Comparison of Different Methods for Delineation of 18F-FDG PET–Positive Tissue for Target Volume Definition in Radiotherapy of Patients

PET with 18F-FDG (18F-FDG PET) is increasingly used in the definition of target volumes for radiotherapy, especially in pa-tients with non–small cell lung cancer (NSCLC). In this context, the delineation of tumor contours is crucial and is currently done by different methods. This investigation compared the gross tumor volumes (GTVs) resulting from 4 methods used for this purpose in a set of clinical cases.Methods: Data on the primary tumors of 25 patients with NSCLC were analyzed. They had 18F-FDG PET during initial tumor staging. Thereafter, additional PET of the thorax in treatment position was done, followed by planning CT. CT and PET images were coregistered, and the data were then transferred to the treatment planning system (PS). Sets of 4 GTVs were generated for each case by 4 meth-ods: visually (GTVvis), applying a threshold of 40 % of the max-imum standardized uptake value (SUVmax; GTV40), and using a