Comparison of the planning target volume based on three-dimensional CT and four-dimensional CT images of non-small-cell lung cancer

Background and purposeTo compare positional and volumetric differences of planning target volumes (PTVs) based on axial three-dimensional CT (3DCT) and four-dimensional CT (4DCT) for the primary tumor of non-small cell lung cancer (NSCLC).Materials and methodsTwenty-eight patients with NSCLC underwent 3DCT and 4DCT scans of the thorax during normal free breathing. PTVvector was defined on 3DCT using the individual tumor motion vector measured by 4DCT accounting for tumor motion; PTV4D 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 PTVvector and PTV4D were evaluated for tumors in different lobes, respectively.ResultsThe median motion vector for tumors located in the upper lobe (group A) and in the middle lower lobe (group B) was 2.8 and 7 mm, respectively. The mean centroid shifts between PTVvector and PTV4D in the LR, AP and CC directions for group A and B were close to zero. The median size ratio of PTV4D to PTVvector was 0.75 and 0.52 for group A and B. The motion vector showed a significant correlation to the ratio of PTV4D to PTVvector for group A and B (p = 0.008 and 0.003). The median DI of PTVvector in PTV4D was 69.19% for group A and 51.60% for group B. The median DI of PTV4D in PTVvector was 98.99% for group A and 99.94% for group B.ConclusionIt 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 derived from 3DCT in treatment planning.