Analysis of uncertainties in Gafchromic EBT film dosimetry of photon beams.

Radiotherapy today employs complex techniques in order to achieve the best possible conformity of the delivered dose to the target volume. When using dynamic dose delivery techniques, it is especially important to verify the delivered dose at as many representative points as possible. As a continuous medium, Gafchromic EBT film offers an excellent spatial resolution which, together with improvements in sensitivity as compared to older types of radiochromic films, make it a promising candidate for this purpose. In this paper we investigated whether EBT films can be used for quantitative dosimetry in photon beams. There are many publications which discuss different aspects of the EBT film dosimetry. Unfortunately, they differ in the used protocols, scanning devices and variables used for the film darkening quantification which makes the sources of uncertainties difficult to compare. Therefore, the overall accuracy and reproducibility of the results which can be reached with Gafchromic EBT films in combination with a commercial flatbed scanner was investigated. Both the film properties and the influence of the readout system were analysed and compared. The total uncertainty in the net optical density determination due to the studied effects was estimated to be 1.6% at 0.3 Gy and 0.8% at 1 Gy for 60Co photons. Based on this analysis of uncertainties, the handling and scanning protocol was optimized and methods to reduce the influence of some of the uncertainties were proposed. Although Gafchromic EBT films have significant advantages, there are certain effects which have to be considered in order to achieve 5% accuracy in the dose delivered to a patient.