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Dose Calculation Method for Dynamic Tumor Tracking Using a Gimbal-Mounted Linac

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S Sugimoto

S Sugimoto1*, S Utsunomiya2 , K Ebe3 , T Inoue1 , C Kurokawa1 , K Usui1 , K Sasai1 , (1) Juntendo University, Bunkyo, Tokyo, Japan, (2) Niigata University, Niigata, Nigata, Japan, (3) Joetsu General Hospital, Joetsu, Niigata, Japan


SU-E-T-465 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Dynamic tumor tracking using the gimbal-mounted linac (Vero4DRT, Mitsubishi Heavy Industries, Ltd., Japan) has been available when respiratory motion is significant. The irradiation accuracy of the dynamic tumor tracking has been reported to be excellent. In addition to the irradiation accuracy, a fast and accurate dose calculation algorithm is needed to validate the dose distribution in the presence of respiratory motion because the multiple phases of it have to be considered. A modification of dose calculation algorithm is necessary for the gimbal-mounted linac due to the degrees of freedom of gimbal swing. The dose calculation algorithm for the gimbal motion was implemented using the linear transformation between coordinate systems.
Methods: The linear transformation matrices between the coordinate systems with and without gimbal swings were constructed using the combination of translation and rotation matrices. The coordinate system where the radiation source is at the origin and the beam axis along the z axis was adopted. The transformation can be divided into the translation from the radiation source to the gimbal rotation center, the two rotations around the center relating to the gimbal swings, and the translation from the gimbal center to the radiation source. After operating the transformation matrix to the phantom or patient image, the dose calculation can be performed as the no gimbal swing. The algorithm was implemented in the treatment planning system, PlanUNC (University of North Carolina, NC). The convolution/superposition algorithm was used. The dose calculations with and without gimbal swings were performed for the 3 x 3 cm² field with the grid size of 5 mm.
Results: The calculation time was about 3 minutes per beam. No significant additional time due to the gimbal swing was observed.
Conclusions: The dose calculation algorithm for the finite gimbal swing was implemented. The calculation time was moderate.

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