[en] A Siemens electronic portal imaging system called Beamview^plusTM was used daily in 15 lung cancer patients for each X-ray field. Out of the 714 expected portal images, 585 (82%), were obtained and printed. We counted 94 errors in the 585 successful controls (16 %). Four main classes of errors were identified: irradiation field placement (47 cases, 50 %), shielding placement (30 cases, 32 %), patients's position on the tilt examining table (14 cases, 15 %), and selection of the energy (3 cases, (3 %). Forty percent of these errors were potentially serious in terms of treatment volume and vital organ protection. Only portal imaging control could easily detect errors because they occurred randomly. We thus consider the use of this daily system helpful for patient management. (author)