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Evaluation of a Two-Dimensional Ionization Chamber Array for Proton Beam Dosimetry

B Poppe

T.S. Stelljes1,2, M. van Goethem3, S. Brandenburg4, K. Willborn1, B. Poppe1,2*, (1) Pius-Hospital, Oldenburg, Germany (2) Carl von Ossietzky University, Oldenburg, Germany (5) Pius Hospital, Oldenburg,(3) University of Groningen, Groningen, Netherlands(4) , Kernfysisch Versneller Instituut, Groningen, Netherlands

SU-E-T-60 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: The application of the two-dimensional ionization chamber array Octavius 729xdr (PTW-Freiburg, Germany) in proton beam dosimetry, consisting of 729 vented ionization chambers arranged in a 27 x 27 cm² matrix, equivalent to the 2D-Array 729 used for photon beams¹, is investigated.

Methods: Measurements were carried out with a double scattered 150 MeV proton beam at the AGOR cyclotron at the KVI (Rijksuniversiteit, Groningen - Netherlands). A water bellow with adjustable depth ranging from 10 to 300 mm was placed in the beam line directly behind the collimator for built-up. The stability, linearity and output factors of the array for circular fields produced using collimators with sizes of 3, 5, 7 and 10 cm in diameter were investigated. PDD profiles with a Farmer chamber (type 30010, PTW-Freiburg, Germany) and the array were measured by stepwise increasing the water bellow's depth.

Results: The variation in measurement signal of the 729xdr array was less than 0.2 % after preirradiation with 2 Gy. The linearity of the array was found to be excellent. Output factors showed the expected small variations for circular field sizes. PDDs measured with the array and a Farmer chamber showed good agreement in the region of the Bragg peak with the array slightly underestimating the dose in the built-up region.

Conclusion: The first measurements with the Octavius 729xdr array show the potential of a two-dimensional ionization chamber array in proton beam dosimetry. The dosimetric properties and the applications of the array in clinical dosimetry will be investigated in more detail in future studies.

¹B. Poppe, A. Blechschmidt, A. Djouguela, "Two-dimensional ionization chamber arrays for IMRT plan verification," Med. Phys. 33, 1005-1015 (2008)

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