Radiotherapy clinical dosimetry audit in The Netherlands and Belgium

In 2008 The Netherlands Commission on Radiation Dosimetry (NCS) published a new code of practice, CoP, for the absorbed dose determination in clinical high energy photon and electron beams used in radiotherapy, NCS-18. VSL contributed to this CoP by measuring so called beam quality correction factors, kQ, using the Dutch primary standard for absorbed dose to water, the VSL water calorimeter. Since the release of the new code of practice most radiotherapy centres in Belgium and the Netherlands are implementing or have implemented the new CoP. In order to monitor and verify the implementation of NCS-18, an NCS audit team was established, consisting of 12 medical physics engineers and medical physicists from various radiotherapy institutes in The Netherlands and Belgium and a metrologist from VSL. The aim of the audit team was to set-up and execute a dosimetry audit for clinical high energy photon beams in radiotherapy. Participation in the audit for any institute was voluntary; however, a total of 25 institutes participated: 20 out of 21 Dutch institutes and 5 out of 26 Belgium institutes.

The aim of the dosimetry audit was to verify local measurements of absorbed dose to water at radiotherapy centres in Belgium and The Netherlands.  This was done by performing measurements using validated methods and calibrated measurement equipment assuring highest accuracy and international traceability. Absorbed dose measurements where done in each radiotherapy institute at a maximum of three photon beams in a single clinical accelerator using the NCS audit equipment. The measurements were always performed by two members of the NCS audit team. The local radiotherapy institute was then given the opportunity to measure the absorbed dose to water using their own equipment and method, an implementation of an accepted (inter)national protocol. The ones that already implemented NCS-18 used this method. Also a routine measurement by the institute was performed. The measurement audit was enclosed by repeating the NCS measurement of the absorbed dose to water.

The audit measurements started in November 2009 and finished in August 2010. Currently the final results are being evaluated.

The audit equipment includes an electrometer (PTW TN10023 Webline), a Farmer type graphite-aluminium ionization chamber (PTW TN30012) and a water phantom (PTW T41033 MP1). Furthermore a digital barometer, digital thermometers and a humidity sensor are used. All measurement equipment was repeatedly calibrated at VSL. The NCS audit dosimetry system (electrometer in combination with ionisation chamber) showed a standard deviation in the established calibration coefficient of less than 0.12 % during one year.

Results with deviations smaller than 1.5 % were considered acceptable. Deviations larger than 1.5 % required further investigation by the audit team. The preliminary results show no deviations larger than 1.3 % from the audit measurements for NCS-18.
The results will be presented at the IAEA International Symposium on Standards, Applications and Quality Assurance in Medical Radiation Dosimetry in Vienna this year.