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Getting the dose right

From the IAEA Division of Human Health, Dosimetry and Medical Radiation Physics Section

The IAEA/WHO dosimetry audit service

For cancer patients at a hospital in Costa Rica in 1996, radiotherapy was a source of hope that their tumours might be eradicated. When they were treated with an incorrectly calibrated machine, they received higher doses of radiation that damaged their health instead of treating their cancer. A total of 115 patients were overdosed, killing 42 within a year and leaving the rest with lasting injuries.

Radiation beams produced by radiotherapy machines need to be calibrated, since the quality and effectiveness of the medical radiation therapy rely on their accuracy. Precise measurement of the dose (or 'dosimetry') is crucial for this calibration.

Thermoluminescent dosimeters (TLDs) and documents provided to participating centres in the IAEA/WHO TLD postal dose audit programme. (Photo from IAEA TLD irradiation video tutorial E. Izewski, IAEA)

The IAEA Dosimetry Laboratory offers a dose audit programme that can help participating Member States to regularly check their radiotherapy facilities, providing feedback on their quality procedures and alerting the centre in case of a problem, before large numbers of patients suffer the consequences of undiscovered errors.

The IAEA's dose audit programme, run jointly with the World Health Organization (WHO) since 1969, is free of charge. Small dose measuring devices called thermoluminescent dosimeters (TLDs) are sent to the requesting centre and a predefined radiation dose is delivered to them as it would be for a patient.

The TLD is then mailed back to the IAEA Dosimetry Laboratory, where it is used to compare the radiation dose the hospital intended to give with the dose it actually gave.

To prevent errors in dosimetry from causing radiation injuries, the Dosimetry Laboratory performs a follow-up procedure if the TLD identifies errors. When a TLD audit detects discrepancies, the centre is alerted accordingly, and asked to repeat the test. If the second TLD audit does not show good results, the follow-up programme ensures the hospital knows how to fix the problem and can do so effectively. Targeted expert missions and more comprehensive medical physics reviews can also be provided upon request to address cases of non-compliance that might be revealed by a TLD audit process.

Medical physicists performing TLD dose measurement. (Photo from IAEA TLD irradiation video tutorial, E. Izewski, IAEA)

Since it became operational, the IAEA/WHO TLD audit programme has revealed a large number of poorly calibrated radiation sources. The records of TLD audit results have been showing a steady increase in hospitals' abilities to get the dose right. More and more low and middle income countries now possess radiation oncology clinics. Most of the over 12,000 radiotherapy beams checked since the inception of the IAEA/WHO TLD programme have been in these low and middle income countries. Since these countries may not be able to afford establishing and maintaining a national audit system, the IAEA/ WHO TLD audit programme offers them this service. Without TLD audits, the concerned Member States would not be able to guarantee accurate calibration of their radiotherapy facilities.

The IAEA/WHO TLD audit service has contributed to improvements in the quality of radiotherapy dosimetry practices worldwide. In over 2,200 radiation oncology clinics in 132 countries that use the TLD service, both clinicians and patients can trust that the radiation beam calibration is correct, leading to reliable and effective treatment.