Variation in Dosimeter Calibration Factor (N_DW) Over a Period of 20 Years

Area of Science:

• Medical Physics
• Radiation Oncology
• Dosimetry

Background:

• COVID-19 pandemic disruptions led to extended calibration intervals for secondary standards radiation dosimeters.
• This raised concerns about the accuracy of absorbed dose to water calibration factors (N_DW) in radiation oncology.
• Uncertainty in N_DW can impact precise dose delivery to cancer patients.

Purpose of the Study:

• To retrospectively analyze the long-term stability of the absorbed dose to water calibration factor (N_DW).
• To assess the impact of delayed calibrations on dosimeter accuracy in radiation oncology.

Main Methods:

• Reviewed 20 years of N_DW calibration data for Farmer (thimble) and Markus (parallel plate) type ion chambers.
• Calibrations were performed at the Secondary Standards Dosimetry Laboratory, Bhabha Atomic Research Centre, Mumbai.
• Analyzed the percentage change in N_DW over the 20-year period.

Main Results:

• The N_DW factor for both ion chamber types exhibited minimal change over two decades.
• The observed change in the N_DW factor was consistently less than ±3%.
• No significant drift or instability was detected in the calibration factors.

Conclusions:

• Radiation oncology dosimeters, specifically Farmer and Markus ion chambers without repair history, demonstrate remarkable stability in their N_DW factors over extended periods.
• The stability suggests that occasional delays in calibration, such as those experienced during the COVID-19 pandemic, may not significantly compromise dose accuracy.
• These findings support the reliability of existing dosimetric equipment for accurate radiotherapy.