an empirical transmitted epid dosimetry method using a backprojection algorithm

Background: the present study aimed to introduce a rapid transmission dosimetry through an electronic portalimaging device (epid) to achieve twodimensional (2d) dose distribution for homogenous environments. material and methods: in this phantom study, first, the epid calibration curve and correction coefficients for field size were obtained from epid and ionization chamber. second, the epid offaxis pixel response was measured, and the greyscale image of the epid was converted into portal dose image using the calibration curve. next, the scattering contribution was calculated to obtain the primary dose. then, by means of a verified backprojection algorithm and the scattertoprimary dose ratio, a 2d dose distribution at the midplane was obtained. results: the results obtained from comparing the transmitted epid dosimetry to the calculated dose, using commercial treatment planning system with gamma function while there is 3% dose difference and 3mm distance to agreement criteria, were in a good agreement. in addition, the pass rates of γ < 1 was 94.89% for the homogeneous volumes. conclusion: based on the results, the method proposed can be used in epid dosimetry.