ESTRO 2020 Abstract Book

S710 ESTRO 2020

PO-1342 An efficient setup for weekly dose output constancy of a 1.5T MR-Linac B. Yang 1 , W.W. Lam 1 , Y.W. Ho 1 , C.C. Ho 1 , C. Hung 1 , K.Y. Cheung 1 , S.K. Yu 1 1 Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Hong Kong, Hong Kong SAR China 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 cm 2 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 cm 3 slab and two 30x30x5 cm 3 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.

For 2%-1 mm γ criteria sensitivity for MLC offset error detection was above 80% and 90% if the plan is considered passing with y>90% and >95%, respectively. Errors in density overrides or HU-ED conversion were detected with γ-criteria of 2%-1mm. Control charts (QACC) were created for the QA with SciMoCa to be used together with measurement QACC (Fig. 2) and γ-values of 94.5% and 92% were derived as new warning and failed plan criteria.

Conclusion The study shows that 3D MC plan verification with tight gamma criteria (2%-2mm and below) provides reliable method to discover clinically relevant errors in beam model and plan parameters, besides rigorous second check of the TPS algorithm. Institution-specific control charts are useful tool to define action limits for calculation-based QA and enhance safety in clinical process.

Conclusion