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Session: Therapy General ePoster Viewing [Return to Session]

Investigating a Correlation Between MLC Positional Errors and IMRT QA Passing Rate

Ahmed E. Yousif1,4, Mahmoud H. Abdelgawad2, Ahmed Eldib2,3*, Somia M. El-Sayed4, M. S. Talaat1, (1) Physics Department, Faculty Of Science, Ain-shams University, Cairo, Egypt(2) Physics Department, Faculty Of Science, Al-Azhar University, Cairo, Egypt(3) Fox Chase Cancer Center, Philadelphia, PA, USA (4) Radiation Oncology Department, Faculty Of Medicine, Ain-shams University,Cairo, Egypt


PO-GePV-T-222 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: Patient-specific quality assurance (QA) has been an essential step prior to treatment with modern radiation therapy techniques such as intensity-modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). It was debated that measurement-based IMRT QA cannot parse the errors from the treatment database, planning system, and linear accelerator. In this work, we studied the use of the trajectory log files in specifying the influence of the linear accelerator multi-leaf collimators (MLC) positional error in the passing rates in our measurement-based QA.

Methods: Sixty-five plans were selected for the current study. Cases included head and neck, lungs, pelvises, craniospinal, and total body irradiation (TBI). IMRT treatment plans were generated using Eclipse Treatment Planning System utilizing Anisotropic Analytical Algorithm dose calculation engine. Radiation is delivered from Varian® linear accelerator UNIQUE which is equipped with 120 Millennium MLC. Trajectory Log files generated during treatment delivery were analyzed using software developed using MATLAB. We determined an MLC matching percentage between the planned positions and the actual delivery positions. Measurement-based QAs for all patients were done and the gamma analysis passing rates were calculated using Verisoft PTW dosimetry system. Then each QA passing rate was compared with the related MLC matching percentage.

Results: The gamma analysis passing rate was in the range of 90% to 99.1% and the matching percentage ranged between 89% to 98.8%. The comparison between the results of both methods showed a p-value <0.05 which indicates a statistically significant correlation. The matching percentage was higher than the value of the corresponding gamma passing percentage in 83% of the cases.

Conclusion: Trajectory log file analysis combined with the measurement-based patient-specific QA can introduce a better understanding of the process and could aid in improving our passing rates in the future.


Quality Assurance, Intensity Modulation, Lacunarity


TH- External Beam- Photons: Quality Assurance - IMRT/VMAT

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