An on-site dosimetry audit for high-energy electron beams.

Background And Purpose: External dosimetry audits are powerful quality assurance instruments for radiotherapy. The aim of this study was to implement an electron dosimetry audit based on a contemporary code of practice within the requirements for calibration laboratories performing proficiency tests. This involved the determination of suitable acceptance criteria based on thorough uncertainty analyses.

Materials And Methods: Subject of the audit was the determination of absorbed dose to water, , and the beam quality specifier, . Fifteen electron beams were measured in four institutes according to the Belgian-Dutch code of practice for high-energy electron beams. The expanded uncertainty ( = 2) for the values was 3.6% for a Roos chamber calibrated in Co and 3.2% for a Roos chamber cross-calibrated against a Farmer chamber. The expanded uncertainty for the beam quality specifier, , was 0.14 cm. The audit acceptance levels were based on the expanded uncertainties for the comparison results and estimated to be 2.4%.

Results: The audit was implemented and validated successfully. All audit results were satisfactory with differences in values mostly smaller than 0.5% and always smaller than 1%. Except for one, differences in were smaller than 0.2 cm and always smaller than 0.3 cm.

Conclusions: An electron dosimetry audit based on absorbed dose to water and present-day requirements for calibration laboratories performing proficiency tests was successfully implemented. It proved international traceability of the participants value with an uncertainty better than 3.6% ( = 2).