Optimising the use of EPIgray for 3DCRT breast treatments

EPIgray is an in-vivo dosimetry system which uses electronic portal images to calculate dose delivered to a point of interest (POI) and the percentage dose difference (%DDiff) from expected dose. For 3D conformal radiotherapy (3DCRT) of breasts, a small shift between patient position on treatment compared to the planning CT is often clinically accepted. However due to the use of the planning CT in the EPIgray back-projection algorithm, acceptable shifts can have undue impact on EPIgray dose so it does not reflect true POI dose. At our centre ± 5.0% %DDiff tolerance is used for all treatment sites, however for breast treatments this effect causes false positive (FP) results, which may mean an actual treatment error is not detected. Patient position can be better represented within EPIgray using a contour correction (CC) method, increasing dose calculation accuracy. A custom breast-lung phantom was developed to validate use of CC, then EPIgray data of 30 breast patients were retrospectively analysed with CC. %DDiff before and after CC identified a FP rate. A process to determine optimal EPIgray tolerances for breast 3DCRT to reduce incidence of FP results is presented, based on analysis of factors influencing %DDiff and a receiver operator characteristic curve analysis of the retrospective study data. This process determined that a reduced tolerance of ± 3.5% would optimise utility of the EPIgray results, but this would require additional clinical resources to investigate the correspondingly increased rate of false negative results. Choice of tolerance requires consideration of workload and aims of the IVD program.