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Technical Note: Design and characterization of a large diameter parallel plate ionization chamber for accurate integral depth dose measurements with proton beams.

PURPOSE
The goal was to develop and test a large diameter parallel plate ionization chamber capable of intercepting at least 98% of the proton beamlets tested with the system.


METHODS
A commercial synchrotron proton therapy system was used for the study (Hitachi, Ltd, Hitachi City, Japan; Model: Probeat-V). The energies investigated were in the range of 100 to 192 MeV. 3 beam spot options available from the system were used. A PTW Bragg peak IC of diameter 84 mm (BP84) (Model PTW34070) was employed for comparison in a scanning water phantom. A prototype of 150 mm diameter was produced (PTW, Freiburg, Germany; model: T34089) and used for the testing. Monte Carlo calculations were also performed with FLUKA to guide the BP150 design and for comparison to the radiological measurements. For comparison, a 40 cm diameter ideal virtual detector was included in the Monte Carlo model.


RESULTS
The measured proton range R90 agrees between the BP84 and BP150 ionization chambers within +0.06/-0.27 mm across the energies 100 - 192 MeV, which is less than the daily experimental setup uncertainty of 0.4 mm. The differences in the absolute integral depth dose curves (IDDs) between the BP84 and BP150 ranged from 0.3% to 1.0% for the spot sizes and beam energies tested. As predicted by the Monte Carlo modeling, the greatest differences were found in the plateau region of the IDDs. Also, the IDDs measured with the BP150 were very similar to those of the ideal 40 cm diameter detector Monte Carlo simulations.


CONCLUSIONS
We conclude that the BP150 offers a small, but a useful reduction in uncertainty from the nuclear halo effect for the system under test.