Level I audits

The Level I mail out audit is an independent measurement of Linac output under reference conditions. Optically Stimulated Luminescent Dosimeters (OSLDs) are sent to a facility, irradiated, and returned to the ACDS for analysis. All photon and electron beams are included in the audit. The Level I audit is conducted at each facility every two years. The ACDS Level I audit is mutually recognised for equivalence to the Imaging and Radiation Oncology Core (IROC) OSLD audit.

At inception, the ACDS performed the Level I audit using thermoluminescent dosimeters (TLDs). Following twelve months of investigations, the ACDS adopted an Optically Stimulated Luminescent Detector (OSLD) on 1 June 2012 for its Level I audit.

Audit measurements

For the ACDS Level I audit, OSL detectors are mailed to the facility enclosed in PMMA blocks. Each photon beam within the facility being audited has a designated block with two OSLDs positioned side by side at approximately dmax. The photon block is placed on a provided platform to reduce scatter effects from the table. The platform is not necessary for electrons as the electron block provides full scatter. The electron block consists of three stacked outer rings and a central acrylic plug containing the OSLD.

OSLD audit kit
Figure 1: The OSLD audit kit with the electron scatter block in the foreground. The photon support platform is shown assembled at the back right. The full audit instructions can be downloaded below.
For both photon and electron audits the facility delivers 100 MU to the OSLD. For each energy, the facility states the absorbed dose to water that would have been delivered to the dose output specification point (DOSP) if the 100 monitor units had been delivered to a water phantom under the facility’s reference conditions. This determination should include the daily output variation, as determined by the facility’s routine protocol for weekly/monthly output checks. At the time of measurements a facility representative should complete, sign and date the ACDS form for this, which is included in the audit kit. The ACDS places no requirement on determination of the stated dose output. However to achieve the best possible comparison it is recommended the measurements be performed with an ionization chamber and ultimately be traceable to the local reference dosimetry.

The irradiated OSLDs are couriered back to the ACDS in the satchel provided and are read out between one week and one month post-irradiation. After evaluation, the participating centre will be provided with a written report detailing the difference between the beam output/quality stated for each beam compared to the ACDS measured beam output/quality. The full audit details can be downloaded below.

ACDS has produced two instructional videos to assist facility staff prepare for OSLD Level I Audits.

Photon beams OSLD Level I audit

Electron beams OSLD Level I audit

ACDS audit Level I documentation