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

Summary

The NCS code of practice for high-energy photon and electron beams [1] has currently been implemented in most of the Dutch and Belgium radiotherapy institutes. Recently, the NCS performed a photon dosimetry audit at a large number of Dutch and Belgium radiotherapy departments [2] that has been continued as a VSL dosimetry service. In order to implement an electron audit rather quickly and efficiently, the NCS decided to establish a small audit group amongst four radiotherapy departments which already implemented the NCS 18 [1] code of practice for electron beams. The goal of this subcommittee was to establish a dosimetry audit for absorbed dose to water, Dw, in electron beams.
The subject of the audit is the determination of absorbed dose to water, Dw in cGy, at reference depth, zref, as well as the beam quality specifier, R_50,dos in cm, in high-energy electron beams in the participant's accelerator facility as performed by local staff. The audit results are expressed as an En-score [3], where the outcome of the audit is either ‘satisfactory' when |En| 1.0 or 'unsatisfactory' when |En| > 1.0.

The so called 'reference values' of R_50,dos and D_w as measured by the audit team were performed according to NCS 18 [1]. Measurements of Dw and the percentage depth ionization curve (PDI) to obtain the beam quality, R_50,dos, were performed with a plane-parallel ionization chamber. Dw was based on a plane-parallel chamber's ⁶⁰Co calibration. Additionally, if an electron energy with R_50,dos > 7 cm was available, a cross-calibration of the plane-parallel chamber against a Farmer type chamber was performed.
The expanded uncertainty for R_50,dos was estimated to be 0.14 cm (k = 2). The relative expanded uncertainty in D_w at zref with a plane-parallel chamber calibrated in ⁶⁰Co was estimated to be 3.6 % (k = 2).For the uncertainty in the D_w audit result, ΔDw, correlations between the measured values by audit team and the participant were taken into account. This resulted in an estimated relative uncertainty for ΔDw of 2.4 %. The uncertainty in the audit result for R_50,dos was estimated to be 0.20 cm.

A total of 15 electron beams were measured in 4 participating institutes. One institute was measured twice, i.e. at the beginning of the audit campaign as an initial test audit and at the end of the audit campaign as an actual audit. Both results have been reported.All R_50,dos audit results, ΔR_50,dos turned out to be satisfactory (i.e. |En| ≤1.0) except for one electron beam with an unsatisfactory En-score of 1.4. It did not influence the audit result on the Dw measurement and therefore the overall audit result of this participant. All Dw audit results, ΔDw, turned out to be satisfactory (i.e. |En| ≤1.0).

Overall it can be concluded that the audit was implemented successfully. Differences of Dw values between the audit team and the participant were in all cases smaller than 1 % and in most cases smaller than 0.5 % with a relative uncertainty of 2.4 % (k = 2). Differences in R_50,dos were in all cases smaller than 0.3 cm and in most cases smaller than 0.2 cm with an uncertainty of 0.20 cm. Except for the first (test) audit, Dw values obtained with a plane- parallel chamber calibrated in ⁶⁰Co and cross-calibrated in a high-energy electron cross- calibration agreed within 0.3 %.

It must be noted that the audit described in this study, has been performed at radiotherapy departments traceable to VSL by applying the same code of practice as the audit team. If this audit would be performed at departments not traceable to VSL or that these departments apply a different dosimetry protocol than the audit team, the uncertainties reported in this study do not apply.This NCS subcommittee had its kick-off meeting at 2 February 2015.