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The Use of Large-Volume Ion Chambers in Out-Of-Field Locations for Radiotherapy Beam Scanning

W Culberson

W Culberson1*, (1) Univ of Wisc Madison, Madison, WI


SU-K-205-14 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 205

Purpose: To investigate the magnitude of ionization current from different types of large-volume ionization chambers at different locations outside the primary beam path for use as a reference detector in beam scanning and small-field commissioning of a clinical linear accelerator.

Methods: Three commercial and one custom-made ion chambers of different geometries were investigated ranging in active volumes from 0.125 cc to 100 cc and placed at several locations around a Varian 21EX medical linear accelerator head. Measurements were performed in the 6MV mode. The field size was also varied to test the effectiveness for use in small field measurements. Recombination was measured using the standard two-voltage technique. The ionization currents from each large-volume chamber and out-of-field location were recorded and compared with the ionization current from a conventional scanning chamber in the primary beam path.

Results: There were certain locations and chamber combinations around the linac head that yielded ionization currents comparable and even much higher than a conventional reference scanning chamber placed in the primary field. Recombination was less than 2% for all chambers and locations tested.

Conclusion: Locations were identified around the head of a linear accelerator where ionization currents exceeded the signal from a standard scanning chamber in the primary beam path. The location of highest ionization current was on the side of the head near the junction of the two panel covers. By using large-volume chambers in out-of-field locations, it becomes possible to perform beam scanning without having to go into the vault and move the reference chamber for each field size change. In addition to time savings, this enables a reference detector to be used for small-field scanning where it may not otherwise be possible to use one.

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