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Assessing the Validity of 4D-CT Based Target Volumes and Free Breathing CBCT Localization in Lung Stereotactic Ablative Radiation Therapy (SABR)

R Badkul

R Badkul*, D Pokhrel , H Jiang, J Park , F Wang , P Kumar , University of Kansas Medical Center, Kansas City, KS


SU-E-J-207 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:Four-dimensional-computed-tomography(4D-CT) imaging for target-volume delineation and cone-beam-tomography(CBCT) for treatment localization are widely utilized in lung-SABR.Aim of this study was to perform a quantitative-assessment and inter-comparison of Internal-target-volumes(ITV) drawn on various phases of breathing-cycle 4D-CT-scans, Maximum-intensity-projection(MIP),average-intensity-projection(AIP)and static CT-scans of lung-motion-phantom to simulate lung-SABR patient geometry. We also analyzed and compared the ITVs drawn on free-breathing-CBCT.

Materials & Methods:4D-CT-scans were acquired on Philips big-bore 16slice CT and Bellows-respiratory monitoring-system using retrospective phase-binning method. Each respiratory cycle divided into 10-phases. Quasar-Phantom with lung-inserts and 3cm-diameter nylon-ball to simulate tumor and was placed on respiratory-motion-platform for 4D-CT and CBCT-acquisition. Amplitudes of motions: 0.5,1.0,2.0,3.0cm in superior-inferior direction with breathing-cycle time of 6,5,4,6sec, respectively used.4D-CTs with 10-phases(0%to90%)for each excursion-set and 3D-CT for static-phantom exported to iPlan treatment-planning-system(TPS).Tumor-volumes delineated in all phases of 4D-CT, MIP,AIP,CBCT scans using fixed-HU-threshold(-500to1000)values automatically.For each 4D-dataset ITV obtained by unifying the tumor-contours on all phases.CBCT-ITV-volumes were drawn in Eclipse-TPS.

Results:Mean volume of tumor contours for all phases compared with static 3D-CT were 0.62±0.08%, 1.67±0.26%, 4.77±0.54% and 9.27±1.23% for 0.5cm,1cm,2cm,3cm excursions respectively. Differences of mean Union-ITV with MIP-ITV were close(≤2.4%).Mean Union-ITV from expected-theoretical values differed from -4.9% to 3.8%.Union-ITV and MIP-ITV were closer within 2.3%. AIP-ITVs were underestimated from 14 to 32% compared to union-ITV for all motion datasets. Differences of -5.9% to -44% and -5% to 6.7% for CBCT-ITV from MIP-ITV and AIP-ITV respectively.Motion excursions and centroid positions were within 2mm for 4D-CT and CBCT-ITVs to that of expected values.

Conclusion:4D-CT MIP-ITV and Union-ITV showed very good agreement that validates that ITV can be fast contoured on MIP.Contouring ITV in AIP must be avoided as it significantly underestimates the volume with all excursions. Free breathing CBCT-ITV showed good agreement with AIP-ITV but underestimated the MIP-ITV. Estimation of excursions and centroid values for 4D-CT and CBCT were in good agreement with expected values.

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