© 1995 SPIE. All rights reserved. A fully automated system is being developed for the portal verification of tangential breast fields in radiation therapy of breast cancer. The automated verification system involves image acquisition, image feature extraction, feature correlation between reference and portal images, and quantitative evaluation of patient setup. In this study, the portal images are acquired using a matrix liquid ion-chamber electronic portal imaging device (EPID), and have a matrix size of 256 × 256 pixels with 12-bit gray levels. A hierarchical region processing technique is developed to extract poor contrast features in the portal image generated by megavoltage photon beams at different levels sequentially. The treatment field is initially extracted from the portal image. The skin line is then extracted from the treatment field. Finally, the lung/soft tissue separation is extracted from the breast region. A Chamfer matching filter is used to correlate features in the portal image with those in the reference image. The resulting parameters for rotation, translation and scaling are used for the setup evaluation of the treatment field.