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

Abstract

Objective To compare positional and volumetric differences between internal target volumes defined on three-dimensional CT (3D-CT),four-dimensional CT (4D-CT) and cone-beam CT (CBCT) images of non-small-cell lung cancer.Methods Thirty-one patients with NSCLC sequentially underwent 3D-CT and 4D-CT simulation scans of the thorax during free breathing.A 3D conformal treatment plan was created based on 3D-CT.The CBCT images were obtained in the first fraction and registered to the planning CT using the bony anatomy registration.All target volumes were contoured with the same protocol by a radiation oncologist.GTVs were contoured based on 3D-CT,maximum intensity projection (MIP) of 4D-CT and CBCT.CTV3D,ITVMIPand ITVCBCTWere defined with a margin of 7 mm accounting for microscopic disease.ITV10mm and ITV5 mm were defined based on CTV3D.ITV10 mm with a margin of 5 mm in LR,AP directions and 10 mm in CC direction,while ITV5 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 compared.Results The median size ratio of ITV10 mm,ITV5mm,ITVMIPto ITVCBCTwere 2.33,1.88,1.03 respectively for tumors in the upper lobe and 2.13,1.76,1.10 respectively for tumors in the middle-lower lobe.The median DSC of ITVMIP and ITVCBCT(0.83) was greater than that of ITV10 mm and ITVcBcT (0.6) and ITV5 mm and ITVCBCT (0.66) for all patients (Z =-4.86,-4.86,P ＜ 0.05).The median percentages of ITVCBCT not included in ITV10 mm,ITV5 mm,ITVMIPwere 0.10％,1.63％ and 15.21％ respectively,while the median percentage of ITV10mm,ITV5mm,ITVMIP,not included in ITVCBCT were 57.08％,48.89％ and 20.04％,respectively.The median percentage of ITVCBCT not included in ITV5 mm was 1.24％ for tumors in the upper lobe and 5.8％ for tumors in the middle-lower lobe.Conclusions The individual ITV based on 4D-CT can't encompass the ITV based on CBCT effectively.The use of the ITV derived from 4DCT in radiotherapy may result in a target miss.The ITVs based on 3D-CT with istropic margins can encompass the ITV from CBCT,but the size might be far greater than the latter. Key words: Non-small-cell lung cancer; Three-dimensional CT; Four-dimensional CT; Cone-beam CT; Internal target volume