Program Information
On the Validity of Lung Target Volume Delineation
D Mohatt*, H Malhotra , Roswell Park Cancer Institute, Buffalo, NY
Presentations
PO-BPC-Exhibit Hall-18 (Saturday, March 18, 2017) Room: Exhibit Hall
Purpose: The standard approach for dealing with target motion in radiotherapy involves accounting for anatomical displacements using time resolved CT imaging. An internal target volume (ITV) is typically defined by the inclusion of 10 GTV phases delineated over the course of one respiratory cycle. While various clinics have used different segmentation techniques to define the ITV, it is the purpose of this study to compare these methods in terms of planning accuracy and delivery efficiency.
Methods: A Quasar respiratory motion phantom was employed to simulate lung tumor movement. Utilizing 4DCT imaging, we created a “gold standard” PTV based on 10 GTV phase composite CT image sets. Four alternative planned PTV’s were created and compared using free breathing (FB), average intensity projection (AIP), maximum image projection (MIP), and a 3 phase composite structure composed of FB and max-inhale/exhale image sets. Statistical analysis was preformed using the Dice similarity coefficient (DSC).
Results: PTV’s derived from the FB image set are the least comparable with the benchmark 10 phase composite target derivation (DSC=0.670-0.305). For target motion greater than 1 cm of movement, the 3 phase estimation and AIP ITV delineation exceeds the 10 phase benchmark by 2% or greater. For all ranges of motion, MIP target segmentation was found to be within 2% agreement with the 10 phase gold standard (DSC>0.877).
Conclusion: In summary, our results indicate the range of motion dictates the accuracy of the defined PTV. We have shown the PTV structure based on the delineation of the free breathing image set yields an inaccurate interpretation of the target treatment density. Among the various techniques used for image segmentation, a judicious balance between accuracy and efficiency is required to account for tumor trajectory, range and rate of mobility.
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