Purpose: To characterize the limitations of fast, spiral computed tomography (CT) when imaging a moving object and to investigate whether positron emission tomography (PET) can predict the internal target volume (ITV) and ultimately improve the planning target volume (PTV) for moving tumors.
Methods and materials: To mimic tumors, three fillable spheres were imaged while both stationary and during periodic motion using spiral CT and PET. CT- and PET-imaged volumes were defined quantitatively using voxel values. Ideal PTVs for each scenario were calculated. CT-based PTVs were generated using margins of 7.5, 10, and 15 mm to account for both organ motion and setup uncertainties. PET-based PTVs were derived with the assumption that motion was captured in the PET images and only a margin (7.5 mm) for setup errors was necessary. Comparisons between CT-based and PET-based PTVs with ideal PTVs were performed.
Results: CT imaging of moving spheres resulted in significant distortions in the three-dimensional (3D) image-based representations, and did not, in general, result in images well representative of either moving or stationary spheres. PET images were similar to the ideal capsular shape encompassing the sphere and its motion. In all cases, CT-imaged volumes were larger than that for the stationary sphere (range of excess volume from 0.4 to 29 cm(3) for stationary volumes of 2.14 to 172 cm(3)), but smaller than that for the true motion volume. PET-imaged volumes were larger than the true motion volume (difference from ideal ranged from 3 to 94 cm(3) for motion volumes of 1.2 to 243 cm(3)) and much larger than the stationary volume. Using CT data, geographic miss of some part of the ideal PTV occurred for 0 of 24 cases, 11 of 24 cases, and 18 of 24 cases using a 15-mm, 10-mm, and 7.5-mm margin, respectively. Geographic miss did not occur in any case for the PET-based PTV. The amount of "normal tissue" included in CT-based PTVs was dramatically greater than that included in PET-based PTVs.
Conclusion: Fast CT imaging of a moving tumor can result in poor representation of the time-averaged position and shape of the tumor. PET imaging can provide a more accurate representation of the 3D volume encompassing motion of model tumors and has potential to provide patient-specific motion volumes for an individualized ITV.