AbstractID: 7572 Title: The Limitations of Spiral CT and the Potential for PET Imaging in Defining the
Internal Target Volume of Moving Targets
Purpose: Knowledge of an individual tumour’s motion would allow a tumour-specific internal target
volume (ITV) to be generated, reducing the risk of geographic miss while minimizing normal tissue
toxicity. Spiral CT acquisitions “freeze” lung tumours at different locations depending upon breathing
motion and the x-ray scan plane resulting in a distorted representation of the tumour. The objectives of this
work was 1) to characterize the geometric distortions caused by spiral CT imaging of a moving target and
2) to determine if PET imaging yields more accurate information about the volume encompassed by that
moving target. Methods: Three spheres, with outer diameters of 16, 32, and 69 mm, filled with the
positron emitter, 22Na, were imaged by spiral CT and hybrid-PET. Images were acquired with the spheres
stationary and moving through a range of 7, 15, or 25 mm in either the cranio-caudal or medial-lateral
directions. Spheres were localized in CT and PET by a fixed CT number and by an adaptive threshold,
respectively.
Results: Marked distortions in shape and volume were seen in spiral CT images. As
expected, distortion was greatest for small objects and large motions, but was significant even with large
objects and moderate motion. Errors in volume estimation ranged from –44% to 32% with CT. PET
images captured the expected shapes and on average, provided more accurate volumes. Conclusions:
Conforming radiation beams to a moving target based on spiral CT is not ideal. PET imaging has potential
to provide both less distortion and a patient-specific ITV.