4D-CT-based target volume definition in stereotactic radiotherapy of lung tumours: Comparison with a conventional technique using individual margins

PurposeTo investigate the dosimetric benefit of integration of 4D-CT in the planning target volume (PTV) definition process compared to conventional PTV definition using individual margins in stereotactic body radiotherapy (SBRT) of lung tumours.Material and methodsTwo different PTVs were defined: PTVconv consisting of the helical-CT-based clinical target volume (CTV) enlarged isotropically for each spatial direction by the individually measured amount of motion in the 4D-CT, and PTV4D encompassing the CTVs defined in the 4D-CT phases displaying the extremes of the tumour position. Tumour motion as well as volumetric and dosimetric differences and relations of both PTVs were evaluated.ResultsVolumetric examinations revealed a significant reduction of the mean PTV by 4D-CT from 57.7 to 40.7 cm3 (31%) (p < 0.001). A significant inverse correlation was found for the motion vector and the amount of inclusion of PTV4D in PTVconv (r = −0.69, 90% confidence limits: −0.87 and −0.34, p = 0.007). Mean lung dose (MLD) was decreased significantly by 17% (p < 0.001).ConclusionsIn SBRT of lung tumours the mere use of individual margins for target volume definition cannot compensate for the additional effects that the implementation of 4D-CT phases can offer.