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

The JSMP01 dosimetry protocol had adopted the provisional ⁶⁰Co calibration coefficient [Formula: see text], 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 [Formula: see text] calibration. In this study, the influence of the calibration shift on the measurement of D _w among users was analyzed.

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.

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 [Formula: see text] calibration.

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.