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Calibration of instruments


Reference dosimetry for external beam radiotherapy is carried out using an ionization chamber connected to an electrometer (dosimeter). The calibration of these dosimeters in terms of air kerma (for kV X-rays) or absorbed dose to water must be traceable to a Primary Standard such as a free air chamber or a calorimeter respectively before they can be used clinically. The calibration coefficient for the dosimeter has units of Gy/C. The dosimeter can be either calibrated at a Primary Standards Dosimetry Laboratory (PSDL), a Secondary Standards Dosimetry Laboratory (SSDL), such as the IAEA Dosimetry Laboratory, or cross-calibrated in a clinic against a secondary standard dosimeter.

The use of a dosimeter having a calibration coefficient for Cobalt-60 gamma rays is a common characteristic of all dosimetry protocols and codes of practice for high energy photons and electrons. These documents, introduced by national and international organizations such as IAEA TRS 398, provide medical physicists with a systematic approach to the dosimetry of external radiotherapy beams.

Important Principles

Calibrations are performed by substitution method in the PSDL or SSDL. Using this method the air kerma/absorbed dose to water is determined at a point using a Primary or Secondary standard. The ionization chamber to be calibrated is then placed at the same point as the standard and the response of the dosimeter is determined. The calibration of the dosimeter is only valid for the reference conditions and beam qualities where it was calibrated. Any difference from these conditions must be taken into account in the clinic such as depth in water, field size, temperature and pressure, humidity, ion recombination and polarity.

It is important that the dosimeter is calibrated for the range of beam qualities that it will be used clinically. For example, a dosimeter used for radiotherapy should not be calibrated in diagnostic or protection qualities.

Calibration of the dosimeters should be performed regularly and there should be a quality control system to ensure that the dosimeters are working correctly and are in good order.


Primary Standards Dosimetry Laboratory (PSDL): A national standardizing laboratory designated by the government for the purpose of developing, maintaining and improving primary standards in radiation dosimetry.

Secondary Standards Dosimetry Laboratory (SSDL): A dosimetry laboratory designated by the competent authorities to provide calibration services, and which is equipped with at least one secondary standard that has been calibrated against a primary standard.

Primary standard: An instrument of the highest metrological quality that permits determination of the unit of a quantity from its definition, the accuracy of which has been verified by comparison with the comparable standards of other institutions at the same level.

Secondary standard: An instrument calibrated by comparison with a primary standard.

Introduction to References

IAEA TRS 469 provides a systematic and standardized approach to the calibration of reference dosimeters used in external beam radiotherapy by SSDLs. The Appendix to the IAEA calibration certificate describes procedures for calibrations at the IAEA Dosimetry Laboratory.