Comparison Of Target Volumes Based On Three-dimensional CT And Four-dimensional CT Images Of Non-small-cell Lung Cancer

Objective: To compare positional and volumetric differences of internal gross tumor volume (IGTV) delineated by three approaches based on four-dimensional CT (4DCT) for the primary tumor of non-small cell lung cancer (NLCLC). For the purpose of accessing the respiratory motion information included in axial three-dimensional CT (3DCT) and the feasibility of treatment planning based on 3DCT comparing to 4DCT, the positional and volumetric differences between 3D and 4D target volume were also compared at the lever of GTV and PTV.Methods: Twenty-one patients with NLCLC underwent big bore axial 3DCT and 4DCT simulation scan of the thorax. IGTVs of the primary tumor were delineated using three approaches as followed: the gross tumor volume (GTV) on each of the ten respiratory phases of the 4DCT image set was delineated and fused ten GTV to produce IGTV₁₀; the GTV delineated separately based on 0% and 50% phase was fused to produce IGTV_EI+EE; the visible tumor on the MIP image was delineated to produce IGTV_MIP. The position of the target center, the volume of target, the degree of inclusion (DI) and the matching index (MI) were compared between IGTVs. Going on collecting the case to 28 cases, the GTV_conv based on 3DCT was contoured. PTV_vector was defined on 3DCT using the individual tumor motion vector measured by 4DCT accounting for tumor motion; PTV_4D was defined on all phases of 4DCT images. In addition, a 7 mm-margin for microscopic disease and a 3 mm setup margin were used for above PTVs, respectively. The differences in target position, volume and coverage between 3D and 4D targets were evaluated at the lever of GTV and PTV for tumors in different lobes, respectively.Results: Average differences between the position of the center of IGTVs on direction of x, y, and z coordinates were less than 1 mm, with no statistically significant difference. The IGTV₁₀ size was larger than IGTV_EI+EE (t=2.37,P=0.028); the IGTV₁₀ size was larger than IGTV_MIP, but the difference was not significant (t=1.95,P=0.065). The ratios of IGTV_EI+EE to IGTV₁₀, IGTV_MIP to IGTV₁₀ were 0.85±0.08 and 0.92±0.11, respectively. DI of IGTV_EN+EE in IGTV₁₀, IGTV_MIP in IGTV₁₀ were 84.78%±8.95% and 88.47%±9.04%. MI between IGTV₁₀ and IGTV_EI+EE, IGTV₁₀ and IGTV_MIP were 0.85±0.09, 0.86±0.09, respectively.Tumors located in the upper lobe (group A) and in the middle-lower lobe (group B) IGTV₁₀ size was larger than GTV_conv(P = 0.001 and 0.002). The size ratio of IGTV₁₀ to GTV_conv was 1.61±0.56,showed a significant correlation to the tumor vecto(rP = 0.052). DI of IGTV₁₀ in GTV_conv and IGTV_conv in GTV₁₀ was 58.32% and 87.06%,DI of IGTV₁₀ in GTV_conv showed a significant correlation to tumor motion vector (r = -0.47, P = 0.007). The median motion vector for tumors located in the upper lobe (group A) and in the middle-lower lobe (group B) was 2.8mm and 7mm respectively. The mean centriod shifts between PTV_vector and PTV_4D in the LR, AP and CC directions for group A and B were close to 0. The median size ratio of PTV_4D to PTV_vector was 0.75 and 0.52 for group A and B. The motion vector showed a significant correlation to the ratio of PTV_4D to PTV_vector for group A and B (p = 0.008 and 0.003). The median DI of PTV_vector in PTV_4D was 69.19% for group A and 51.60% for group B. The median DI of PTV_4D in PTV_vector was 98.99% for group A and 99.94% for group B.Conclusions: The center displacement of the IGTVs delineated separately by the three different techniques based on 4DCT images are not obvious; IGTV_EI+EE and IGTV_MIP can not replace IGTV₁₀, however, IGTV_MIP is more close to IGTV₁₀ comparing to IGTV_EI+EE. The ratio of IGTV_EI+EE to IGTV₁₀ is correlated to the tumor motion vector. As the vector increases, the ratio of IGTV_EI+EE with IGTV₁₀ decreases, especially for small tumors.The IGTV₁₀ size is significant larger than GTV_conv, the size ratio of IGTV₁₀ to GTV_conv tends to increasing as the tumor vector increasing. The coverage of IGTV₁₀ by GTV_conv is poor. It is necessary to expand the internal margin isotropically in a single direction for 3DCT treatment planning due to the uncertainty of the 3DCT-based target position. The 3DCT-based PTV using individual margins provides a good coverage of the 4DCT-based PTV, meanwhile encompasses relatively large normal tissues, especially for middle and lower lobe tumors. We should be cautious about the use of the individual PTV in treatment planning...