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TH‐AB‐201‐02: A TG‐61 Calibration Method for the Zeiss Intrabeam with Spherical Applicators
Author(s) -
Siochi R
Publication year - 2016
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4958030
Subject(s) - calibration , backscatter (email) , imaging phantom , ionization chamber , reproducibility , optics , materials science , physics , nuclear medicine , mathematics , engineering , statistics , medicine , ion , telecommunications , quantum mechanics , wireless , ionization
Purpose: To develop an independent in‐air TG‐61 based calibration method for the Zeiss Intrabeam 50 kV x‐ray source and spherical applicators. Methods: We used the Zeiss supplied parallel plate chamber (TN34013), calibrated in water, and a micrometer staged water phantom to verify the annual factory water based calibration per Zeiss protocol. For independence, a second TN34013 chamber was calibrated at an ADCL at various energies in air. TG‐61 HVL measurements were performed to determine this chamber's calibration factor. The backscatter factor was determined by two independent means: (1) measurement with correction for chamber holder and stem, and (2) analysis of scatter geometry and buildup based on ratios of applicator transfer functions for various applicator sizes. To compare in‐air data against water data, a correction from the backscatter setup to the clinical setup was measured and corrected for geometry differences. To compare the water data against the factory calibration, a factory supplied correction factor for geometry and measurement setup differences was applied. Results: The HVL is 0.89 mm‐Al, consistent with published literature. The backscatter factors were 1.124 and 1.128 for methods 1 and 2 respectively, so we used their average, 1.126. The correction from the backscatter setup to the clinical setup was 0.9992 for the specific calibration geometry we selected (4 cm diameter applicator with a source to detector distance of 22 mm). The resulting TG‐61 dose rate, corrected to water, was 0.5771 Gy/min. The Zeiss protocol in water gave 0.5803 Gy/min. The Zeiss calibration value, corrected for the output of the day, was 0.5768 Gy/min. These are all within 1% of each other. Conclusion: An independent in‐air TG‐61 based method was successfully developed and shown to be in excellent agreement with a water based protocol and the factory calibration.