Energy response of the new EBT2 radiochromic film to x-ray radiation

Gafchromic EBT2, Radiochromic film is assessed for its change in optical density response to x-ray radiation over a broad energy range, from low energy kilovoltage to megavoltage x-rays. A small energy dependence was found with variations in the change in optical density when scanned in the red component of a desktop scanner light source per unit dose of 6.5% from 50 kVp to 10 MV. This produces a slightly smaller and thus even more energy independent film than its predecessor, EBT film whose response varied by 7.7% over the same energy range. The energy response peaked at 100 kVp with a 5% over response compared to 6 MV x-rays and the minimum response found at both 50 kVp and 250 kVp being a 1.5% under response. It should be noted that the shape of the energy dependence response curve increases from 50 kVp up to 100 kVp followed by a decrease through to higher energies whilst the original EBT was found to increase in response from 50 kVp through to 10 MV. A reflected net optical density change of 0.215 ± 0.006 OD for the first Gray of radiation was found for EBT2 analysed in reflection mode at 6 MV x-ray energy. The minimal energy dependence of the EBT2 film provides further enhancement compared to EBT for its accuracy with respect to spectral changes in the beam to measure beams such as IMRT where complex field and multileaf collimator configurations exist causing small spectral changes to occur over the treatment field or at depth where spectral changes also occur.