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Study On the Effective Depth of Measurement for Gafchromic EBT2 and EBT3 Films

N Corradini

N Corradini1*, S Presilla2, E Sterpin3, (1) Clinica Luganese, Lugano, Switzerland,(2) Ente Ospedaliero Cantonale, Bellinzona, Switzerland,(3) Center of molecular imaging, Radiotherapy and Oncology, Universite catholique de Louvain, Brussels, Belgium

SU-E-T-79 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To determine the effective depth of measurement for Gafchromic EBT2 and EBT3 films

Methods: Absolute dose point measurements were made with EBT2 and EBT3 film on the surface of flat phantom and at various depths. The films were cut to an approx. 1-2 mm square size and placed on the beam central axis. Measurements with an Exradin A1SL ion chamber were performed for each irradiation. A dose of 1 Gy was delivered to Dmax=1.5 cm. A previously validated Monte Carlo (MC) model was used for simulation and comparison to measurements. The MC simulation data was collected at 0.1 mm steps along the beam central axis. Film measurements were converted to dose using a post-irradiation signal consisting of the red and blue color channels from an Epson Expression 10000XL scanner. Film analysis was performed using ImageJ software and MATLAB. Measurements were converted to PDD and then compared to MC data to determine an effective depth of film measurement.

Results: EBT2 measurements showed a systematic upstream shift of 0.10 mm with respect to the MC data while EBT3 showed the shift to be slightly less at 0.07 mm. The error in film measurement was estimated to be 0.04 mm (2σ). All measurements were within one STD of the MC error of 0.33 mm.

Conclusion: The PDD measurements of EBT2 and EBT3 were consistent amongst themselves and agreement with MC was excellent. The observed shift in the results and the measurement error are smaller than the error in MC data, a possible indication that the nominal depth of measurement can be considered as the effective depth of measurement. A study with more accurate geometry for comparison is needed to determine a conclusive depth of measurement.

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