Assessment of a 2D EPID-based Dosimetry Algorithm for Pre-treatment and In-vivo Midplane Dose Verification

Introduction: The use of electronic portal imaging devices (EPIDs) is a method for the dosimetric verification of radiotherapy plans both pretreatment and in-vivo. The aim of this study was to test a 2D EPID-based dosimetry algorithm for dose verification of some plans inside a homogenous and anthropomorphic phantom and in-vivo, as well. Materials and Methods: Dose distributions were reconstructed from EPID images using a 2D EPID dosimetry algorithm inside a homogenous slab phantom for a simple 10×10 cm2 box technique, the 3D conformal (prostate, head-and-neck and lung) and an IMRT prostate plans inside an anthropomorphic (Alderson) phantom and the patients (one fraction in-vivo) for the 3D conformal plans (prostate, head-and-neck and lung). Results: The Planned and EPID dose difference at isocenter, on average, was 1.7% for the pretreatment verification and was less than 3% for all in vivo plans except a head-and-neck which was 3.6%. The mean γ values for a 7-field prostate IMRT plan delivered to the Alderson phantom varied from 0.28 to 0.65. For 3D conformal plans applied for the Alderson phantom, all γ1% values were within the tolerance level for all plans and in both AP-PA beams. Conclusion: The 2D EPID-based dosimetry algorithm provides an accurate method to verify the dose of a simple 10×10 cm2 fields in two dimensions inside a homogenous slab phantom as well as for IMRT prostate plan and 3D conformal plans (prostate, head-and-neck and lung plans) applied using an anthropomorphic phantom and in-vivo.