Dosimetry and Medical Radiation Physics
The IAEA/WHO SSDL Network
Guidelines for Member States on the Designation of SSDLs
These guidelines were first published under the title "Guidelines for Member States Concerning Radiation Measurements Standards and Secondary Standard Dosimetry Laboratories" and is available from the Network Secretariat
1. Introduction
In the early nineteen-sixties an acute need developed for higher dosimetric accuracy in radiation therapy, particularly in developing countries. This need led to the establishment of a number of dosimetry laboratories around the world, specialising in the calibration of radiation measuring instruments and dosimeters.
In order to co-ordinate the provision of guidance and assistance to such laboratories, the International Atomic Energy Agency (IAEA) and the World Health Organization (WHO) set up a Network of Secondary Standard Dosimetry Laboratories (SSDLs) under their joint aegis. The present guidelines deal with the functions and status of SSDLs, in particular with the need for recognition and support by the competent national authorities.
1.1. The need for measurement standards
The highest possible accuracy is needed in radiation therapy, where success or failure of the treatment is at stake. In radiation protection measurements lower accuracy is acceptable, particularly in environmental monitoring. However, when dosimeters are used to determine doses received by individuals under working conditions, such measurements need to be traceable though an unbroken chain of comparisons to national and international standards. Such traceability is needed to ensure accuracy and reliability, and also on account of legal and economic implications. This aspect is of acute relevance as it may be expected that many SSDLs will become engaged in dosimetric measurements of persons and the environment, and in instrument calibrations in connection with unintentional radiation exposures.
Traceability and confidence in known levels of measurement are also required for radiation processing facilities , such as those used in sterilising pharmaceutical products and the treatment of food.
1.2. SSDLs and metrology
The prime function of an SSDL is to provide a service in metrology. As holder of a secondary standard instrument, it provides an essential link to the international measurement system which is itself based on the comparison of standards held by primary standards laboratories under the aegis of the International Bureau of Weights and Measures (BIPM). The secondary standard may constitute a country’s national standard (for a particular quantity), and the laboratory may be part of a larger metrology organization. The functions and status of a particular SSDL are determined by national or local circumstances but, in all cases, necessary recognition and support by the competent national authorities are crucial to the success of the SSDL in practice. Indeed, such support is a prerequisite for holding full membership in the SSDL Network.
At present, two types of SSDLs exist. Firstly, those laboratories that fulfil a nationwide metrological function based on traceability to approved measurement standards. This includes the provision of certified calibrations for instruments used in radiation therapy and other fields. Secondly, those calibration laboratories that take care of a particular radiation therapy centre, or group of such centres, without a formal national mandate. Clearly, the latter type of SSDL must also possess a calibrated secondary standard instrument, though it may not have been designated as the national measurement standard. It is anticipated that the latter type of SSDL may evolve into the former type, though both large and small laboratories are of equal concern to the Secretariat of the IAEA/WHO Network of SSDLs, provided they are operational, participate in dose intercomparison measurements organized by the network secretariat, and their official status is recognized and supported by the competent national authorities.
2. Principles and Recommendations
2.1. Measurement standards
It is a basic principle of metrology that measurements of physical quantities should be traceable to approved measurement standards, thus providing assurance that the accuracy of measurements is adequate for the purpose.
Every country in which ionizing radiation is used should either maintain a national measurement standard, which may be a primary or a secondary standard, for each relevant quantity, or make arrangements for ready access to such standards established and maintained in another country, or the IAEA/WHO Network of SSDLs, for the calibration of relevant instruments.
National measurement standards may be maintained by a primary standard dosimetry laboratory (PSDL), or if no PSDL exists, in an SSDL. Such national standards may be calibrated at the International Bureau of Weights and Measures (BIPM).
For a particular country, there should be only one national measurement standard for a given quantity, and this should be recognized in a regulatory form by the competent national authorities. This standard should be compared periodically with other national standards forming part of the international measurement system under the aegis of the BIPM.
The competent national authorities may designate one SSDL as the holder of the national measurement standard for a specified quantity. If a county has more than one SSDL, the working standards of the other SSDL must be traceable to the national standard, and it is recommended that the SSDLs should be grouped into a national SSDL organization.
2.2. SSDLs and the Network
The competent national authorities may nominate a single SSDL, or an SSDL organization, for participation in the SSDL Network. Establishment of an SSDL organization allows a country to have as many dosimetry laboratories as are deemed necessary or desirable. Because of the metrological nature of the work of SSDLs, it is essential that any SSDL be legally identifiable, and it is preferable that the SSDL organization be linked to the national Metrology Office.
If, in a country, the establishment of an SSDL organization is not practicable, the IAEA/WHO Network of SSDLs may accept, upon request from the competent national authorities, more than one SSDL for participation in the Network. Such arrangements are, however, exceptional and for a limited period only.
National recognition and support of an SSDL are prerequisite for participation in the SSDL Network. However, such participation does not depend on the designation of the secondary standard held at the laboratory as a national standard.
Participation in the SSDL Network does not constitute a prerequisite for obtaining assistance through the IAEA Technical Co-operation programme. The provision of such assistance will be based on a request from the competent national authorities and will take into account the priorities set by them, within the limits of resources available for the implementation of that programme.