Verifying Setup Position with Fiducial Markers and a Standard Electronic Portal Imaging Device

Abstract

Purpose/Objective: Advances in highly conformal radiation therapy require more precise treatment setups. The purpose of this study is to determine whether metal fiducials on the patient′s skin and a commercial Electronic Portal Imaging Device (EPID) can improve setup verification. Materials/Methods: On the day of CT simulation and on each day of treatment, we placed three gold seed fiducial markers on the skin of two patients. Six physicians reviewed ten sets of orthogonal port film pairs and compared the difference in the position of isocenter between DRR and port film. We used in-house software to record the direction and magnitude of isocenter shifts in the x, y, and z axes under two conditions: the standard practice using visual anatomy and the fiducial-EPID system. Results: 1)Interobserver variation with standard practice of visual comparison of DRR and EPID: Variation in isocenter shifts was large in comparison with the level of precision required for highly conformal radiation therapy. The median standard deviation of the distance of the movement of the position of isocenter was 2.6 mm (range 2.3–3.8 mm).2)Feasibility of the Fiducial-EPID system: The physicians were able to locate all fiducial markers on every view on every image. The variation in isocenter shift was much smaller with the fiducial-based system than for standard practice. The median standard deviation was1.2 mm (range 0.5–2.7 mm) vs 32.6 mm (2.3–3.8 mm), p=0.0009.3)Ease of using the Feducial-EPID system: The fiducial system was easier to use than the standard system. The rating of easy or very easy was 76% vs 25%, p < 0.0001. The time for the total verification process was slightly longer with the fiducial system at median 2.1 minutes vs. 1.2 minutes, p < 0.0001. Conclusions: A simple software program makes it possible to visualize and record the position of metal fiducials on standard EPID images. This Fiducial-EPID system is likely to improve the precision of patient setup in a way that is both time efficient and user friendly.