Technical Note: A simple algorithm to convert EPID gray values into absorbed dose to water without prior knowledge

    loading  Checking for direct PDF access through Ovid



Full integration of EPID-based dosimetry in a global quality control workflow is still complicated. All the actual solutions are based on a relation between image gray-level signal and total linac-delivered dose. In this study, we propose a simple algorithm relying pixel gray-level of EPID image with average linac delivered dose per acquisition frame.


Calibration models are constructed for Varian and Elekta linacs including scattering conditions and EPID-arm backscatter-specific corrections. Only simple homogeneous fields are required to establish the EPID dose conversion model for each x-ray beam. Then, the model was evaluated by comparing calculated and converted dose distributions for homogeneous and modulated beams using gamma maps.


To fit average dose per frame (Dfnorm) vs pixel gray value (Ngnorm) of each EPID image, a logarithmic curve Symbol, has been chosen where A, B and C are constants depending on beam energy. Gamma comparison (2%, 2 mm, threshold 15%) between converted images and calculated dose distributions for linac control and pretreatment patient fields led to a gamma pass rate higher than 97% for all the analyzed fields.


Without a prior irradiation settings knowledge except the incident energy beam, we use EPID as a reliable dose to water detector for both homogeneous and modulated beams.

Related Topics

    loading  Loading Related Articles