Analysis of fiducial marker-based position verification in the external beam radiotherapy of patients with prostate cancer

Purpose Evaluate the fiducial marker-based position verification in the external-beam radiotherapy of patients with prostate cancer. Methods Four hundred and fifty-three patients with prostate cancer received an IMRT treatment combined with fiducial marker-based position verification. Portal images were taken in all 35 treatment fractions. This database was used to study the accuracy of detecting the prostate position as well as the presence of time trends and the effectiveness of commonly used off-line correction protocols. Results The variation in inter-marker distance shows that the prostate position can be detected with an accuracy better than 0.6 mm. Significant time trends in prostate position occurred in 35%, 18% and 48% of the patients in the vertical, lateral and longitudinal directions, respectively, with 34%, 9% and 35% deviating more than 3 mm over the course of the treatment. Off-line correction protocols that estimate a deviation only in the first fractions of the treatment (shrinking action level (SAL), no action level (NAL)) are not effective in following these trends. With daily off-line position correction using an adapted SAL protocol we reduced systematic positioning errors in clinical practice to less than 0.8 mm in all directions. Conclusion Fiducial markers are a reliable tool for prostate position verification. Time trends occur frequently. Correction procedures must take such trends into account.