Comparison of internal target volumes defined on 3-dimensional, 4-dimensonal, and cone-beam CT images of non-small-cell lung cancer

PURPOSE: The purpose of this study was to compare the positional and volumetric differences of internal target volumes defined on three-dimensional computed tomography (3DCT), four-dimensional CT (4DCT), and cone-beam CT (CBCT) images of non-small-cell lung cancer (NSCLC). MATERIALS AND METHODS: Thirty-one patients with NSCLC sequentially underwent 3DCT and 4DCT simulation scans of the thorax during free breathing. The first CBCT was performed and registered to the planning CT using the bony anatomy registration during radiotherapy. The gross tumor volumes were contoured on the basis of 3DCT, maximum intensity projection (MIP) of 4DCT, and CBCT. CTV(3D) (clinical target volume), internal target volumes, ITV(MIP) and ITV(CBCT), were defined with a 7 mm margin accounting for microscopic disease. ITV(10 mm) and ITV(5 mm) were defined on the basis of CTV(3D): ITV(10 mm) with a 5 mm margin in left–right (LR), anterior–posterior (AP) directions and 10 mm in cranial–caudal (CC) direction; ITV(5 mm) with an isotropic internal margin (IM) of 5 mm. The differences in the position, size, Dice’s similarity coefficient (DSC) and inclusion relation of different volumes were evaluated. RESULTS: The median size ratios of ITV(10 mm), ITV(5 mm), and ITV(MIP) to ITV(CBCT) were 2.33, 1.88, and 1.03, respectively, for tumors in the upper lobe and 2.13, 1.76, and 1.1, respectively, for tumors in the middle-lower lobe. The median DSCs of ITV(10 mm), ITV(5 mm), ITV(MIP), and ITV(CBCT) were 0.6, 0.66, and 0.83 for all patients. The median percentages of ITV(CBCT) not included in ITV(10 mm), ITV(5 mm), and ITV(MIP) were 0.1%, 1.63%, and 15.21%, respectively, while the median percentages of ITV(10 mm), ITV(5 mm), and ITV(MIP) not included in ITV(CBCT) were 57.08%, 48.89%, and 20.04%, respectively. CONCLUSION: The use of the individual ITV derived from 4DCT merely based on bony registration in radiotherapy may result in a target miss. The ITVs derived from 3DCT with isotropic margins have a good coverage of the ITV from CBCT, but the use of those would result in a high proportion of normal tissue being irradiated unnecessarily.