document: A method for geometrical verification of dynamic intensity modulated radiotherapy using a scanning electronic portal imaging device In order to guarantee the safe delivery of dynamic intensity modulated radiotherapy -LRB- IMRT -RRB- , verification of the leaf trajectories during the treatment is necessary . Our aim in this study is to develop a method for on-line verification of leaf trajectories using an electronic portal imaging device with scanning read-out , independent of the multileaf collimator . Examples of such scanning imagers are electronic portal imaging devices -LRB- EPIDs -RRB- based on liquid-filled ionization chambers and those based on amorphous silicon . Portal images were acquired continuously with a liquid-filled ionization chamber EPID during the delivery , together with the signal of treatment progress that is generated by the accelerator . For each portal image , the prescribed leaf and diaphragm positions were computed from the dynamic prescription and the progress information . Motion distortion effects of the leaves are corrected based on the treatment progress that is recorded for each image row . The aperture formed by the prescribed leaves and diaphragms is used as the reference field edge , while the actual field edge is found using a maximum-gradient edge detector . The errors in leaf and diaphragm position are found from the deviations between the reference field edge and the detected field edge . Earlier measurements of the dynamic EPID response show that the accuracy of the detected field edge is better than 1 mm . To ensure that the verification is independent of inaccuracies in the acquired progress signal , the signal was checked with diode measurements beforehand . The method was tested on three different dynamic prescriptions . Using the described method , we correctly reproduced the distorted field edges . Verifying a single portal image took 0.1 s on an 866 MHz personal computer . Two flaws in the control system of our experimental dynamic multileaf collimator were correctly revealed with our method . First , the errors in leaf position increase with leaf speed , indicating a delay of approximately 0.8 s in the control system . Second , the accuracy of the leaves and diaphragms depends on the direction of motion . In conclusion , the described verification method is suitable for detailed verification of leaf trajectories during dynamic IMRT
doc_bio_tags: O O O O O O B I I I O O O O O O O O O O O O B I O B I I I O O O O O O O B I O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O B I O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O
extractive_keyphrases: dynamic intensity modulated radiotherapy dynamic multileaf collimator safe delivery leaf trajectories diaphragm positions motion distortion effects reference field edge distorted field edges control system
abstractive_keyphrases: geometrical verification method on-line verification scanning read-out liquid-filled ionization chambers leaf positions dose distributions treatment planning