Evaluation of the robustness of 3-dimensional conformal technique with MLC position control into the planning target volume in stereotactic body radiotherapy for lung cancer

The purpose of this study was to evaluate the robustness of 3-dimensional conformal technique with MLC position control into the planning target volume (PTV) in stereotactic body radiotherapy for lung cancer. Two techniques using fixed beams were compared; one technique involved setting the MLC position outside the PTV and was referred to as Plan “O.” Another technique involved setting the MLC position inside the PTV and was referred to as Plan “I.” Two tumor motions were simulated: (1) tumor motion on the internal target volume (ITV) boundary and (2) tumor motion on the PTV boundary. Ten-phase CT images that captured the tumor in respiratory motion were generated for 2 simulations. Then, 4-dimensional (4D) treatment planning was performed by using deformable image registration. The gross tumor volume (GTV) dose changes between the 4D accumulated dose and treatment planning dose were evaluated for Plan “O” and Plan “I,” respectively. For the simulation of tumor motion on the ITV boundary, the changes in GTV D50% were −0.10 ± 0.31% and −0.22 ± 0.26% (p < 0.05) for Plan “O” and Plan “I,” respectively. In the same manner, for the simulation of tumor motion on the PTV boundary, the changes in GTV D50% were −3.37 ± 2.16% and −3.68 ± 1.71% (p < 0.05). Our result suggested that the dose change would be negligible in a clinical situation where the tumor moves within the ITV margin for both techniques, while Plan “O” showed better robustness.