S2328
Physics - Quality assurance and auditing
ESTRO 2026
energies, the higher the energy of the electrons, the less likely they are to be scattered and absorbed, and the less influence of the observed effect. This is what we see in practice.
Digital Poster 401
Correction of pressure dependence low energy electrons output factor for Machine Performance Check (MPC) for accelerator. Vladimir Stserbakov Radiotherapy, North-Estonia Medical Center Foundation, Tallinn, Estonia Purpose/Objective: Machine Performance Check (MPC) test of the True Beam (Varian) accelerator. To measure the output factor of 6 E electron as a function of atmospheric pressure and explain effect. Material/Methods: We observed simultaneously MPC test and atmospheric pressure during approximately two months. In extreme low or high atmospheric pressure, we observed shift into the “red zone” ( ±2 %).In our case we independently measured output factor by daily check of Sun Nuclear (Daily check 3D), which did not show remarkable changing of electron output factor for every date of measurement period. Results: At the Figure 1 you can see that changing output factor [ Δ Output = (Output - Output Calibration )/ Output Calibration, in %] only for 6 E electron has clear dependence from atmospheric pressure: Δ Pressure [ Δ Pressure=(Pressure - Pressure norm ) in units] and Δ Output values are „opposite in phase”. So, if the pressure changes by ~-4 units, the output of 6 E electrons will change by ~+3%.
Figure 2. Comparing the volumes of air involved in the scattering and absorption of the outgoing radiation field of electrons. A) VMPC – air volume in case of MPC test. B) VDC – air volume in case of Daily check. Conclusion: 1. Calibration of MPC output 6 E electrons test should perform at average atmospheric pressure for location of hospital. 2. It is necessary to correct the MPC readings: MPC reading x (Pat time of measurement/P average for hospital location) Keywords: quality control, output factor, electrons Relationship between treatment plan complexity and quality assurance results in single-lesion HyperArc radiosurgery Albert Bartrés Salido, Melanie Erzilbengoa, Verónica Alba Escorihuela, Pedro Liñán Rodríguez, Carlos Fernandez Leira, Marta Sanchez Casi, Nagore Garcia Apellániz, Sergio Panzuela, Miren Lamaison Bidarte, Maddalen Alonso Etxarri, Francisco José Lozano Flores Medical Physics, Osakidetza, Donostia, Spain Purpose/Objective: Varian TrueBeam accelerators, both in the standard and HD versions, allow for the generation of stereotactic radiosurgery plans using predefined templates within the planning system. This study evaluates two parameters influencing plan complexity: Digital Poster 456 the Aperture Shape Controller (ASC), designed to generate larger MLC openings while avoiding excessively small segments, and the total monitor unit (MU) limitation, which indirectly impacts aperture size.
Figure 1.Changing of MPC output factors Δ Output for electrons and atmospheric pressure Δ Pressure at same dates. The reason for the difference between the MPC output test and the daily check can be understood by comparing the air volumes involved in scattering and absorbing the outgoing radiation field of the lower energy 6 E electrons (Figure 2). In the case of the MPC test, this is ~0.33x29x38x145= 52 731 cm3 (VMPC), while for the daily check it is 0.33x20x20x100=13 200 cm3 (VDC). The volumes ratio is 52 731/13 200 =~ 4.0, i.e. in the MPC test we have ~4 times more air molecules to scatter and absorb the radiation, which explains why the dependence of the output signal on pressure for the daily test (<1%) is ~4 times smaller than for the MPC test. For other electron
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