Abstract
BackgroundThe “gold standard” approach for defining an internal target volume (ITV) is using 10 gross tumor volume (GTV) phases delineated over the course of one respiratory cycle. However, different sites have adopted several alternative techniques which compress all temporal information into one CT image set to optimize work flow efficiency. The purpose of this study is to evaluate alternative target segmentation strategies with respect to the 10 phase gold standard.MethodsA Quasar respiratory motion phantom was employed to simulate lung tumor movement. Utilizing 4DCT imaging, a gold standard ITV was created by merging 10 GTV time resolved image sets. Four alternative planed ITV’s were compared using free breathing (FB), average intensity projection (AIP), maximum image projection (MIP), and an augmented FB (FB-Aug) set where the ITV included structures from FB plus max-inhale/exhale image sets. Statistical analysis was performed using the Dice similarity coefficient (DSC). Seventeen patients previously treated for lung SBRT were also included in this retroactive study.ResultsPTV’s derived from the FB image set are the least comparable with the 10 phase benchmark (DSC = 0.740-0.408). For phantom target motion greater than 1 cm, FB and AIP ITV delineation exceeded the 10 phase benchmark by 2% or greater, whereas MIP target segmentation was found to be consistently within 2% agreement with the gold standard (DSC > 0.878). Clinically, however, the FB-Aug method proved to be most favorable for tumor movement up to 2 cm (DSC = 0.881 ± 0.056).ConclusionOur results indicate the range of tumor motion dictates the accuracy of the defined PTV with respect to the gold standard. When considering delineation efficiency relative to the 10 phase benchmark, the FB-Aug technique presents a potentially proficient and viable clinical alternative. Among various techniques used for image segmentation, a judicious balance between accuracy and efficiency is inherently required to account for tumor trajectory, range and rate of mobility.
Highlights
The “gold standard” approach for defining an internal target volume (ITV) is using 10 gross tumor volume (GTV) phases delineated over the course of one respiratory cycle
In 2004, Allen et al described an estimated composite target volume on the basis of the inhale and exhale CT image sets [3]. It is noted in some clinics an “augmented” ITV has further been refined to include the union targets delineated on the free breathing (FB) image set along with the inhale and exhale CT images
With the exception of FB, Fb-Aug, average intensity projection (AIP) and maximum intensity projection (MIP) methods clearly show a linear increase in target volume for increasing range of motion along the superior-inferior direction
Summary
The “gold standard” approach for defining an internal target volume (ITV) is using 10 gross tumor volume (GTV) phases delineated over the course of one respiratory cycle. In 2004, Allen et al described an estimated composite target volume on the basis of the inhale and exhale CT image sets [3] It is noted in some clinics an “augmented” ITV has further been refined to include the union targets delineated on the free breathing (FB) image set along with the inhale and exhale CT images. Rietzel et al took this ideology one step further by including the contours of the GTV’s from all 10 phased data sets [4] This straightforward fusion of GTVs at all instances over the course of respiratory cycle to generate the ITV has since become the “gold standard” approach in the treatment planning process. The fundamental objective of this study is to determine if efficient ITV delineation strategies are a valid representation for the irradiated target volume
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