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, D-w, and the beam quality specifier, R-50,R-dos. Fifteen electron beams were measured in four institutes according to the Belgian-Dutch code of practice for high-energy electron beams. The expanded uncertainty (k=2) for the D-w values was 3.6% for a Roos chamber calibrated in Co-60 and 3.2% for a Roos chamber cross-calibrated against a Farmer chamber. The expanded uncertainty for the beam quality specifier, R-50,R-dos, 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 D-w audit results were satisfactory with differences in D-w values mostly smaller than 0.5% and always smaller than 1%. Except for one, differences in R-50,R-dos 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% (k=2).