Phantom Measurements to Quantify the Accuracy of a Commercially Available Cone-Beam CT Gray-Value Matching Algorithm Using Multiple Fiducials

To assess the accuracy of the gray-value matching algorithm (XVI (TM), Elekta) when multiple iodine-125 ((125)I) seeds are used as fiducials. A phantom, consisting of a plastic box filled with water-dense material containing about 50 dummy seeds, developed primarily as a manual-skill trainer for (125)I seed implantation was used (Figure 1). The phantom was scanned first with a planning CT (PCT) at a slice thickness of 1 mm, 3 mm and 5 mm and with cone-beam CT (CBCT) to be associated with each reference PCT. Matching was performed with the XVI (TM) gray-value algorithm. The isocenter was marked with external markers at PCT. After matching, residual error was determined as the difference between planned isocenter and the isocenter that would have been treated based on the matching process. The procedure was performed twice, once without any manipulation (Figure 2) and once with deformation of the seed-bearing dummy prostate by inserting a plug into the phantom aperture that mimics the rectum (Figure 3). For the undeformed phantom the maximal residual error regarding the isocenter after gray-value matching around the seed-bearing region was 0.0 mm in x, y and z directions in case of the PCT with 1 mm thickness. The range of residual error was 0-0.4 mm in case of the PCT with 3 mm and 0-0.8 mm in x, y and z directions in case of 5 mm slice thickness, respectively (Figure 4). For the deformed phantom similar results were obtained (maximum error: 1.1 mm). The residual error after seed-based matching regarding the phantom isocenter was < 1.1 mm in all cases and for the clinical situation (3 mm slice thickness) always < 0.4 mm. The algorithm is therefore appropriate for precision radiotherapy.