The International Atomic Energy Agency (IAEA) started a postal dosimetry service in 1966 using lithium fluoride (LiF) thermoluminescent dosimeters (TLD). The World Health Organization (WHO) joined the programme in 1968. Dosimeters were sent on 29 occasions to primary standards laboratories or to the 'Bureau International des Poids et Measures' to guarantee the traceability to primary standards of absorbed dose. The mean deviation was -0.26 +/- 0.18% (1% SD of the mean). One SD for a single result was about 1%. The number of intercomparisons made during the whole period is about 2,000 from about 700 radiotherapy centres in 89 countries. To date only 60Co machines have been included. The mean deviation of all the results from the centres was -0.25, and 1% SD for a single result 6.7%. About 5% of the centres had deviations larger than 30%, at least on one occasion. It is shown that the accuracy in dosimetry improves considerably for those departments participating more than once in the TL dosimetry service.

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