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ESTRO 2020

Session Item

Physics track: Dose measurement and dose calculation
9319
Poster
Physics
00:00 - 00:00
An efficient setup for weekly dose output constancy of a 1.5T MR-Linac
BIN YANG, Hong Kong (SAR) China
PO-1342

Abstract

An efficient setup for weekly dose output constancy of a 1.5T MR-Linac
Authors: Kin Yin Cheung.(Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Hong Kong), Chin Chak Ho.(Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Hong Kong), Yick Wing Ho.(Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Hong Kong), Chenyu Hung.(Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Hong Kong), Wai Wang Lam.(Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Hong Kong), Bin Yang.(Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Hong Kong), Siu Ki Yu.(Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Hong Kong)
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Purpose or Objective

Water phantom has been widely used for output check of MR-Linac. However, dose measurement in a water phantom is time consuming. This aim of this study is to investigate the efficacy and practicality of a simple measurement technique using a solid water phantom for weekly output constancy of MR-Linac.

Material and Methods

7MV flattening filter free beams in a transverse magnetic field were delivered by an Elekta 1.5T MR-Linac. A field size of 10x10 cm2 at the isocenter was used for both weekly and daily output constancy in our center. A tissue equivalent solid phantom (Gammex Solid Water HE) was used which consisted of one 30x30x1 cm3 slab and two 30x30x5 cm3 slabs. The 1cm thick slab for inserting the farmer chamber was sandwiched between two 5 cm thick slabs. Before dose measurement, water was filled in the chamber hole to avoid possible air gap after the chamber was inserted. Measurements were performed with a PTW30013 MR compatible waterproof chamber, which was placed vertically into the phantom, perpendicular to both radiation beams at gantry 90 and 270 degree and the magnetic field. The index bar and sagittal laser at the couch end were used for setup alignment. The benchmark was obtained immediately after dose calibration in a water phantom. After MV images at three gantry angles (0, 90 and 270 degree) were taken to verify the location of chamber, the phantom position was then marked on the treatment couch. Output deviations at gantry 90 and 270 degree were measured, respectively, and the mean represented the weekly benchmark value. 1% dose deviation was set as the tolerance of weekly output constancy. The records of daily output constancy were also presented for analysis. For ease of setup, the daily output constancy was performed with a vertical beam given at gantry 0 degree. The phantom was lying flat on the couch and the chamber was inserted into the phantom in parallel with the magnetic field without filling with water.

Results

Figure 1 shows the data charts of both weekly and daily output constancy. Trends for both tests are very similar while daily check has larger variation, which is mainly due to the air gap. The average difference between results of gantry 90 and 270 degree in weekly output constancy is 0.2%, which accounts for the setup uncertainty, output stability and angular dependence of chamber. The daily and weekly output data obtained during the study period demonstrates that the tolerance of 2% and 1% are reasonable.
Figure 1. Data charts of both weekly and daily output check. Daily output check was performed at gantry 0 degree while weekly output check was measured at both gantry 90 and 270 degree. Three points in purple circles and the purple dash dot lines indicate the dates when we adjusted the machine output. Red dash lines represent the tolerance of weekly and daily output check, respectively.

Conclusion

An efficient measurement setup using a solid water phantom for weekly dose output constancy of a MR-Linac has been successfully developed. The setup has been shown to be stable and suitable for routine quality assurance.