A feasibility study of dynamic verification for tumor target delineation and dose delivery using a six degrees of freedom motion phantom

Background: The dynamic phantom is one of the best tools to study the impact of motion on tumor target delineation and absorbed dose verification during dose delivery. Materials and Methods: this study, a 6-DOF (degrees of freedom) phantom was designed following the stacked serial kinematics and assembled by six commercial motion stages to generate 6-DOF motion, which were RotX (pitch, around X), RotY (roll, around Y), TransZ (anterior–posterior), RotZ (yaw, around Z), TransY (superior–inferior) and TransX (left–right). Tumor targets were designed by six plastic spheres for the delineation test. Also, an ionization chamber array detector and RW3 solid water were combined to measure the absorbed dose for dose verification tests.  Results: The maximum translation speeds for LineX and LineY were 50mm/s and 35mm/s for LineZ, while the maximum rotation speeds for RotX, RotY, RotZ were 5.33° per second, 6° per second and 15° per second respectively. Spiral-CT and 4D-CT images acquired in the static and dynamic states successfully showed the influences of tumor motion on target delineation. In the absorbed dose verification, all cases did not pass the gamma test; the pass rate for the 6-DOF motion case was only 34.2% and the pass rates of all other cases were less than 90%. Conclusion: The phantom designed in this study is able to simulate complex tumor motion and can be used to study the influence of tumor motion in radiotherapy.