Impact of a common user-derived electrometer calibration coefficient on absorbed dose-to-water determination: a multicenter uncertainty study

Background : Accurate absorbed dose-to-water determination in radiotherapy requires electrometers calibrated by a primary standards laboratory (PSL). Recent studies have suggested that electrometers of the same model may exhibit sufficiently similar sensitivity characteristics to allow the use of a common electrometer calibration coefficient for quality-control purposes. However, the effect of this approach on absorbed dose-to-water uncertainty has not been quantitatively evaluated. Purpose : To evaluate the uncertainty contribution associated with the use of a common electrometer calibration coefficient and to assess its influence on absorbed dose-to-water determination. Methods A multicenter study was conducted involving 12 radiotherapy facilities and 24 combinations of linear accelerators, ionization chambers, and electrometers. All electrometers possessed valid PSL-issued calibration coefficients. Absorbed dose-to-water values were calculated using both the PSL calibration coefficient ( k elec ) and a common electrometer coefficient ( k elec,user ). Dose monitor unit values and uncertainty budgets were compared for 6-MV and 10-MV photon beams. Uncertainty analysis was performed according to the Guide to the Expression of Uncertainty in Measurement (GUM). Results : Only small differences were observed between dose monitor unit values calculated using ( k elec ) and ( k elec,user ). The user-dependent uncertainty increased from 0.17% to 0.23% ( k = 1), corresponding to an increase of approximately 0.06 percentage points. However, this contribution remained substantially smaller than the dominant uncertainty sources in reference dosimetry. Consequently, the combined standard uncertainty and expanded uncertainty of absorbed dose-to-water determination remained unchanged. Conclusion : The uncertainty contribution associated with the common electrometer calibration coefficient was small and had no meaningful influence on the overall uncertainty budget of absorbed dose-to-water determination. These findings support the use of a common coefficient for electrometer sensitivity verification and quality-control applications within a given electrometer type. Although periodic PSL calibration remains essential for maintaining traceability, common-coefficient-based verification may provide a useful complementary approach for routine electrometer quality assurance.