Analysis of the advantage of individual PTVs defined on axial 3D CT and 4D CT images for liver cancer

The purpose of this study was to compare positional and volumetric differences of planning target volumes (PTVs) defined on axial three dimensional CT (3D CT) and four dimensional CT (4D CT) for liver cancer. Fourteen patients with liver cancer underwent 3D CT and 4D CT simulation scans during free breathing. The tumor motion was measured by 4D CT. Three internal target volumes (ITVs) were produced based on the clinical target volume from 3DCT ([Formula: see text]): i) A conventional ITV ([Formula: see text]) was produced by adding 10 mm in CC direction and 5 mm in LR and and AP directions to [Formula: see text]; ii) A specific ITV ([Formula: see text]) was created using a specific margin in transaxial direction; iii) [Formula: see text] was produced by adding an isotropic margin derived from the individual tumor motion vector. [Formula: see text] was defined on the fusion of CTVs on all phases of 4D CT. PTVs were generated by adding a 5 mm setup margin to ITVs. The average centroid shifts between PTVs derived from 3DCT and [Formula: see text] in left–right (LR), anterior–posterior (AP), and cranial–caudal (CC) directions were close to zero. Comparing [Formula: see text] to [Formula: see text] , [Formula: see text] , and [Formula: see text] resulted in a decrease in volume size by 33.18% [Formula: see text] , 24.95% [Formula: see text] , 48.08% [Formula: see text] , respectively. The mean degree of inclusions (DI) of [Formula: see text] in [Formula: see text] , and [Formula: see text] in [Formula: see text] , and [Formula: see text] in [Formula: see text] was 0.98, 0.97, and 0.99, which showed no significant correlation to tumor motion vector ([Formula: see text] , 0.259, and 0.244; [Formula: see text] , 0.371, and 0.401). The mean DIs of [Formula: see text] in [Formula: see text] , [Formula: see text] in [Formula: see text] , and [Formula: see text] in [Formula: see text] was 0.66, 0.73, and 0.52. The size of individual PTV from 4D CT is significantly less than that of PTVs from 3DCT. The position of targets derived from axial 3DCT images scatters around the center of 4D targets randomly. Compared to conventional PTV, the use of 3D CT‐based PTVs with individual margins cannot significantly reduce normal tissues being unnecessarily irradiated, but may contribute to reducing the risk of missing targets for tumors with large motion. PACS number: 87