ESTRO 2020 Abstract book

S759 ESTRO 2020

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.

without the magnetic field. Calculated surface dose by Monaco is systematically higher than our measurements using chamber but shows a similar trend as field size varies. PDDs measured with Advanced Markus chamber in solid water phantom agree well with data collected by the microDiamond detector in water. Especially for the data with the magnetic field, the deviation at most depth is less than 0.5% except the region from depth 25 mm to 40 mm, which is probably due to the air gap between phantom slabs that caused the decrease of chamber reading. The surface dose measured by the film was averaged based on three color channels and is very close to the result of chamber with less than 1% deviation on average, which is reasonable after taking the thickness of entrance foil of Advanced Markus chamber and polyester base of EBT3 film into account.

Conclusion Compared with measurements before the magnetic field was on, the surface dose with magnetic field for field size larger than 8x8 cm 2 is much smaller because of the purging of the majority of lepton contamination. For small field size (less than 8x8 cm 2 ), the larger surface dose with magnetic field could be due to the skewed average direction of the secondary particles. The change of surface dose with field size with magnetic field is not as drastic as conventional Linac while the surface dose of maximum field size is 16.6% for the 1.5T MR-Linac. PO‐1344 Patient‐specific QA using DVH analysis provides better clinical relevance of complex VMAT plans A. Yoosuf 1 , S. AlShehri 1 , A. AlHadab 1 , M. Alqathami 1 1 King Abdulaziz Medical City, Department of Oncology, Riyadh, Saudi Arabia Purpose or Objective In this work, we report our clinical experience using Dose- volume-histogram (DVH), with ionization based transmission detector and model-based verification system, as a pre-treatment patient-specific quality assurance (QA) tool to determine the clinical relevance of complex volumetric modulated arc therapy (VMAT) plans which is difficult in traditional conventional gamma based analysis. Material and Methods 73 subsequent treatment plans grouped into four clinical sites (Head and Neck, Thorax, Abdomen, and Pelvis) were evaluated. The average dose (D avg ) and dose received by 1% (D 1 ) of the planning target volumes (PTVs) and organs at risks (OARs) calculated (Monte Carlo algorithm) using the treatment planning system (TPS) were compared to a model-based computation (collapsed cone algorithm , COMPASS) and reconstructed dose (utilizing measured fluence, DOLPHIN detector) using DVH analysis. The correlation between traditional gamma analysis (3% 3mm) and DVH based analysis for targets was evaluated. Results Linear regression affirmed good correlation between TPS plans and computed dose using a model-based verification system ( r 2 =1). The average percentage difference between TPS calculated and reconstructed dose for PTVs achieved using DVH analysis are as follows: Head and Neck – 0.57%±2.8 (D avg ) & 2.6%±2.7 (D 1 ), Abdomen – 0.19%±2.8 & 1.64%±2.2, Thorax – 0.24%±2.1 & 3.12%±2.8, Pelvis

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. PO‐1343 Measurement of surface dose in a 1.5 T MR‐ Linac using plane‐parallel ionization chamber B. Yang 1 , W.W. Lam 1 , K.K. Tang 1 , W.K. Law 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 This study is to investigate the surface dose in a 1.5T MR- Linac and compare the measured data with those collected without the magnetic field. Material and Methods 7 MV flattening filter free beams with field size ranging from 2×2 to 57×22 cm 2 at gantry 0 degree (G0) were delivered by an Elekta 1.5T MR-Linac. A plane-parallel ionization chamber (PTW 34045 Advanced Markus) placed in solid water phantom was used for measuring the surface doses, which were normalized to Dmax at depth of 1.3 cm. For evaluating the performance of Advanced Markus chamber in MR environment, percentage depth dose (PDD) for 10x10 cm 2 field was measured and compared with data collected in water using a microDiamond detector. Surface dose was also measured with EBT3 Gafchromic film and solid water phantom. The films were placed on the surface and at depth of dmax, perpendicular to the radiation beam at G0. Water was filled in between the film and phantom to avoid any air gap. Dose calculation on virtual phantom was performed by Monaco 5.40 with a calculation grid of 1mm and exported for analysis. Results As shown in figure 1, the surface dose changes from 12.8% to 16.6% while the field size varies from 2x2 to 57x22 cm 2 , which is much less than the variation of surface dose