Room: Stars at Night Ballroom 4
Purpose: To systematically study the motion interplay effect of VMAT and IMRT for lung SBRT treatment using a dynamic virtual patient model
Methods: Based on an in-house developed dynamic patient model, motion interplay between change of the beam aperture and a moving lung tumor was simulated for SBRT lung treatments (8Gy/fraction). This model generated mean CT and virtual 4DCT for tumor at the discretized positions along a sinusoidal breathing trajectory (4 seconds), along with their corresponding contours. The initial plan on the mean CT was fed to the model to generate 3D plans for tumor at these discretized positions. The corresponding 3D doses were computed in Pinnacle. All the 3D doses were then deformed and accumulated at the reference phase to obtain 4D accumulated GTV dose. Different tumor sizes (5mm and 2cm) and motion amplitudes (1cm and 3cm) were studied for the tumor in the middle of lung. Sixteen initial plans were designed with combinations of (1) technique: VMAT (two half arcs, 90 control points) or Step-and-shoot IMRT (7 beams, 20 total segments) (2) beam energy: 6FFF or10FFF, (3) iso-center: at or not at the PTV center-of-mass and (4) heterogeneous or homogeneous PTV dose distribution.
Results: All the 4D accumulated GTV minimum doses (9.0Â±0.7Gy) were above the prescription dose except for the plans treating 5mm tumor (1cm motion) requiring homogeneous dose distribution using IMRT with 10FFF beams (-9.06% and -7.87%). VMAT achieved higher minimum GTV dose (9.4Â±0.5Gy) delivered to tumor than IMRT technique (8.7Â±0.6Gy).
Conclusion: Based on a novel model, 4D dosimetry was simulated to investigate motion interplay effect in lung SBRT. Generally, the interplay effect did not result in tumor under-dosing except for the small tumor with moderate motion that had homogeneous PTV dose planned using IMRT and 10FFF beams.