Emerging role of PET/CT in osteosarcoma

Introduction: FDG PET/CT has been used to evaluate various malignancies including those of the musculoskeletal system. It is an imaging technique that provides information about the metabolic changes associated with cancer. PET imaging uses molecules that are labelled with radio nuclides. In clinical PET practice the principal radio-isotope used is the positron emitting 18 F-FDG which is a glucose analogue labelled with radionuclide 18 F. FDG is injected intravenously and is transported from the plasma to the cells by glucose transporters (GLUT 1 & GLUT 4). It then undergoes phosphorylation within the cell by the enzyme hexokinase and is converted to FDG-6-phosphate. FDG-6phosphate does not get further metabolised and gets trapped in the cell. Cancer cells demonstrate increased anaerobic glycolysis (Warburg effect) which is believed to be due to upregulation of glucose transporters and hexokinase and reduced levels of glucose-6phosphatase thus limiting further metabolism of the tracer in cancer cells [1]. Thus FDG PET provides unique functional information by taking advantage of the propensity of malignant tumors to demonstrate increased glucose utilization (metabolism) as compared with normal tissue. A semiquantitative measure of the metabolic activity (corrected for the amount of radiotracer injected per kilogram of body weight and for blood glucose level) is known as the standard uptake value (SUV) . Although the degree to which both benign and malignant tumors accumulate FDG can be quite variable, malignant tumors tend to have higher SUV values. In general significant difference is noted in the SUV of benign and malignant primary bone tumors. It should be noted that the interpretation of absolute SUV values can be misleading. No cutoff value of SUV can be established in clinical practice to reliably differentiate benign from malignant primary bone tumors. Osteosarcomas (OGS) and Ewings sarcomas tend to be FDG avid whereas chondrosarcomas and soft tissue sarcomas showing a wide range of uptake depending on the histology and grade of the tumor. Since OGS is FDG concentrating tumor, FDG PET can be used for staging and restaging. FDG PET can also be used to monitor response to neo-adjuvant chemotherapy and as a prognostic and predictive marker.