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Kompeito-Shot: Development of a Novel Verification System for 3D/4D Beam Alignment

M Tsuneda

M Tsuneda*, T Nishio , A Saito , S Tanaka , T Suzuki , D Kawahara , K Hioki , Y Ochi , T Okumura , S Ozawa , K Karasawa , Y Nagata


MO-DE-FS1-6 (Monday, July 31, 2017) 1:45 PM - 3:45 PM Room: Four Seasons 1

Purpose: High accuracy of beam axis is required for high-precision radiation therapy. However, the gantry head sags under the weight of MLC and jaws. Furthermore, recent machine such as Cyber Knife swing the irradiation port for pursuing irradiation and this technique depends on temporal axis. The users have some following problems: (a) unable to quantitatively and directly evaluate the sagging effect of the gantry head; (b) no quantitative verification method for non-isocentric irradiation; and (c) unable to evaluate 3D isocentericity including the temporal axis. To resolve these problems, we developed a novel verification system “Kompeito-shot”. In this study, we examined the concept of this system and verify the radiation isocenter in 3D coordinate.

Methods: This system was composed of a column-typed plastic scintillator, a truncated cone-shaped mirror and a CCD camera. The system was irradiated with a 6-MV photon beam and the scintillation light was measured using the CCD camera. To examine the accuracy of calculating the gantry rotation axis, the gantry angle (GA) was set from 250 to 290 degrees to mimic the sagging angle (SA). And, to examine the 3D isocentricity, we irradiated photon beam every 30-degree interval from 180e to 180 degrees. We developed the analysis algorithm for the calculation of the beam axis in 3D coordinate.

Results: The relation between irradiated and measured GAs had good linearity and good accuracy. However, the difference between irradiated and measured GAs tended to increase with increasing in irradiated angle. SA was 0.616, -0.564 and -0.776 degrees for the gantry angle of 0, 180 and 180e degrees, respectively. This result included the setup error and manufacture error.

Conclusion: We developed a prototype system for evaluating 3D beam alignment. The concept of this system works for verifying the 3D isocentricity and evaluating the trend of sagging effect.

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