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Absorbed Dose Rate-To-Water at the Surface of a Beta-Emitting Planar Ophthalmic Applicator with a Planar, Windowless Extrapolation Chamber

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A Riley

A Riley1*, C Soares2 , J Micka3 , W Culberson4 , L DeWerd5 , (1)University of Wisconsin Medical Radiation Research Center, Madison, WI, (2) NIST (Retired), Gaithersburg, MD, (3) University of Wisconsin Medical Radiation Research Center, Madison, WI, (4) University of Wisconsin Medical Radiation Research Center, Madison, WI, (5) University of WI-Madison/ADCL, Madison, WI


WE-A-17A-1 Wednesday 7:30AM - 9:30AM Room: 17A

Purpose: Currently there is no primary calibration standard for determining the absorbed dose rate-to-water at the surface of β-emitting concave ophthalmic applicators and plaques. Machining tolerances involved in the design of concave window extrapolation chambers are a limiting factor for development of such a standard. Use of a windowless extrapolation chamber avoids these window-machining tolerance issues. As a windowless extrapolation chamber has never been attempted, this work focuses on proof of principle measurements with a planar, windowless extrapolation chamber to verify the accuracy in comparison to initial calibration, which could be extended to the design of a hemispherical, windowless extrapolation chamber.

Methods: The window of an extrapolation chamber defines the electrical field, aids in aligning the source parallel to the collector-guard assembly, and decreases the backscatter due to attenuation of lower electron energy. To create a uniform and parallel electric field in this research, the source was made common to the collector-guard assembly. A precise positioning protocol was designed to enhance the parallelism of the source and collector-guard assembly. Additionally, MCNP5 was used to determine a backscatter correction factor to apply to the calibration. With these issues addressed, the absorbed dose rate-to-water of a Tracerlab 90Sr planar ophthalmic applicator was determined using National Institute of Standards and Technology’s (NIST) calibration formalism, and the results of five trials with this source were compared to measurements at NIST with a traditional extrapolation chamber.

Results: The absorbed dose rate-to-water of the planar applicator was determined to be 0.473 Gy/s ±0.6%. Comparing these results to NIST’s determination of 0.474 Gy/s yields a -0.6% difference.

Conclusion: The feasibility of a planar, windowless extrapolation chamber has been demonstrated. A similar principle will be applied to developing a primary calibration standard for concave applicators and plaques.

Funding Support, Disclosures, and Conflict of Interest: This research is funded by the customers of the University of Wisconsin Accredited Dosimetry Calibration Laboratory

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