Comparison of uniformity of dose response of double layer radiochromic films (MD-55-2) measured at 5 institutions

In recent years, double-layer radiochromic films (MD-55-2) also known as new improved Gafchromic Films (NMD-55), have been used for measuring dose distributions of radiation fields. It is reported that the response of radiochromic film is affected by the scanning densitometry systems used to read the film. To quantify the response of MD-55-2 films, two sheets (12.5 cm × 12.5 cm) from different batches, were irradiated to 900 cGy and 2000 cGy, using uniform flat photon beams. The films were sent to 5 institutions for response evaluation using 5 different densitometry systems with narrow band light source centered at nominal 633 to 665 nm, which is near the major absorption peaks of the film sensitive emulsion. The dose response curve was established for each dens tometer. The one-dimensional film responses were obtained for specified directions. A set of fiducial marks, located at ∼1.5 cm from the film edges, was used for identification of scan direction and alignment. The local fluctuations were assessed by comparing the film response with the mean response and its relative (percentage) standard deviation (RSD) in the region of interest. The regional non-uniformity was measured by examining the difference between the maximum and minimum responses within the region of interest. Our data indicates that the RSD, as obtained by the 5 institutions, varied from 2.4% to 5.8% for the film irradiated at low (∼900 cGy) dose and from 1.2% to 4.3% for the film irradiated at the higher dose (∼2000 cGy). The regional non-uniformity was also improved with increased dose and was less in longitudinal direction of the film that is parallel to the direction of coating application. Data comparison for regional non-uniformity indicates that the film responses were affected not only by the wavelength of analyzing source, but also by other instrumentation factors such as step size and sampling size. High-resolution scanners may also have more noise that should not be attributed to film non-uniformity.