ARCHIVED - Setting the standard for cancer therapy
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August 01, 2009— Ottawa, Ontario
About 150,000 Canadians are diagnosed with cancer every year, roughly half of whom ultimately receive radiation treatment to destroy their tumours. For newly diagnosed patients, one of the last things on their minds is whether they’re receiving the optimum dose to cure their illness, yet it’s a vitally important part of their treatment.
Canadian hospitals deliver hundreds of thousands of radiation treatments per year using "linac" machines. "The margin for error in radiation therapy is very small," says Dr. Carl Ross, who leads the ionizing radiation standards group at the NRC Institute for National Measurement Standards (NRC-INMS) in Ottawa. "When physicians treat a tumour, they need to know the accuracy of the radiation dose within 5 percent. If the error is larger, it can have serious implications: a patient could receive a dose so low that it provides little benefit, or a dose so high that it causes a lot of damage to healthy tissue."
In Canada, the first step in ensuring the accuracy of radiation therapy involves NRC. "As a standards laboratory, our role is to provide a calibration service to cancer clinics that allows them to determine the output of their linacs," says Dr. Ross. "There are about three dozen cancer clinics across the country that we work with."
Did you know?
The impact of ionizing radiation on human tissue is measured by the "absorbed dose." The SI unit of absorbed dose is the "gray," which equals one joule of energy per kilogram of mass. This unit is named after Dr. Louis Harold Gray, who worked at the Cavendish Laboratory in the University of Cambridge, UK with the Nobel laureate Sir Ernest Rutherford, on the absorption of gamma rays in matter.
"Medical physicists working in these clinics usually calibrate their linacs on an annual basis," adds his colleague, Dr. Malcolm McEwen. "The instrument they use for this purpose — called an ionization chamber — is in turn calibrated by NRC every two years, using a calorimeter."
Improving international consistency
Dr. Ross’s team recently participated in an international program to improve the consistency of radiation therapy around the world. According to Dr. McEwen, this initiative is important because physicians determine the optimal doses for treating cancer based on clinical trials conducted around the world. "If they’re all treating the same type of cancer," he says, "they need to deliver the same dose. But if radiation standards vary between countries, it’s difficult to ensure that patients are getting the best treatment available."
The international initiative was launched by the Bureau International des Poids et Mesures (BIPM) in France, whose mandate is to ensure the worldwide uniformity of measurements and their traceability to the International System of Units (SI). The BIPM has developed a portable graphite calorimeter to measure the radiation beam from a clinical linac. It plans to take this calorimeter to seven national metrology institutes — in Canada, Germany, France, Switzerland, the U.K., the U.S. and Australia — and then compare its results with the radiation standards maintained by each institute.
"When new areas of metrology open up, one of the great challenges is to establish the worldwide system for equivalence and recognition of national standards," says Professor Andrew Wallard, Director of the BIPM. "Accelerator-based dosimetry calibrations are emerging as the technique of choice for hospitals, and national metrology institutes are therefore investing in linear accelerator-based national standards for these measurements."
To kick off its program, BIPM staff visited NRC in June 2009. Preliminary results from the testing showed a high level of consistency between NRC and BIPM measurements, with less than 0.5 percent difference between the two standards. "We were happy to have the BIPM come here because it provides an independent test of how well we're doing and gives us assurance that the calibrations we’re doing for Canadian cancer clinics are correct," says Dr. Ross.
Calculating the right dose
In addition to its role in maintaining radiation standards, NRC has simplified the radiation treatment process. To ensure that only cancerous tissue is destroyed, medical physicists must determine the precise dose of radiation a patient needs. Conventional methods for calculating the dose involve significant approximations, but NRC scientists have developed a much more accurate approach. Their desktop software program considers a patient’s anatomy, the size and depth of the cancer, the type of treatment machine and other key details. The program was licensed for commercial use in 2000 and NRC continues to train medical physicists from Canada and abroad to calculate radiation doses. For more information about the NRC software, see: “Physics finds a better way to target cancer” and “Radiation transport modeling.”
For more information about ionizing radiation standards at NRC-INMS, click here.
For more information about the BIPM, please visit its website.
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