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September 2, 2015 | Direct dosimeter calibration for greater precision

CEA LIST laboratory LNHB (Laboratoire national Henri-Becquerel) now offers the first-ever medium-energy X-ray photon calibration method based on the absorbed dose to water, slashing measurement uncertainty threefold.

Current reference dosimetry measurements for medium-energy X-rays (80 kV to 300 kV) used in radiotherapy are generally stated as air kerma. The absorbed dose to water—the reference unit for radiotherapy—is extrapolated from the air kerma measurement, with typical uncertainty levels of 1.9% to 3.6%.

Calorimètre pour la mesure absolue de la dose absorbée dans l’eau en radiothérapieCalorimeter for the absolute measurement of absorbed dose to water in radiotherapy

Water calorimeter developed by LNHB for calibration

LNHB designed and built a water calorimeter that reduced uncertainty to less than 0.8%. This primary instrument, which takes measurements at a depth of 2 cm as required by the IAEA protocol, can be used for the direct calibration of radiotherapy dosimeters..

The device was first used by the Institut de Cancérologie de l’Ouest René Gauducheau, in Nantes, France to calibrate an ionization chamber; a calibration certificate was granted for three medium-energy X-ray beams.

Because the calibration coefficient for this type of chamber varies by more than 10% for the energy range considered, depending on beam characteristics, having the LNE(France’s national measurement and testing lab)-LNHB primary reference measurements is crucial. This latest advance will position LNHB to offer calibration services to medical physicists across France.