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Positioning and Dosimetric Uncertainties in Image-Guided Radiation Therapy with Respiratory Gating

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I Ali

I Ali*, S Hossain , O Algan , E Syzek , S Ahmad , University of Oklahoma Health Sciences Center, Department of Radiation Oncology, Oklahoma City, OK


SU-G-JeP3-14 (Sunday, July 31, 2016) 5:00 PM - 5:30 PM Room: ePoster Theater

Purpose: To investigate quantitatively positioning and dosimetric uncertainties due to 4D-CT intra-phase motion in the internal-target-volume (ITV) associated with radiation therapy using respiratory-gating for patients setup with image-guidance-radiation-therapy (IGRT) using free-breathing or average-phase CT-images.
Methods: A lung phantom with an embedded tissue-equivalent target is imaged with CT while it is stationary and moving. Four-sets of structures are outlined: (a) the actual target on CT-images of the stationary-target, (b) ITV on CT-images for the free-moving phantom, (c) ITV’s from the ten different phases (10-100%) and (d) ITV on the CT-images generated from combining 3 phases: 40%-50%-60%. The variations in volume, length and center-position of the ITV’s and their effects on dosimetry during dose delivery for patients setup with image-guidance are investigated.
Results: Intra-phase motion due to breathing affects the volume, center position and length of the ITVs from different respiratory-phases. The ITV’s vary by about 10% from one phase to another. The largest ITV is measured on the free breathing CT images and the smallest is on the stationary CT-images. The ITV lengths vary by about 4mm where it may shrink or elongated depending on the motion-phase. The center position of the ITV varies between the different motion-phases which shifts upto 10mm from the stationary-position which is nearly equal to motion-amplitude. This causes systematic shifts during dose delivery with beam gating using certain phases (40%-50%-60%) for patients setup with IGRT using free-breathing or average-phase CT-images. The dose coverage of the ITV depends on the margins used for treatment-planning-volume where margins larger than the motion-amplitudes are needed to ensure dose coverage of the ITV.
Conclusions: Volume, length, and center position of the ITV’s change between the different motion phases. Large systematic shifts are induced by respiratory-gating with ITVs on certain phases when patients are setup with IGRT using free-breathing or average-phase CT-images.

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