The WED of a batch was determined e

The WED of a batch was determined experimentally due to the complex chemical composition of the film. Since the films are very thin, determination of the water equivalent thickness was performed experimentally by measuring a stack of 75 films. Regarding film analysis of the measured dose the multichannel dosimetry with red, blue, and green channels was performed.31,32 In general the signal in the red channel is more sensitive at lower dose. From the blue channel theinformation of the uniformity was obtained by the built-in marker dye in the film.33 For calibration, the radiochromic films were cut into 5 9 5 cm2 pieces and placed in beam direction between RW3 plates at the reference depth of 3 cm to avoid the build-up effect using a 150 MeV proton beam.29 Absorbed doses in the range of 0.5 Gy–5 Gy in 0.5 Gy steps were applied. A fit of a rational function of the form X (D) = A + B/(D _ C) with the fit parameters A, B, C was applied [D = dose (Gy), X(D) = color channel (%)]. The used software was FilmQA ProTM 2014 (Ashland, Convington, USA) together with a flatbed document scanner Epson Expression 10000XL (Epson, Suwa, Japan). We followed the recommendations on handling or avoiding uncertainties in GafchromicTM films from other publications using protons. 34,35