- Ottawa, Ontario
Radiation is energy in the form of waves or streams of particles travelling through space or matter. It is often associated with atomic energy and nuclear power, when in fact there are many forms of radiation in our everyday environment. We use or experience non-ionizing radiation all the time―just on the drive home from work there's the radio station we listen to, the cell phone we use to call a friend, the garage door opener we activate on the driveway and even the sunlight coming through the windows. Ionizing radiation is distinguished from non-ionizing types because it not only transmits energy but has the capacity to modify materials at the atomic level.
Radiation therapy is the application of ionizing radiation to the treatment of cancer and has been used since the discovery of x-rays and radioactivity at the end of the 19th century. In radiation therapy, an oncologist prescribes a series of treatments to deliver a known absorbed dose to a patient's tumour;an absorbed dose is defined as the amount of the radiation energy absorbed by an organ or tissue. Medical physicists in cancer centres use calibrated instruments to determine the dose delivered to the patient, traceable to primary standards (these are accurate measurements used in metrology for quantities such as mass, length and time). Experts use water as a substitute for soft tissue and primary standards around the world are designed to accurately measure absorbed doses to water.
In 2021, experts from the National Research Council of Canada (NRC) Metrology Research Centre began the construction of a new primary standard water calorimeter for the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) laboratory, which provides calibrations to cancer centres across Australia and New Zealand. The ARPANSA was looking to develop a new standard in a short timeframe, so they sought the help of the NRC to produce a water calorimeter. A design based on the NRC standard was chosen.
The calorimeter is a complex device that must be operated at 4°C to accurately measure the very small temperature rises produced by the interaction of ionizing radiation beam with the water. For a typical radiation therapy prescription, this is less than 1/1000th of a degree, requiring a very sensitive detection system capable of sub-micro degree measurement. The heart of the calorimeter is a glass vessel filled with hyper-pure water, saturated with hydrogen gas. This provides a very well-controlled medium in which the absorbed dose can be measured accurately.
The calorimeter commissioning at the NRC was completed in July 2022 and shipped to ARPANSA. Then, in September, NRC researchers Claudiu Cojocaru and Malcolm McEwen travelled to Australia for the final testing, and training of the ARPANSA staff who would use the device.
The primary standard was successfully set up in the beam of the linear accelerator (LINAC) in the Primary Standard Dosimetry Laboratory at ARPANSA. This LINAC is exactly the same as those found in cancer centres in both Canada and Australia, which means the measurements with the standard are directly relevant to the end-user applications.
A successful series of measurements was obtained, consistent with the data collected at the NRC during the commissioning phase. The ARPANSA staff can now continue investigations with the primary standard and apply it to the needs of users of ionizing radiation for cancer treatments in Australia.
"The ARPANSA-NRC water calorimeter project has been a very positive experience for us here at ARPANSA," said Dr. Peter Harty, Senior Radiation Scientist at the Australian Radiation Protection and Nuclear Safety Agency. "We appreciated the expertise the NRC was able to provide, with Dr. Malcolm McEwen and Dr. Claudiu Cojocaru being world experts in this field. We look forward to continuing our collaboration with them in designing a future water calorimeter system for electron and proton measurements."
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