SU‐E‐T‐95: Impact of Chamber Support Device On In‐Air Profile Scans of High‐Energy Electron Beams

Purpose: In‐air profile measurements are required for commissioning the Electron Monte Carlo Algorithm in Eclipse (Varian Medical Systems, Palo Alto CA). The purpose of this study was to investigate the impact of the ion chamber support device on in‐air profile scans of high‐energy electron beams. Methods: An IBA Blue Phantom² with CC13 (IBA Dosimetry, Germany) ion chamber was used to record in‐air one‐dimensional profiles of electron beams from a Varian Clinac 21EX with no applicator and collimator jaws set to maximum opening (40cm). The chamber was set at 95cm from the source using the ODI and lateral movement verified to be level. Due to the large field size, only half profile scans were obtained.A chamber support device was manufactured using rigid foam and attached to the chamber holder included with the tank. This relocated the chamber to approximately 25cm away from the metallic crossbar and plastic chamber holder. Profile scans were collected for all available beam energies (6MeV, 9MeV, 12MeV, 15MeV, 18MeV) both with and without the use of the additional foam support. Results: Measurements with and without the use of the foam support device resulted in minimal change to the recorded plots. For profiles normalized to CAX, the mean absolute difference between corresponding points along the profile was found to be less than 1% for all energies measured. The maximum deviation at any point was less than 1% for all but two energies (9MeV=3.5%, 15MeV=4.5%). The difference at 80% of the field size was less than 1.5% for all energies. Conclusion: The ion chamber support device included with the IBA Blue Phantom2 appears to have minimal impact on in‐air profile measurements for high energy electron beams. Additional air‐like support material should therefore not be necessary for measuring such profiles.