ARPANSA - Australian Radiation Protection and Nuclear Safety Agency

ARPANSA has two electron linear accelerators (linacs) - an Elekta Synergy Platform and a refurbished Vickers (Radiation Dynamics)

Elekta Synergy Platorm

An Elekta Synergy Platform medical radiotherapy treatment linac was installed in October 2008 in order to develop absorbed dose standards at megavoltage energies and to provide direct calibration of reference ionisation chamber dosemeters.

The medical standards linac will be able to provide direct calibration of therapy reference dosemeters at megavoltage beam qualities close to those used by radiotherapy treatment centres. These reference dosemeters are used in the centres to calibrate treatment linac beams in terms of monitor unit (MU) outputs. Up to seven X-ray photon beams (between 4 and 25 MV) and ten electron beams (between 4 and 22 MeV) are available. A set of three photon beams (6, 10 and 18 MV) have been selected to best match the range of treatment linacs used in Australia. A dose scanning water tank is used to establish depth dose and profile characteristics of the beams (see Figure 1) which are compared with Monte Carlo modeling calculations to ensure relevant reference conditions.

Absolute absorbed dose rates from the ARPANSA medical standards linear accelerator are determined from measurements with the ARPANSA absorbed dose primary standard graphite calorimeter. Figure 2 shows the linac beam undergoing absolute measurement of the output dose. Two graphite calorimeters (one is on loan from the IAEA) are on the recently installed patient support table. The third device at far left, is a graphite phantom mounted with a transfer standard ionization chamber for absorbed dose to graphite measurements. This is an intermediate stage to deriving absorbed dose to water required for reference dosemeter calibration.

Other triads of photon beams are matched to linacs in other national standards laboratories (the National Physical Laboratory (UK) uses 6, 10 and 15 MV), in order to undertake international comparisons that will provide international traceability to the SI. Other sets will be used for research activities.

Direct calibrations for megavoltage photons and electrons

• Therapy chambers will be calibrated using beams similar to those in the clinic. Correction factors for ⁶⁰Co are not required, and the process of calibrating linear accelerator output will be more accurate and simpler.
• ⁶⁰Co calibration coefficients will continue to be provided once the service is running. However, now the energy correction factor (“k_Q”) can be measured for each ionisation chamber instead of using a generic value calculated for a particular chamber type. The measured k_Q will reduce the uncertainty of linac output measurements.
• For electron beams, a direct calibration will also simplify the calibration process in the clinic because a cross- calibration is not required – parallel plate chambers can be calibrated directly at the energies needed.
• The service is expected to be available in 2011.

Postal TLD audit for megavoltage photon and electron beams

The Level I postal audit is a simple, independent check of accelerator output. By extending the scope of the audit to include electron beams, the audit will cover all external megavoltage treatments in Australia.

TLD’s are irradiated in a water phantom at a radiotherapy clinic and evaluated at ARPANSA to obtain an independent check of the dose (nominally 2 Gy) and/or beam quality (TPR20,10 or D20,10) for a linear accelerator. Currently available only for photon beams, with the aid of the medical standards linac, the audit could be extended to electrons in 2011.

With implementation of the Australian Clinical Dosimetry Service due to start early in 2011, the postal TLD audit service will be carried out under its auspices and will be now fully funded. While the service is still voluntary, there will be no cost to the participating centre.

Research Opportunities

The medical standards accelerator is available for research use where the benefit of the research is improved medical use of radiation. Please contact the megavoltage dosimetry team to discuss your project.

Vickers (Radiation Dynamics) electron linear accelerator

ARPANSA has a refurbished Vickers (Radiation Dynamics) electron linear accelerator similar to those in use at the NPL in the UK and the National Research Council of Canada (NRCC). Its primary use is to provide high dose/dose-rate irradiations.

The research linac's electron beam energy can be varied continuously between 7 and 21 MeV. Pulse repetition frequencies are available from 3 to 400 pulses per second. There are two beam lines, one for high dose and dose-rate irradiation applications with electron beams, including a single shot mode available for pulse radiolysis available under a MoU with AINSE, and the other for energy analysis and dosimetry measurements.

For further information, contact:

Dr Duncan Butler
Megavoltage Dosimetry Group
ARPANSA
619 Lower Plenty Road
Yallambie VIC 3085

Ph: +61 3 9433 2274
Email: duncan.butler@arpansa.gov.au

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