EBT2 film as a depth-dose measurement tool for radiotherapy beams over a wide range of energies and modalities.

ABSTRACT

PURPOSE: One of the fundamental parameters used for dose calculation is percentage depth-dose, generally measured employing ionization chambers. There are situations where use of ion chambers for measuring depth-doses is difficult or problematic. In such cases, radiochromic film might be an alternative. The EBT-2 model GAFCHROMIC™ film was investigated as a potential tool for depth-dose measurement in radiotherapy beams over a broad range of energies and modalities. METHODS: Pieces of the EBT-2 model GAFCHROMIC™ EBT2 film were exposed to x-ray, electron, and proton beams used in radiotherapy. The beams employed for this study included kilovoltage x-rays (75 kVp), (60)Co gamma-rays, megavoltage x-rays (18 MV), electrons (7 and 20 MeV), and pristine Bragg-peak proton beams (126 and 152 MeV). At each beam quality, film response was measured over the dose range of 0.4-8.0 Gy, which corresponds to optical densities ranging from 0.05 to 0.4 measured with a flat-bed document scanner. To assess precision in depth-dose measurements with the EBT-2 model GAFCHROMIC™ film, uncertainty in measured optical density was investigated with respect to variation in film-to-film and scanner-bed uniformity. RESULTS: For most beams, percentage depth-doses measured with the EBT-2 model GAFCHROMIC™ film show an excellent agreement with those measured with ion chambers. Some discrepancies are observed in case of (i) kilovoltage x-rays at larger depths due to beam-hardening, and (ii) proton beams around Bragg-peak due to quenching effects. For these beams, an empirical polynomial correction produces better agreement with ion-chamber data. CONCLUSIONS: The EBT-2 model GAFCHROMIC™ film is an excellent secondary dosimeter for measurement of percentage depth-doses for a broad range of beam qualities and modalities used in radiotherapy. It offers an easy and efficient way to measure beam depth-dose data with a high spatial resolution.