Effect of metal implants and metal artifacts on back‐projected two‐dimensional entrance fluence determined by EPID dosimetry

PURPOSE: To evaluate the errors caused by metal implants and metal artifacts in the two‐dimensional entrance fluences reconstructed using the back‐projection algorithm based on electronic portal imaging device (EPID) images. METHODS: The EPID in the Varian VitalBeam accelerator was used to acquire portal dose images (PDIs), and then commercial EPID dosimetry software was employed to reconstruct the two‐dimensional entrance fluences based on computed tomography (CT) images of the head phantoms containing interchangeable metal‐free/titanium/aluminum round bars. The metal‐induced errors in the two‐dimensional entrance fluences were evaluated by comparing the γ results and the pixel value errors in the metal‐affected regions. We obtained metal‐artifact‐free CT images by replacing the voxel values of non‐metal inserts with those of metal inserts in metal‐free CT images to evaluate the metal‐artifact‐induced errors. RESULTS: The γ passing rates (versus PDIs obtained without a phantom in the beam field (PDI(air)), 2%/2 mm) for the back‐projected two‐dimensional entrance fluences of phantoms containing titanium or aluminum (BP(Ti)/BP(Al)) were reduced from 92.4% to 90.5% and 90.6%, respectively, relative to the metal‐free phantom (BP(metal‐free)). Titanium causes more severe metal artifacts in CT images than aluminum, and its removal resulted in a 0.0022 CU (median) reduction in the pixel value of BP(Ti artifact‐free) relative to BP(Ti) in the metal‐affected region. Moreover, the mean absolute error (MAE) and root mean square error (RMSE) decreased from 0.0050 CU and 0.0063 CU to 0.0034 CU and 0.0040 CU, respectively (vs. BP(metal‐free)). CONCLUSION: Metal implants increase the errors in back‐projected two‐dimensional entrance fluences, and metals with higher electron densities cause more errors. For high‐electron‐density metal implants that produce severe metal artifacts (e.g., titanium), removing metal artifacts from the CT images can improve the accuracy of the two‐dimensional entrance fluences reconstructed by back‐projection algorithms.