OBJECTIVE: To determine an appropriate threshold value for delineation of the target in positron emission tomography (PET) and to investigate whether PET can delineate an internal target volume (ITV), a series of phantom studies were performed.METHODS: An ellipse phantom (background) was filled with 1028 Bq/ml of [(18)F] fluoro-2-deoxyglucose ((18)FDG), and six spheres of 10 mm, 13 mm, 17 mm, 22 mm, 28 mm, and 37 mm in diameter inside it were filled with (18)FDG activity to achieve source-to-background (S/B) ratios of 10, 15, and 20. In static phantom experiments, an appropriate threshold value was determined so that the size of PET delineation fits to an actual sphere. In moving phantom experiments with total translations of 10 mm, 20 mm, and 30 mm and a period of oscillation of 4 s, the maximum size of PET delineation with the appropriate threshold value was measured in both the axial and sagittal planes.RESULTS: In the static phantom experiments, the measured maximum (18)FDG activities of spheres of less than 22 mm were lower than 80% of the injected (18)FDG activity, and those for the larger spheres ranged from 90% to 110%. Appropriate threshold values determined for the spheres of 22 mm or more ranged from 30% to 40% of the maximum (18)FDG activity, independent of the S/B ratio. Therefore, we adopted an appropriate threshold value as 35% of the measured maximum (18)FDG activity. In moving phantom experiments, the maximum (18)FDG activity of spheres decreased significantly, dependent on the movement distance. Although the sizes of PET delineation with 35% threshold value tended to be slightly smaller (<3 mm) than the actual spheres in the axial plane, the longest sizes in the sagittal plane were larger than the actual spheres.CONCLUSIONS: When a threshold value of 35% of the measured maximum (18)FDG activity was adopted, the sizes of PET delineation were almost the same for static and moving phantom spheres of 22 mm or more in the axial plane. In addition, PET images have the potential to provide an individualized ITV.
Artifacts Fluorodeoxyglucose F18