Feasibility of automatic marker detection with an a-Si flat-panel imager

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