Room: Exhibit Hall
Purpose: Measurements were made with conventional ionization chambers and two Â¹â?°â?¶Ru eye plaques to test a method for transferring surface dose rate results from a convex windowless extrapolation chamber to clinics.
Methods: Surface dose rate was previously measured for two curved Â¹â?°â?¶Ru episcleral plaques using a convex windowless extrapolation chamber developed for this work. To test a proposed transfer technique, measurements were made with conventional ionization chambers (A26, A1SL, A20) held in a clinic QA fixture composed of PMMA that was machined in-house. During measurements, each chamber was centered in front of the plaque with the chamber tip in contact with the concave source face. One Â¹â?°â?¶Ru plaque was designated as the reference source used to generate a calibration coefficient (mGy sâ?»Â¹ pAâ?»Â¹) which transferred the extrapolation chamber result. Measurements were also carried out with the second Â¹â?°â?¶Ru plaque in the same fixture, and the dose rate predicted from each chamber calibration was compared with the expected dose rate from the extrapolation chamber.
Results: For ionization chamber measurements with the test plaque, surface dose rate predicted from the transfer technique was 2.5â€“3.8% higher than the expected value from convex extrapolation chamber measurements. This agreement was within the total uncertainty for the extrapolation chamber measurement estimated as Â±6.8% (k=2). An offset in chamber response (<3%) was observed between the two sources that was attributed to differences in penetration within the ionization chamber walls, since the change corresponded with depth dose curves previously measured in this study using a film stack phantom. The change was likely brought about by minor differences in self-attenuation between the two plaques.
Conclusion: Conventional ionization chambers within a clinic QA fixture can be used to transfer the extrapolation chamber result for surface dose rate through a calibration coefficient generated with a reference Â¹â?°â?¶Ru plaque.