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

Abstract

Purpose To 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 methods Two different PTVs were defined: PTV conv 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 PTV 4D 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. Results Volumetric examinations revealed a significant reduction of the mean PTV by 4D-CT from 57.7 to 40.7 cm 3 (31%) ( p < 0.001). A significant inverse correlation was found for the motion vector and the amount of inclusion of PTV 4D in PTV conv ( 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). Conclusions In 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.