TH‐AB‐201‐02: A TG‐61 Calibration Method for the Zeiss Intrabeam with Spherical Applicators

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.