RT Journal Article SR Electronic T1 Comparison of Different Methods for Delineation of 18F-FDG PET–Positive Tissue for Target Volume Definition in Radiotherapy of Patients with Non–Small Cell Lung Cancer JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 1342 OP 1348 VO 46 IS 8 A1 Ursula Nestle A1 Stephanie Kremp A1 Andrea Schaefer-Schuler A1 Christiane Sebastian-Welsch A1 Dirk Hellwig A1 Christian Rübe A1 Carl-Martin Kirsch YR 2005 UL http://jnm.snmjournals.org/content/46/8/1342.abstract AB PET with 18F-FDG (18F-FDG PET) is increasingly used in the definition of target volumes for radiotherapy, especially in patients 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 methods: visually (GTVvis), applying a threshold of 40% of the maximum standardized uptake value (SUVmax; GTV40), and using an isocontour of SUV = 2.5 around the tumor (GTV2.5). By phantom measurements we determined an algorithm, which rendered the best fit comparing PET with CT volumes using tumor and background intensities at the PS. Using this method as the fourth approach, GTVbg was defined. A subset of the tumors was clearly delimitable by CT. Here, a GTVCT was determined. Results: We found substantial differences between the 4 methods of up to 41% of the GTVvis. The differences correlated with SUVmax, tumor homogeneity, and lesion size. The volumes increased significantly from GTV40 (mean 53.6 mL) < GTVbg (94.7 mL) < GTVvis (157.7 mL) and GTV2.5 (164.6 mL). In inhomogeneous lesions, GTV40 led to visually inadequate tumor coverage in 3 of 8 patients, whereas GTVbg led to intermediate, more satisfactory volumes. In contrast to all other GTVs, GTV40 did not correlate with the GTVCT. Conclusion: The different techniques of tumor contour definition by 18F-FDG PET in radiotherapy planning lead to substantially different volumes, especially in patients with inhomogeneous tumors. Here, the GTV40 does not appear to be suitable for target volume delineation. More complex methods, such as system-specific contrast-oriented algorithms for contour definition, should be further evaluated with special respect to patient data.