QUANTITATIVE-EVALUATION OF RADIOCHROMIC FILM RESPONSE FOR 2-DIMENSIONAL DOSIMETRY

Radiochromic film (RCF) is attractive as a thin, high resolution, 2D p lanar dosimeter. We have studied the uniformity, linearity, and reprod ucibility of a commercially supplied RCF system (model MD-55). Forty 1 2 cm long strips of RCF were exposed to uniform doses of 6 MVxrays. Op tical density (OD) distributions were measured by a helium-neon scanni ng laser (633 nm) 2D densitometer and also with a manual densitometer. All film strips showed 8%-15% variations in OD values independent of densitometry technique which are evidently due to nonuniform dispersal of the sensor medium, A double exposure technique was developed to so lve this problem. The film is first exposed to a uniform beam, which d efines a pixel-by-pixel nonuniformity correction matrix. The film is t hen exposed to the unknown dose distribution, rescanned, and the net O D at each pixel corrected for nonuniformity. The double exposure techn ique reduces OD/unit dose variation to a 2%-5% random fluctuation. RCF response was found to deviate significantly from linearity at low dos es (40% change in net OD/Gy from 1 to 30 Gy); a finding not previously reported. To study the tradeoff between statistical noise and spatial resolution, OD was averaged over blocks of adjacent 50 mu m pixels (r anging from 1x1 to 10x10 pixels). Reproducibility, defined as the stan dard deviation of repeated single-pixel measurements on separate film pieces, was 2% at 30 Gy for a resolution of 0.25 mm. With careful corr ection for nonlinearity and nonuniformity, RCF is a promising quantita tive 2D dosimeter for radiation oncology applications. (C) 1997 Americ an Association of Physicists in Medicine.