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Constancy Check of Longitudinal Beam Profile for Helical Tomotherapy Using ArcCHECK

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B YANG*, W Wong , H GENG , W Lam , K Cheung , S Yu , Hong Kong Sanatorium & Hospital, 2 Village Road, Happy Valley, Hong Kong


SU-I-GPD-T-449 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To develop an alternative method to monitor the consistency of the longitudinal beam profiles for a tomotherapy machine using ArcCHECK.

Methods: AAPM TG148 recommends monthly check of the consistency of the longitudinal beam profiles for all commissioned slice widths. Three topographic profile procedures with static gantry angle at 0° and constant couch speed of 1mm/s were used in this study for verifying 10mm, 25mm and 50mm field width respectively. The ArcCheck was isocentrically set up and adjusted for couch sag. Movie files which took a snapshot exposure every 50ms were recorded. During one delivery, total 22 diodes in the central region under irradiation were selected to measure the longitudinal profiles and a mean profile was calculated to present the final result. For comparison, the standard measurements suggested by AAPM and the manufacturer for this constancy check using ionization chamber and solid water phantom were also performed except the position of chamber was adjusted to at the same level of the diode in ArcCHECK. Benchmarks for both ArcCHECK and ionization chamber based methods were taken after annual QA. Several measurements were performed on different day and gamma analysis was applied with criteria of DTA: 1% of FWHM and 2% dose difference.

Results: ArcCHECK measurements had smaller penumbra than ion chamber due to the smaller size of diode. The ArcCHECK measurements were more robust to the setup uncertainty and output fluctuation. For 10mm field width, the maximum and mean gamma index from ion chamber measurements, which were 0.48 and 0.17 respectively, were almost twice larger than those from ArcCHECK data. Measurements on different days had similar results which were consistent with the ionization chamber based method.

Conclusion: Our proposed method based on ArcCHECK has been proved to be reliable and convenient in the constancy check of longitudinal beam profile for tomotherapy.

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