4D-CT-based plan target volume (PTV) definition compared with conventional PTV definition using general margin in radiotherapy for lung cancer

Abstract

To investigate the dosimetric benefit of 4D-CT in the planning target volume (PTV) definition process compared with conventional PTV definition using general margin in radiotherapy of lung cancer. A set of 4D-CT images and multiphase helical CT scans were obtained in 10 patients with lung cancer. The radiotherapeutic plans based on PTV determined by 4D-CT and in addition of general margin were performed, respectively. The 3D motion of the centroid of GTV and the 3D spatial motion vectors were calculated. The differences of the two kinds of PTVs, mean lung dose (MLD), V5,V10,V15,V20 of total lung, mean heart dose (MHD), V30 and V40 of heart, D99 and D95 were compared, and the correlation between them and the 3D spatial motion vector was analyzed. The PTV4D in eight patients were smaller than PTVconv, with a mean reduction of (13.0 ± 8.0)% (P = 0.018). In other two patients, whose respiration motion was great, PTV4D was larger than PTVconv. The mean 3D spatial motion vector of GTV centroid was (0.78 ± 0.72)cm. By using 4D-CT, the mean reduction of MLD was (8.6 ± 9.9)% (P = 0.037). V5, V10, V15, V20 of total lung were decreased averagely by (7.2 ± 10.5)%, (5.5 ± 8.9)%, (6.5 ± 8.4)% and (5.7 ± 7.4)%, respectively (P < 0.05 for all). There was a significant positive correlation between PTV4D/PTVconv and the 3D spatial motion vector of the GTV centroid (P = 0.008). A significant inverse correlation was found between D994D/D99conv and the 3D spatial motion vector of the GTV centroid (P = 0.002). D994D/D99conv, (MLDconv-MLD4D) /MLDconv, total lung (V5conv-V54D)/V5conv, total lung (V10conv-V104D)/V10conv, (MHDconv-MHD4D)/MHDconv, heart (V30conv-V304D)/V30conv were inversely correlated with PTV4D/PTVconv (P < 0.05 for all). 4D-CT can be used to evaluate the respiration motion of lung tumor accurately. The 4D-CT-based PTV definition and radiotherapeutic planing can reduce the volume of PTV in patients with small respiration motion, increase the intra-target dose, and decrease the dose of normal tissue sequentially. For patients with large respiration motion, especially those more than 1.5-2 cm, this method can avoid target miss, meanwhile, not increase the dose of normal tissue significantly.