SU-E-T-97: A Methodology for Using Gafchromic EBT2-Films for Accurate Relative 2D-Dosimetry Without the Need of An Accurate Calibration Curve

Abstract

Purpose: Calibration curve (CC) for EBT2 film dosimeters is mainly dependend on film batch, film scanning and analysis conditions. CC errors are translated to 2D dosimetry errors. Aiim of study is to present a methodology for using EBT2‐films for accurate 2D relative dose measurements without need of an accurate CC. Methods: A batch of EBT2 films has been used for calibration irradiations using doses up to 2500 cGy. An arbitrary parameter (AP) (inverse pixel intensity using ImageJ software) was used as the dependent variable in a film dose response relationship. Linear dose‐response region was evaluated. Within this dose region, the relative changes of dose (D%) equals the relative changes of the APnet = [(AP‐APbackground)%]. A film from same batch was irradiated using a Stereotactic Radiosurgery treatment plan (Maximum dose lied within the linear film dose range). The film derived relative dose map was compared against corresponding TPS calculations using Gamma Index (2%, 2mm)Results: Linear Dose AP response was observed for doses up to ∼400 cGy. Therefore, within this dose range, (D%) = (APnet%). 2D map of measured SRS irradiated EBT2 film (Apnet%) values have been measured and compared to corresponding TPS calculation (D%) values. Satisfying agreement between the two data sets was observed (gamma <1 for ∼96% of the pixels). Conclusion: EBT2 films can be used for accurate 2D relative dose measurements as long as they are irradiated within the dose range of 0 to 400 cGy and their batch linear dose response is ensured. There is no need to ensure a certain slope of the linear response, but just the linearity of response itself. Since films are mainly useful for relative rather than absolute dose measurements, the proposed methodology could offer a simple solution for 2D dosimetry in a large number of radiotherapy QA and/or plan verification purposes.