Accuracy of GafChromic EBT Film as Dose Meter in Radiotherapy QA

Abstract

Contemporary radiation therapy dose delivery methods like e.g. IMRT and RapidArc needs high-accurate quality assurance dose detection methods. For this purpose GafChromic EBT film is available. No developing process is required like for radiographic film, while the read out of the EBT films can be performed with cheap flatbed scanners. However, before clinical use the film - flatbed scanner (Epson 1680 Expression Pro as well as Epson 10000XL) combination should be tested on their characteristics and achievable overall dosimetric accuracy. Hereto comparison measurements in water with ionization chamber, diode (array) and EBT film were performed for standard and IMRT radiation fields. We observed that absolute dose measurement with EBT film can be performed with an uncertainty up to 1.8% (1 SD) for a single film and 1.4% (1 SD) applying per measurement 2 films simultaneously, and when precautions are taken. Precautions which should be considered are: 1) inhomogeneity effects in film thickness; 2) strictness in film orientation during calibration and film scanning; 3) variation in optical density read out over the scan window of the scanner which depends furthermore on the dose delivered to the EBT film. The first aspect is related to film production processes. The second aspect is related to the orientation of the polymer molecules in the film. The third aspect is related to light polarization capacity of the EBT film polymer molecules in combination with an inherent creation of polarized light in the scanner itself during transport of the light ray from each point in the light tube to the CCD chip.