Changes in deviation of absorbed dose to water among users by chamber calibration shift

Purpose The JSMP01 dosimetry protocol had adopted the provisional 60 Co calibration coefficient N D,w,Q 0 , namely, the product of exposure calibration coefficient N C and conversion coefficient k D,X . After that, the absorbed dose to water D w standard was established, and the JSMP12 protocol adopted the N D,w,Q 0 calibration. In this study, the influence of the calibration shift on the measurement of D w among users was analyzed. Materials and methods The intercomparison of the D w using an ionization chamber was annually performed by visiting related hospitals. Intercomparison results before and after the calibration shift were analyzed, the deviation of D w among users was re-evaluated, and the cause of deviation was estimated. Results As a result, the stability of LINAC, calibration of the thermometer and barometer, and collection method of ion recombination were confirmed. The statistical significance of standard deviation of D w was not observed, but that of difference of D w among users was observed between N C and N D,w,Q 0 calibration. Conclusion Uncertainty due to chamber-to-chamber variation was reduced by the calibration shift, consequently reducing the uncertainty among users regarding D w . The result also pointed out uncertainty might be reduced by accurate and detailed instructions on the setup of an ionization chamber.