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 brachythera py sources and small radiation fields. In order to optimize the measur ement sensitivity and thus improve precision, we describe a method to calculate the dose response curves (net optical density at a give wave length or spectrum versus absorbed dose) for different densitometer li ght sources using measured GC him absorption spectra. Comparison with measurements on the latest version of GC film (model MD-55-2) using fo ur types of densitometers [He-Ne laser, broadband (white light) densit ometer, and two LED (red-light) filtered densitometers] confirm the ac curacy of this predictive model. The linearity and sensitivity of the dose response curves are found to be highly dependent on the light sou rce spectrum. Initial slope is a function of the average weighted abso rbance. 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 n arrow bandpass filter centered at 671 nm near the major absorption pea k 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. (C) 1997 American Association of Physicists in Medicin e.