Validation of a precision radiochromic film dosimetry system for quantitative two-dimensional imaging of acute exposure dose distributions

We present an evaluation of the precision and accuracy of image-based radio chromic film (RCF) dosimetry performed using a commercial RCF product (Gafc hromic MD-55-2, Nuclear Associates, Inc.) and a commercial high-spatial res olution (100 mum pixel size) He-Ne scanning-laser film-digitizer (Personal Densitometer, Molecular Dynamics, Inc.) as an optical density (OD) imaging system. The precision and accuracy of this dosimetry system are evaluated b y performing RCF imaging dosimetry in well characterized conformal external beam and brachytherapy high dose-rate (HDR) radiation fields. Benchmarking of image-based RCF dosimetry is necessary due to many potential errors inh erent to RCF dosimetry including: a temperature-dependent time evolution of RCF dose response; nonuniform response of RCF; and optical-polarization ar tifacts. In addition, laser-densitometer imaging artifacts can produce syst ematic OD measurement errors as large as 35% in the presence of high OD gra dients. We present a RCF exposure and readout protocol that was developed f or the accurate dosimetry of high dose rate (HDR) radiation sources. This p rotocol follows and expands upon the guidelines set forth by the American A ssociation of Physicists in Medicine (AAPM) Task Group 55 report. Particula r attention is focused on the OD imaging system, a scanning-laser film digi tizer, modified to eliminate OD artifacts that were not addressed in the AA PM Task Group 55 report. RCF precision using this technique was evaluated w ith films given uniform 6 MV x-ray doses between 1 and 200 Gy. RCF absolute dose accuracy using this technique was evaluated by comparing RCF measurem ents to small volume ionization chamber measurements for conformal external -beam sources and an experimentally validated Monte Carlo photon-transport simulation code for a Ir-192 brachytherapy source. Pixel-to-pixel standard deviations of uniformly irradiated films were less than 1% for doses betwee n 10 and 150 Gy; between 1% and 5% for lower doses down to 1 Gy and 1% and 1.5% for higher doses up to 200 Gy. Pixel averaging to form 200-800 mum pix els reduces these standard deviations by a factor of 2 to 5. Comparisons of absolute dose show agreement within 1.5%-4% of dose benchmarks, consistent with a highly accurate dosimeter limited by its observed precision and the precision of the dose standards to which it is compared. These results pro vide a comprehensive benchmarking of RCF, enabling its use in the commissio ning of novel HDR therapy sources. (C) 2000 American Association of Physici sts in Medicine. [S0094-2405(00)00510-1].