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Ion Recombination for Ionization Chamber Dosimetry in a Pencil Beam Scanning Proton Therapy Beam

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J Sorriaux

J Sorriaux1*, P Takoukam2, D Bertrand3, C Baumer4, J Lee5, S Vynckier6, E Sterpin7, (1) Universite catholique de Louvain, Brussels, ,(2) R&D IBA Dosimetry GmbH, 90592 Schwarzenbruck, ,(3) R&D IBA, Louvain-la-neuve, ,(4) West German Proton Therapy Center Essen, Essen, ,(5) Universite Catholique de Louvain, Brussels, ,(6) Cliniques Univ St. Luc, Hoeilaart, ,(7) ,Brussels,

WE-E-141-5 Wednesday 2:00PM - 3:50PM Room: 141

Purpose: IAEA TRS-398 provides recipes and formulas to compute ion recombination correction factors for continuous and pulsed broad proton beams. However, those formulas may not be optimal for pencil beam scanning modalities (PBS). This work aims at evaluating appropriately ion recombination correction for different ionization chamber types for PBS delivery.

Methods: Ion chamber measurements were performed in a water phantom (BluePhantom², IBA Dosimetry GmbH) irradiated by a 10x10 cm² uniform field (2.5mm spot spacing, IBA PBS dedicated nozzle) for Extradin T1, FC65-G, CC01 and PPC05 at different beam energies, beam current and polarizing voltages. The Boutillon formalism was used in order to separate the contributions from initial and general recombination. The recombination correction factor was computed using the two-voltage method for continuous, pulsed, and pulsed-scanned beams as well. Chamber-dependent conditions such as depth and relative position to spot mapping were also evaluated.

Results: The formulas for continuous beams in TRS-398 lead to an underestimation of the correction factors (ks) of 0.4% compared to Boutillon analysis for EXTRADIN T1. An overestimation of 0.2% is observed considering the beam as pulsed. For PPC05 using the two-voltage methods, (ks) difference of 0.3% is found compared to Boutillon's value. For FC65-G using the two-voltage correction (ks) is 0.4% underestimated in continuous beam and can be 3% overestimated using pulsed beam formula. (ks) values is computed for various depth positions, energies and beam currents. Plotting inverse ionizing charge versus inverse squared voltage shows that initial recombination is not negligible for Extradin T1, FC65-G and PPC05 at low residual range.

Conclusion: We have determined recombination correction factors for 4 ion chambers using various practical and yet accurate methods in the specific case of PBS delivery. Significant differences in recombination correction factors can appear if recipes from IAEA TRS-398 are applied blindly for proton PBS delivery.

Funding Support, Disclosures, and Conflict of Interest: Jefferson Sorriaux is financed by the Walloon Region under the project name InVivoIGT, convention number 1017266. Jefferson Sorriaux is sponsored by a public-private partnership IBA - Walloon Region

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