[Evaluation of relative electrometer calibration factor with user beam.]

Area of Science:

• Medical Physics
• Radiation Dosimetry

Background:

• Establishing traceability of absorbed dose to water is critical in radiotherapy.
• Current Japanese practice calibrates ionization chamber and electrometer systems together.
• Solely calibrating electrometers is increasingly desired for flexibility with multiple ionization chambers.

Purpose of the Study:

• To develop a method for relative electrometer calibration at the user site.
• To assess the feasibility and uncertainty of user-site electrometer calibration.
• To address the lack of a domestic nano-ampere electrometer calibration service in Japan.

Main Methods:

• A relative electrometer calibration factor was determined by comparing a calibrated system with a non-calibrated electrometer under identical irradiation conditions.
• Uncertainties were estimated by comparing calibration factors derived from ionization current (linac photon beam) and DC current (precision DC source).

Main Results:

• The variation in electrometer readings for pulsed ionization current was found to be negligible under steady linac photon beam output.
• Relative electrometer calibration factors obtained under user-site irradiation conditions were deemed valid.
• The study provides a practical solution in the absence of a dedicated domestic calibration service.

Conclusions:

• Relative electrometer calibration at the user site is a valid method for ensuring accurate absorbed dose measurements in radiotherapy.
• This approach is essential until a domestic nano-ampere electrometer calibration service becomes available.
• The findings support the continued use of electrometers for precise dosimetry in radiation oncology.