitivity and whole body capability. It not only can confirm but can delineate the extent of recurrent disease. The impact of PET on management, avoiding unnecessary surgery, allowing more complete surgery forms the basis of cost effectiveness.

FOG PET can be helpful in evaluation of therapeutic response well before morphologral decrease of tumour mass can be demonstrated by conventional imaging. Early determination ()f therapeutic resistance is also important to avoid the toxicity of an ineffective therapy and to allow selection of a new therapeutic regimes.

Thus PET has a tremendous potential for diagnosis and decision making in a complex oncological problem. Extensive clinical research are bring undertaken to find out the sensitivity and specificity in various situations and define its precise role.