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

PURPOSE: The JSMP01 dosimetry protocol had adopted the provisional60 Co calibration coefficient [Formula: see text], namely, the product of exposure calibration coefficient NC and conversion coefficient kD,X . After that, the absorbed dose to water Dw  standard was established, and the JSMP12 protocol adopted the [Formula: see text] calibration. In this study, the influence of the calibration shift on the measurement of Dw among users was analyzed.

MATERIALS AND METHODS: The intercomparison of the Dw using an ionization chamber was annually performed by visiting related hospitals. Intercomparison results before and after the calibration shift were analyzed, the deviation of Dw 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 Dw was not observed, but that of difference of Dw among users was observed between NC and [Formula: see text] calibration.

CONCLUSION: Uncertainty due to chamber-to-chamber variation was reduced by the calibration shift, consequently reducing the uncertainty among users regarding Dw . The result also pointed out uncertainty might be reduced by accurate and detailed instructions on the setup of an ionization chamber.