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EURAMET.RI(I)-S7 Interlaboratory Comparison - Measurement of the absorbed dose to water by means of alanin/ESR in the therapeutic dose range for gamma radiation

09.09.2011

The Laboratoire National Henri Becquerel (LNE-LNHB), the National Physical Laboratory (NPL) and the Physikalisch-Technische Bundesanstalt (PTB) have compared their secondary standard measuring systems within the scope of the European imeraPlus project JRP7 "External Beam Cancer Therapy". These standards are based on the identification of radiation-induced free radicals in the amino acid alanine by means of electron spin resonance (ESR).

This trilateral comparison was carried out in such a way that each institute irradiated dosimeters for the others, both in the 60Co reference radiation field and with a photon radiation quality from a medical accelerator (10 MVX or 12 MVX). In turn, each institute had to analyze two sets of detectors which had been irradiated by one of the other institutes. The agreement was that, for each of the two qualities, each time four probes would be irradiated with approx. 5 Gy and four other probes with approx. 10 Gy. The exact value of the applied dose had to lie in a range between 4.5 Gy and 5.5 Gy, or between 9 Gy and 11 Gy, respectively. At first, the values were not communicated to the other institutes, but only to the BIPM as a neutral body. The comparison was aimed at assessing the complete measuring system, consisting of the irradiation support, the detector, the spectrometer and the analytical system. All the results were available to the BIPM in November 2010. The status of the publication is presently "Draft B".

The first analysis showed a rather large deviation (3.4 %) between the value determined by the LNHB and the dose applied by PTB for 60Co. This was, however, attributed to the ambient humidity during the analysis at the LNHB which deviated from the standard conditions. The measurement was repeated, and the agreement was within 1 % for a total uncertainty of 1.7 %. The agreement between PTB and NPL is excellent: the largest deviation between the applied and the determined doses was below 0.3 %. All in all, the comparison can be considered as successful, since the agreement was very good within the uncertainties - apart from the exception mentioned above.

The results of the comparison are compiled in Table 1 below. The column "Quality" indicates the type of radiation used; the columns "Irradiation" and "Analysis" indicate the institutes which irradiated/analyzed the probes. The column "MW ratio" states the weighted mean value (over the 8 different dose values) of the ratio of determined dose to applied dose; u is the relative standard measurement uncertainty, and Δrms is the mean square error. In most cases, the mean square error is clearly lower than the measurement uncertainty.

The report will soon be submitted to Metrologia for publication.

QualityIrradiationEvaluationMW
ratio
u
(ratio)
Δrms
(ratio)
EvaluationMW
ratio
u
(ratio)
Δrms
(ratio)
60CoLNHBNPL0.9960.0090.004PTB1.0090.0060.010
NPLLNHB1.0040.0100.006PTB0.9990.0070.002
PTBLNHB1.0350.0080.034NPL0.9960.0080.004
LNHB
( repeat)
1.0100.0070.010
12 MVXLNHBNPL0.9980.0130.004PTB1.0010.0110.002
10 MVXNPLLNHB1.0090.0110.011PTB1.0030.0090.003
10 MVXPTBLNHB0.9980.0080.006NPL0.9990.0090.005

Table : Results of the EURAMET interlaboratory comparison EURAMET.RI(I)-S7. Explanations can be found in the text.