WE‐AB‐207A‐05: New Dosimetry Methods for CBCT

Abstract

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 dosimetryMethods: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.