On-line aSi portal imaging of implanted fiducial markers for the reduction of interfraction error during conformal radiotherapy of prostate carcinoma.

PURPOSE: An on-line system to ensure accuracy of daily setup and therapy of the prostate has been implemented with no equipment modification required. We report results and accuracy of patient setup using this system. METHODS AND MATERIALS: Radiopaque fiducial markers were implanted into the prostate before radiation therapy. Lateral digitally reconstructed radiographs (DRRs) were obtained from planning CT data. Before each treatment fraction, a lateral amorphous silicon (aSi) portal image was acquired and the position of the fiducial markers was compared to the DRRs using chamfer matching. Couch translation only was used to account for marker position displacements, followed by a second lateral portal image to verify isocenter position. Residual displacement data for the aSi and previous portal film systems were compared. RESULTS: This analysis includes a total of 239 portal images during treatment in 17 patients. Initial prostate center of mass (COM) displacements in the superior, inferior, anterior, and posterior directions were a maximum of 7 mm, 9 mm, 10 mm and 11 mm respectively. After identification and correction, prostate COM displacements were <3 mm in all directions. The therapists found it simple to match markers 88% of the time using this system. Treatment delivery times were in the order of 9 min for patients requiring isocenter adjustment and 6 min for those who did not. CONCLUSIONS: This system is technically possible to implement and use as part of an on-line correction protocol and does not require a longer than standard daily appointment time at our center with the current action limit of 3 mm. The system is commercially available and is more efficient and user-friendly than portal film analysis. It provides the opportunity to identify and accommodate interfraction organ motion and may also permit the use of smaller margins during conformal prostate radiotherapy. Further integration of the system such as remote table control would improve efficiency.