Program Information
New Dosimetry Methods for CBCT
M Lawless*, D Anderson , W Culberson , L DeWerd , Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison
Presentations
WE-AB-207A-5 (Wednesday, August 3, 2016) 7:30 AM - 9:30 AM Room: 207A
Purpose: To determine the absorbed dose to water in the moderately filtered (M-series) x-ray beams at the UWADCL and assess the feasibility of transferring this dose to water to a clinical CBCT environment.
Methods: The dose was determined in the M-series beams using three dosimetry methods: the TG-61 protocol, a ⁶⁰Co absorbed dose to water calibration, and through the use of a Monte Carlo (MC) model of the x-ray tube.
The dose to water in the M-series beams determined by the MC model was used to calibrate an ionization chamber. The calibrated chamber was used to measure the dose to water from a Varian CBCT system. The dose from the CBCT beams was also measured using the same methods as for the M-series beams.
Results: The measured surface doses in the M-series beams determined by MC method and TG-61 agreed within 3% for all beams. The dose to water a depth of 2 cm in the M-series beams as determined by the TG-61, ⁶⁰Co, and MC methods, agreed within 2%.
The dose in the CBCT beams was determined using the same methods as for the M-series beams and using the newly determined x-ray dose to water calibration coefficient. At the phantom surface, the dose values agreed within 1.5% and at a 2 cm depth the values agreed within 3%.
Conclusion: New techniques to determine the absorbed dose to water in standard x-ray beams were developed. The dose values were in good agreement with previous techniques, but with much lower uncertainties. The dose to water in the M-series beams was used to calibrate an ionization chamber and the dose in the CBCT beams was measured using this calibration. The CBCT dose at a depth of 2 cm was measured in this manner and it agreed with other dose determination methods.
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