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Characterization of EBT3 Films in Photon and Proton Beams

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J Sorriaux

J Sorriaux1*, S Rossomme1, A Kacperek2, D Bertrand3, S Vynckier1,4, E Sterpin1, (1) Molecular Imaging Radiotherapy & Oncology, Catholic University of Louvain, Belgium (2) Douglas Cyclotron, Clatterbridge Center for Oncology, Wirral, UK (3) Ion Beam Applications s.a, Louvain-la-Neuve, Belgium (4) Cliniques Universitaires Saint-Luc, Brussels, Belgium

SU-E-T-118 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall

Purpose: To measure the calibration curves of EBT3 dosimetry films in photon and proton beams and to quantify the related uncertainties from one beam type to another.

Methods: EBT3 Gafchromic films have similar properties than EBT2 with a symmetric construction and a matte polyester substrate to prevent Newton's ring artefacts. Films from a same batch were exposed in three different beam qualities, an Elekta SL25 6 MV photon beam, a 100 MeV 5x5cm² proton beam delivered by pencil-beam scanning dedicated system from IBA and a 60 MeV fixed proton beam (2.5cm in diameter) at Clatterbridge Center for Oncology (CCO), UK. The films were read using an EPSON 10000 XL/PRO scanner. Film calibration curves were acquired for all modalities within a range of 0.05 to 20 Gy. Influence of increasing linear-energy transfer (LET) on film response was investigated by comparing dose measured by EBT3 to a silicon diode detector in depth for a fully-modulated beam using the CCO beam line (homogeneous dose with distal end at 3.1cm in water). A comprehensive uncertainty budget (reproducibility, uniformity'¦) was estimated on films irradiated by Elekta SL25.

Results: The main source of uncertainty was the non-uniformity of the scanner response. By placing all the irradiated films at the center of the scanner, the uncertainty could be reduced from 5.8% to 1.9% (1 sigma). For all beams and energies, the calibration curves were matched within uncertainties. Along the fully-modulated depth dose curve, diode and EBT3 measurement were in a 4% agreement point-to-point, indicating films weak dependence with LET.

Conclusions: The weak influence of LET, beam type and energy on film response as well as its small uncertainty make EBT3 suitable for relative dosimetry and a promising candidate for measuring correction factors (quality, recombination,'¦) for reference dosimetry with ion chambers of non-standard beams (e.g pencil-beam scanning proton-therapy).

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