Variation in dosimeter calibration factor (N_dw) over a period of 20 years

Long delays in renewal of calibration of secondary standards radiation dosimeters in radiation oncology centers due to the COVID19 pandemic have aroused concerns regarding accuracy in dose delivery to patients. The concerns are mainly due to the uncertainty in the absorbed dose to water calibration factor (N_DW) over a period of time. In this study, the N_DW factor for two ion chambers, thimble type (Farmer) and parallel plate type (Markus), used in most of the radiotherapy centers, were retrospectively reviewed for 20 years. The calibration on all occasions except once was carried out at the Secondary Standards Dosimetry Laboratory, Bhabha Atomic Research Centre, Mumbai. The change in the N_DW factor over this period was < ±3%. We, therefore, believe that a dosimeter with no history of repairs showed reasonable stability in the NDW factor over a long period. (author)