IAEA-WHO cobalt-60 teletherapy dosimetry service using mailed LiF dosemeters. A survey of results obtaining during 1970-75.

The IAEA-WHO postal dose comparison service for cobalt-60 teletherapy dosimetry is an ongoing programme aimed at improving dosimetric accuracy in IAEA-WHO Member States. LiF dosemeters were irradiated under specified conditions in a water phantom at the participating radiotherapy centres and returned to the IAEA's Dosimetry Labotatory for evaluation. The participants filled in a data sheet giving numerical values of physical factors (such as per cent depth dose, tissue-air ratio, etc.) that they employed. Nearly 65% of the comparisons showed deviations within +/- 5%, a figure which is believed to represent the overall uncertainty of the method itself. Almost 10% of the measurements deviated more than +/- 15%. It appears that participants applying 'in phantom' output measurements have better control over the dose they administer. Many of the errors observed are due to the use of faulty values for the various physical factors employed, and to the fact that often a check of the output just before the irradiation of the LiF dosemeters was not carried out.