Introduction: Tumor control and treatment quality depend on accuracy of delivered dose. Various methods have been studied for treatment dosimetry verification. Among these methods, the use of electronic portal imaging device (EPID) is highly considered. Many efforts are underway to improve the results of this context. Materials and Methods: In this study, a new method for pretreatment dose verification is reported using the transmitted signal of a camera based EPID. Lateral scatter within imager has been considered as field size dependent additive operation. Lateral scatter description within the imager were obtained from measurements with a 2D array ionization chambers for a set of different field sizes. The water-equivalent thickness of phantom material was defined for dose reconstruction, by means of the treatment planning system, according to the patient's data in CT scans along the beam central axis. Results: The method was tested for pretreatment verification of 3D conformal prostate cancer treatment plans. There are no significant differences between the EPID and ionization chamber responses (P value = 0.37). Conclusion: This study proposes a practical method with easier and less calculations in comparison to the deconvolution approaches that can be applied for overall dosimetric assessments of treatments in every fraction.