Optical density dependence on postirradiation temperature and time for MD-55-2 type radiochromic film

Various forms of GAFChromic film (GC) have been used for several years as radiographic media for measuring dose distributions of brachytherapy sources and small radiation fields. A new double layer film, referred to as MD-55-2 is now offered and is useful for clinical dose ranges. In this paper we study the effect of postirradiation storage and the reversible effects of temperature during optical density measurements for this new radiochromic film. The reversible change in optical density measured with a He-Ne laser densitometer (632.8 nm) was found to vary by less than 1% over the range 18-30 degrees C, but with densitometers whose light source falls on the left-hand side (i.e., shorter wavelengths) of the GC film's major absorbance peak the dependence was found to be as high as 2%/degrees C. The rate at which the film darkens irreversibly (postirradiation) was found to depend on the storage temperature. In typical clinical conditions (20-25 degrees C), the initial rate at which the him darkens can vary by as much as a factor of 1.5. Furthermore, the storage temperature was found to affect the asymptotic value for the optical absorbance. For storage temperatures of 30-35 degrees C, the limiting values of the absorbance were approximately 4% higher than samples stored at lower temperatures, which had been irradiated to the same dose. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)00103-0].