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Orthovoltage Beam Comparison After X-Ray Tube Change

K McCollough*, C Blackwell , Mayo Clinic, Rochester, MN


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: Orthovoltage X-ray tubes do not allow users to “tune� the beam parameters to achieve a specified output or percent depth dose (PDD). Thus, when a tube is replaced, a complete new set of beam parameters must be acquired to characterize the new X-ray energies. The purpose of this work was to determine if the timer set program needed to be modified for the new tube.

Methods: Beam characterization parameters were measured using conventional methods at four beam energies: 60, 100, 180, and 250 kVp. These included half-value layer (HVL), output (cGy/min), and percent depth dose (PDD). After measurement, the new parameters were compared with the old and differences determined.

Results: HVL differences ranged from less than 1% to 2.3% (absolute difference 0.03mm AL and 0.01mm Cu between the two tubes). PDDs values at 60 kVp were compared from surface to 2.0 cm depth, and differences were less than 1.0% across all field sizes. Calibration / output changed by a maximum of 3.8%.

Conclusion: It was decided in 2016, at the time of the installation, to utilize published BJR beam data (PDD, HVL, etc.) for timer set calculations. Without the ability to tune the beam to match the first x-ray tubes beam parameters we were cautious about the outcome. The second x-ray tube’s parameters agreed to within 2% of the original x-ray tube’s data and demonstrated improved agreement with the BJR data. Only small changes in reference outputs (cGy/min) were required in the new timer set program.


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