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Purpose Glass rod dosimeters(GRD)are useful to measure patients' entrance treatment doses on skin because of their high reproducibility and little energy dependence.In contrast with conventional thermal-luminescence detectors,the GRD can be used repeatedly without any zeroing process that may be very useful in clinical sites.In this study,uncertainties of repeatedly measured doses are evaluated to verify their reliabilities.Method First,as a single exposure group,every three GRDs were irradiated as much as 0.5,1,2,4 Gy with 6 and 15 mega-voltage(MV)beams using a linear accelerator(totally 24 GRDs).Second,every three GRDs were repeatedly irradiated and read with 0.5,1,2,4 Gy using same energy beams(totally 6 GRDs).Finally,every three GRDs were repeatedly and read with 0.5 Gy three times using same energy beams(totally 6 GRDs).We calculated the absolute error and standard deviation of the data from three exposure groups,and compared the uncertainties between the single exposure data and multi exposure data.Result The maximum absolute errors of single exposure,and two multi exposure groups were 5%,12%,23%,respectively.Also,inter-GRD deviations in every groups were 3,4,6%each.Especially in multi-exposure groups,the absolute dose errors increase as accumulated doses increase.The R2 values were 0.594 and 0.664 in two multi-exposure groups.Conclusion In single exposure,absolute dose errors and deviations were relatively low.However,in multi-exposure,they cannot be ignored any more.In spite of the clinical convenience of skipping the zeroing process,we recommend the zeroing of GRDs before every measurements for accurate treatment dose evaluation.