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Use of a 2-Dimensional Ion Chamber Array to Measure Head Leakage of a Varian Truebeam Linear Accelerator

S Taneja1*, J Teruel1, L Hu2, J Xue1, D Barbee1, (1) NYU Langone Medical Center, New York, NY (2) New York Proton Center, New York, NY


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: During linear accelerator acceptance, leakage radiation is measured to evaluate the adequacy of shielding of the treatment head with a radiation limit of 0.2% according to the NCRP and IEC. Conventional measurements involve wrapping the gantry in film to isolate the point of maximum leakage, then using an ion chamber to verify the dose at 1 m. This method is challenging due to film processing limitations and the accuracy of ion chamber positioning during absolute dose measurement. This work evaluates the use of a two-dimensional (2D) planar ion chamber array to characterize leakage radiation in comparison with conventional measurement.

Methods: A Varian Truebeam® with a maximum photon energy of 15 MV was used for all measurements. The machine was wrapped in Portal Pack for Localization (PPL) film (Carestream Health Inc.) to locate areas of increased leakage. A calibration curve was developed and used to determine dose-to-film. Leakage radiation was measured using an ion chamber array (IC Profiler™) manufactured by Sun Nuclear Corporation at three locations. All measurements were normalized to leakage at 100 cm from the target relative to the central axis. A comparison of leakage (%) measured by PPL film and the IC Profiler was performed.

Results: Three locations were investigated, including the top of the gantry, the Varian logo, and the side of the gantry. PPL film and the IC Profiler showed good agreement for each location, with measurements of 0.142% and 0.131%, 0.036% and 0.030%, and 0.014% and 0.019%, respectively.

Conclusion: Current methods for evaluating leakage radiation is challenging due to the cost and availability of film and film processors. This work validated the accuracy of leakage measurements using an IC Profiler against a conventional measurement technique. Future work will aim to develop protocols for a full leakage characterization using only an ion chamber array.


Commissioning, Acceptance Testing, Ionization Chamber


TH- External Beam- Photons: Calibration protocol and primary standards

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