Dosimetric verification of IMRT and 3D conformal treatment delivery using EPID.

Purpose: Electronic portal imaging devices (EPIDs) could potentially be useful for either in-vivo or pre-treatment dosimetric verification of external beam radiation therapy. The accuracy of EPID for dosimetric purposes is highly dependent on the specific method used for the determination of dose-response characteristics. The aim of this study was to develop a simple and time-saving EPID back-projection dosimetry algorithm for 2D dose verification in 3D conformal and intensity-modulated beams.

Methods: The procedure of dose reconstruction includes a first calibration step using ionization chamber measurements to convert the Electronic Portal Image (EPI) pixel values into an absorbed dose in water. Subsequently, several corrections were applied to the Portal Dose Images (PDIs) for the effect of field size, attenuator thickness, scattering radiation, beam hardening and EPID off-axis response. Furthermore, to consider tissue inhomogeneity for accurate dose reconstruction, the patient's water equivalent path length (WEPL) was calculated using a range of digitally reconstructed radiographs (DRRs) obtained at various thicknesses by Plastimatch software. The EPID-derived dose maps accuracy was assessed by comparing with the treatment planning system (TPS) calculated dose in the prostate region of Alderson phantom irradiated with 3D conformal and intensity-modulated beams.

Results: The gamma analysis for the dose plane showed agreements of 96.95% and 93.5% for 3D conformal and IMRT fields, respectively, with 3%/3 mm acceptance criteria.

Conclusion: The presented algorithm can provide accurate absolute 2D dose maps for clinical use in the context of 3DCRT or IMRT Quality Assurance (QA) programs.