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,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 (60)Co and 3.2% for a Roos chamber cross-calibrated against a Farmer chamber. The expanded uncertainty for the beam quality specifier, R(50,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,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).