Predicting optical densitometer response as a function of light source characteristics for radiochromic film dosimetry.

Various forms of GAFChromic (GC) film have been used for several years as radiographic media for measuring dose distributions of brachytherapy sources and small radiation fields. In order to optimize the measurement sensitivity and thus improve precision, we describe a method to calculate the dose response curves (net optical density at a give wavelength or spectrum versus absorbed dose) for different densitometer light sources using measured GC film absorption spectra. Comparison with measurements on the latest version of GC film (model MD-55-2) using four types of densitometers [He-Ne laser, broadband (white light) densitometer, and two LED (red-light) filtered densitometers] confirm the accuracy of this predictive model. The linearity and sensitivity of the dose response curves are found to be highly dependent on the light source spectrum. Initial slope is a function of the average weighted absorbance. Early saturation and decreased linearity of the dose response curves are ascribed to the nonuniform transmission of the light source through the GC film. We found that an LED (red-light) source with a narrow bandpass filter centered at 671 nm near the major absorption peak achieves nearly the maximum possible sensitivity (almost four times more sensitive than He-Ne laser, 632.8 nm) and may be suitable for in vivo dosimetry.