SU‐E‐T‐328: Method for Verifying the Air Kerma Strength of Pre‐Assembled I‐125 Plaques for Ocular Melanoma

Abstract

Purpose: To develop a method of easily verifying the activity or air kerma strength of pre‐assembled COMS and Eye Physics EP917 eye plaques. Methods: A Capintec CRC‐7 Dose Calibrator with its standard vial/syringe sample holder was used to measure the activity of pre‐assembled eye plaques using IsoAid Advantage I‐125 seeds. Plaque activity measurements were made by placing the plaque face up in the center of a 10 cm tall Styrofoam insert in the source holder. The source holder was rotated to four angles (0°, 90°, 180°, and 270°). The average of these four values was compared to the assay air kerma strength, decayed to the plaque measurement date, to establish a plaque calibration factor. The average and standard deviation of several plaque measurements were then calculated to determine the repeatability of the process. Activity measurements were also made to determine the contribution of each seed position to the total measured activity of the EP 917 plaque. Results: Activity measurements were taken on seven vendor‐preloaded Eye Physics EP917 eye plaques. For the EP917, the average plaque calibration factor was 0.344 with a standard deviation of 0.012. For the COMS plaques with a silastic carrier, the plaque calibration factors ranged from 0.237 for a COMS 16N to 0.281 for a COMS 20. The average plaque calibration factor was 0.256 and the standard deviation was 0.015 The individual seed measurements were made using three different EP‐917 plaques. The data from these measurements show that the contribution of a single seed to the total plaque activity is approximately 5.9% or 1/17th of the total activity. Conclusions: Plaque calibration factors based on this methodology may be used to verify the activity of preassembled plaques to within 5%.