Feasibility of automatic marker detection with an a-Siflat-panel imager

Abstract

Here we study automatic detection of implanted gold markers relativeto the field boundary in portal images for on-line position verification.Portal images containing 1-2 MU were taken with an amorphous siliconflat-panel imager. The images were obtained with lateral field at 18 MV. Boththe detection success rate and the localization accuracy of markers of 1.0 and1.2 mm diameter were determined with the help of a marker detection methodbased on a marker extraction kernel. A method for determining a fiducialreference point related to the field boundary was developed.Detection success rates were 0.99, 0.90 and 0.95 for markers of 1.2 mmdiameter and 5 mm length, 1.0 mm diameter and 5 mm length and 1.0 mm diameterand 10 mm length respectively. The localization accuracy appeared to be betterthan 0.3 mm. The reference point could be reproduced with an accuracy equal to1 pixel (0.5 mm at isocentre) within one fraction. During the first fewseconds of a treatment fraction the field edge was not stable, which appearedto be an effect of the motion of the radiation source.Thanks to the use an a-Si flat-panel imager, on-line positionverification using implanted gold markers becomes clinicallyfeasible. We can use a clinically acceptable marker diameter assmall as 1.0 mm. These markers can be automatically detected inportal images obtained with 1-2 MU relative to a stable referencepoint related to the field boundary.