Room: Exhibit Hall
Purpose: Validate that a 2D ionization chamber array (ICA) combined with a double-wedge phantom (DWP) can track changes in electron beam energy well within the 2mm precision recommended by TG-142 for monthly quality assurance (QA).
Methods: Electron beam profiles of 4â€“22 MeV were measured for a 25Ã—25cm2 cone using an ICA with a DWP placed on top of it along one diagonal axis. The relationship between the measured field size created by DWP and the depth of 50% dose (R50) for a given ICA/DWP combination is calibrated beams of know energies (R50 values). Once this relationship is established the ICA/DWP system will report the R50 directly. We calibrated the ICA/DWP on a linear accelerator with energies of 6, 9, 12, 16, 20 and 22MeV. The R50 values of these beams and 8 other beams with different R50s were measured and compared with the R50 measured in water. In addition, for the higher energy (â‰¥12MeV) beams, we also used 0.5- and 1.0-cm solid-water to shift the energy giving us 9 additional R50s. To check the long-term reproducibility of ICA/DWP we measured R50 on a monthly basis for a period of 3-years.
Results: For all 25 R50s studied over a range of 2â€“8.7cm, the R50 reported by the ICA/DWP system agreed with that measured in water within 0.4Â±0.6mm (meanÂ±1Ïƒ) and maximum discrepancy of 1.4mm. If we exclude the beams created with the solid-water range shifters the agreement improves to 0.0Â±0.5mm with maximum discrepancy of 1.0mm. Reproducibility results show the ICA/DWP system to be stable with measured R50s agreeing with their base lines within 0.4mm over 3-years.
Conclusion: Comparison of R50 measured using ICA/DWP and in water scanned and long-term reproducibility of ICA/DWP measured R50 indicates that the ICA/DWP can be used for monitoring of electron beam energy constancy.