Validation and Reliability of Free Breathing Cone Beam Computed Tomography (FB-CBCT) Imaging Guidance in Stereotactic Ablative Radiation Therapy (SABR) of Lung Tumors

Abstract

Purpose/Objective(s): FB-CBCT has been widely adapted as effective image guidance for SABR. The aim of this work was to assess interfractional variation of tumor volume on FB-CBCT in comparison of planning internal target volume (ITV) on 4-D planning CT scan and to determine the reliability, consistency and reproducibility of FB-CBCT. Materials/Methods: One hundred eighty-two FB-CBCT images from 35 patients with 39 lung tumors treated with SABR in 3-5 fractions were evaluated. The gross tumor volume (GTV) on FB-CBCT was contoured using a referenced CT window. The variations of GTV volume and its geometric deformation on FB-CBCT were calculated. The correlation between GTV on FB-CBCT and ITV was analyzed. Interfractional variability was assessed by variation coefficient, reliability coefficient and conformity index (CI). Results: The mean GTV volume on FB-CBCT from all tumors was 12.1cc, which was 12.9% smaller than ITV volume of 13.91 cc. The mean GTV volume was highly correlated (R Z 0.96) to the ITV volume. The mean GTV volume in fraction 1 to 5 was 11.73, 13.20, 12.17, 12.01, and 11.22 cc, respectively. This GTV volume variation during each treatment session was not statistically significant. The variation coefficient for the population was 0.17 0.08 with normal distribution. The values of the interfractional reliability coefficient (0.99 for 5 fractions and 0.97 for 3 fractions) indicated high reproducibility of results. The Jaccard index of conformity was 0.32 0.13. The CI. of the central tumors was better than that of the peripheral tumors but the difference of the means was not statistically significant (p Z 0.58). A certain degree of deformation in the volume of GTVon FB-CBCTwas observed. Mean volume deformation of GTV was 24.9% ( 14.21). For centrally located tumors it was 23.2% ( 9.91) and for peripherally located tumors it was 25.1% ( 14.83). However, this difference was not statistically significant (p Z 0.678). The regression analysis indicated that the distance of GTV from vertebrae and their locations was significantly correlated to the deformation of GTV (p Z 0.013 and 0.004, respectively). Conclusion: FB-CBCT is a valid tool for image guidance in SABR with high correlation of GTV volume to ITV, lower interfractional variation coefficient and high reliability coefficient. The relative large degree of GTV deformation on FB-CBCT may warrant the consideration of geometric volume missing when non volumetric image guidance is used for SABR. Author Disclosure: F. Wang: None. S. Das: None. J. Park: None. R. Badkul: None. S. Hus: None. P. Kumar: None.