Diagnostic Reference Levels are values which are usually easy to measure and have a direct link with patient doses. They are therefore established to aid efficient dose management and to optimize patient doses. If such doses are found to exceed the corresponding reference dose, possible causes should be investigated and corrective action taken accordingly, unless the unusually high doses could be clinically justified.
The ICRP publications recommended that values should be determined by
professional medical bodies, reviewed at intervals that represent a
compromise between the necessary stability and the long-term changes in
observed dose distributions and be specific to a country or region. The
concept of Diagnostic Reference Level is beginning to be a well-defined
tool in many countries and is used to reduce patient dose during
medical interventions and examinations.
The aim of this article is to present the status of the different concepts of Diagnostic Reference Levels in Europe in the following countries:
The methods used to establish reference levels for medical examination and interventions and to enforce them in surgeries and hospitals as well as training developed for the medical staff are presented. This article also gives information on the periodicity and the methods used to update the DRLs as well as on the future outlook.
MEDICAL APPLICATIONS FOR WHICH DRLs ARE DEFINED
In France, Diagnostic Reference Levels are established for 21 x-ray examinations and for 10 nuclear medicine examinations. The levels apply to radiography examinations (fluoroscopy is excluded) of standard-sized adult patients. Examination for which DRLs have been proposed include:
- 9 types conventional x-ray including mamography on adult patients
- 2 types of conventionnal x-ray (thorax and pelvis) for children 0 to 1 5 years old
- 7 types of conventionnal x-ray for children 5 years old
- 4 types of CT examination on adult patients
- 10 nuclear medicine examinations including 18F-PET
In Germany, Diagnostic Reference Levels are established for x-ray and nuclear medicine examinations. In particular DRLs are established for:
- 12 types of radiograph for adult patients
- 5 types of radiography/fluoroscopy examinations for adult patients
- 7 types of CT examination for adult patients
- 2 types of fluoroscopically-guided interventional procedure for adult patients
- 6 types of radiograph for paediatric patients (2-5 years old)
- 1 type of radiography/fluoroscopy examination for paediatric patients (4 years old)
- 17 types of diagnostic nuclear medicine procedures for adult patients and conversion factors for children
The requirement for the establishment and application of Diagnostic Reference Levels is imposed by the Greek Radiation Protection Regulations. The Greek Atomic Energy Commission (GAEC) as the national authority for radiation protection, is responsible for the establishment and enforcement of the national DRLs. DRL values for mammography and 12 types of nuclear medicine examinations have already been approved by GAECs board. DRL values for 7 types of computed tomography examinations are in the process of approval, while DRLs for 10 conventional radiography and for fluoroscopy examinations are expected to be determined in the near future.
In Italy, Diagnostic Reference Levels are established and applied to:
- 7 types of conventional x-ray on adult patients
- 4 types pf conventional x-ray on infant patients (≤ 5 years old)
- 1 type of mammography examination
- 4 types of CT-examinations on adult patients
- 48 types of diagnostic nuclear medicine procedures on adult patients and, based on scaled values taking into account the body mass, on pediatric patients
The Decree on Radiation Protection of 2001 stipulates that the Minister of Health, Welfare and Sport shall promote the establishment and use of DRLs, but it has not lead to the implementation of DRLs in the Netherlands yet.
In Sweden, Diagnostic Reference Levels are established for 12 x-ray examinations and for 19 nuclear medicine examinations. The levels apply to complete examinations of standard-sized adult patients. Examination for which DRLs have been established include:
- 6 types conventional x-ray on adult patients
- 4 types of CT examination on adult patients
- 2 types of mammography examination
- 19 nuclear medicine examinations
In Switzerland, Diagnostic Reference Levels are applied to conventional radiology, interventional radiological procedures, computer tomography and nuclear medicine, for adult, and in many cases also for infant, patients. DRLs are established for:
- 9 types of conventional x-ray on adult patients
- 1 type of mammography examination
- 8 types of interventional procedures in radiology on adult patients
- 4 types of interventional procedures in cardiology on adult patients
- 8 types of CT examination on adult patients
- 4 types of CT examination on infant patients
- 47 types of diagnostic nuclear medicine procedure on adult patients and infant patients
A Department of Health DRL Working Party has been set up in the UK to formally adopt national DRLs in compliance with the requirements of the Ionising Radiation (Medical Exposure) Regulations 2000. The Working Party will consider proposals for DRLs from relevant professional groups and organisations (primarily NRPB/HPA and ARSAC) based on published patient dose data from UK national surveys. Medical applications for which DRLs had been proposed by 2005 include:
- 13 types of individual radiograph on adult patients
- 15 types of radiography/fluoroscopy examination on adult patients
- 12 types of CT examination on adult patients
- 5 types of fluoroscopically-guided interventional procedure on adult patients
- 3 types of radiography/fluoroscopy examination on paediatric patients (5 years old)
- 2 types of CT examination on paediatric patients (3 years old)
- 96 types of diagnostic nuclear medicine procedure on adult patients
METHODS AND MEANS USED TO DETERMINE THE DRLs
The first step consisted of making a list of the most common radiological procedures and in writing down the corresponding standardised protocols with the French Society of Radiology (SFR) , the Institute of Radiation protection and Nuclear Safety (IRSN) and ASN. On the basis of protocols and data sheets established with the French Society of Medical Physics (SFPM). TLD measurements (entrance dose) and examinations data (parameters or Dose length product) were measured, recorded or calculated. The data were collected in 24 volunteer centers and 8 examinations have been selected: 4 in conventional radiology and 4 in computed tomography. Mean dose values and third quartile values were determined for approximately 1300 patients in conventionnal radiology and 600 in CT. In conventionnal radiology, it was first concluded that the DRLs proposed by the European Commission can be applied in conventionnal radiology but for CT the European DRLs can be lowered. For nuclear medicine, the value of activity recommended in the marketing autorization for radiophamaceuticals was choosen as first value for the reference levels.
The initial values of the German DRLs in diagnostic radiology were proposed by an expert group of physicians and medical physicists chaired by the Federal Office for Radiation Protection (BfS), including representatives of the professional medical societies. For radiographs of adult patients, the European DRLs were adopted accordingly. For fluoroscopy examinations, a restricted survey of current practices in university hospitals, and for CT examinations, a national survey of CT practice performed in 1999 were used to derive the DRLs. For diagnostic nuclear medicine procedures, BfS had proposed national DRLs based on the results of a national survey on frequencies and administered activities in diagnostic nuclear medicine, on recommendations of national and international societies and on proposals for DRLs in other countries. The BfS proposal was finally discussed with members of the German Radiation Protection Commission (SSK). The quantities used to express the DRLs are:
- Dose-area-product (DAP) for conventional x-ray examinations (for radiographs, the entrance surface air Kerma (ESAK) and entrance surface dose (ESD) can be used alternatively)
- Computed tomography dose index (CTDIVol) and dose-length product (DLP) for computed tomography
- Entrance surface dose (ESD) for mammography
- Administered activity for nuclear medicine
The determination of DRLs is based on the data collected during the on-site inspections performed by GAEC in radiology and nuclear medicine laboratories. The on-site inspections are carried out as a part of the licensing procedure of the laboratories every 2 years for nuclear medicine and 5 years for radiology laboratories respectively. As it concerns the radiological examinations, adequate dosimetric measurements are performed for the different types of examinations performed, while for nuclear medicine examinations the administered activities for each diagnostic procedure are considered as the appropriate quantity. The DRL for each examination is determined as the rounded 3rd quartile value of the distribution of the corresponding dosimetric or activity values registered. More specifically, the quantities used to express DRLs are:
- Entrance surface dose (ESD) for conventional x-ray
- Computed tomography dose index (CTDI) for computed tomography
- Entrance surface dose (ESD) and Average glandular dose (AGD) for mammography, and
- Administered activity for nuclear medicine examinations
The values of the DRLs were established on the basis of a survey of data reported in the literature, with particular regard to Guidelines published by the EC. The quantities used for the DRLs are:
- Entrance skin dose for conventional x-ray examinations and mammography
- Dose length product (DLP) and weighted computed tomography dose index (CTDIw) for computed tomography
- Administered activity for diagnostic nuclear medicine
For all examinations for which a DRL exists, hospitals have to determine the dose or administered activity for a standard sized patient, whose values are compared with the corresponding DRL. If the level is exceeded actions have to be taken in order to reduce the dose.
The present DRLs were determined by studying the radiation dose levels in hospitals. A national survey of doses for x-ray examinations was carried out in 1999. For nuclear medicine examinations the dose situation was roughly known from the nominal administered activities that have been reported each year. The DRLs have been established on the basis of the resulting dose distributions. The quantities used for the DRLs are:
- Dose-area-product for conventional x-ray examinations
- Dose-length-product and the volume computed tomography dose index for computed tomography
- Mean glandular dose for mammography and
- Administered activity for nuclear medicine
For all examinations for which a DRL exists hospitals have to determine the radiation dose or administered activity for a standard sized patient. This standard dose or administered activity is compared with the corresponding DRL - if the level is exceeded actions have to be taken in order to reduce the dose, if possible.
The method adopted to determine the Diagnostic Reference Levels (DRLs) varied according to the modality. In 2002, Switzerland took part in a Europe-wide survey on computed tomography (CT). In this case, data from Swiss hospitals were used to establish the DRLs in the CT area. In the following years 2003 and 2004 the Institute of Applied Radiophysics (IRA) was commissioned by the Swiss Federal Office of Public Health (SFOPH) to study high-dose applications in interventional radiology and cardiology. For nuclear medicine, Basel University Hospital was commissioned by the SFOPH in 2004 to conduct a nationwide survey of administered activities. For conventional radiography, the SFOPH adopted the values recommended by the European Commission. A programme currently under way is designed to provide a broader basis for the DRLs in interventional radiology and cardiology. While the DRLs in this area have previously only been based on data obtained from university hospitals, a representative selection of all the Swiss centres where such procedures are performed is now being taken into account. It will be interesting to note any difference that may emerge, e.g. how patients exposure levels are influenced by factors such as investigation frequency or operators experience.
For x-ray imaging procedures, DRLs are based on national surveys of patient doses conducted by NRPB/HPA or the National Health Service Breast Screening Programme (for mammography). National reference doses are set at the rounded 3rd quartile values of the distribution of mean doses seen on representative samples of patients at each hospital in large national surveys. For diagnostic nuclear medicine procedures, national DRLs are based on DRLs recommended by the Department of Healths Administration of Radioactive Substances Advisory Committee (ARSAC).
The quantities used to express the DRLs are:
- Entrance surface dose (ESD) and dose-area-product (DAP) for conventional x-ray examinations
- Computed tomography dose index (CTDI) and dose-length product (DLP) for computed tomography
- Mean glandular dose for mammography
- Administered activity for nuclear medicine
WAYS AND MEANS USED TO ENFORCE THE DRLs IN SURGERIES AND HOSPITALS
The DRLs are set in the ministerial order of 12 February 2004 as a part of the tranposition into French regulation of the European directive 97/43/Euratom. According to this order, each radiologist or nuclear medicine practitionner must evaluate every year for 20 standard patients (or on an anthropomorphic phantom) and for 2 types of procedures defined in the order, the parameter choosen for quantifyng DRLs (Entrance skin bose, dose length product or activity). The procedures must be different every year and the data must be sent to IRSN, who is in charge of data collection and analysis and determine the possible need to change DRLs.
With the amendment of the Radiation Protection Ordinance (Strahlenschutzverordnung, StrlSchV) in 2001 and the X-Ray Ordinance (Röntgenverordnung, RöV) in 2002, the requirements of the European Directive 97/43/Euratom were adopted into German legislation. StrlSchV and RöV demand that DRLs, to be established and published by the BfS, have to be considered for X-ray and nuclear medicine examinations of humans. The so called Ärztliche Stellen (ÄS), Medical Authorities which are already involved in the process of quality control concerning image quality and compliance with the guidelines of the Federal Medical Board, must check compliance of the average patient exposure in the various radiological installations with the DRLs. The normal control period is about 2 years. If the ÄS find that the DRLs are exceeded without medical justification, they give advice for optimization and reduction of patient doses. In this case, the control period is shortened to about half a year. According to StrlSchV and RöV, the ÄS are obliged to report any consistent, unjustified exceeding of DRLs to the competent Authorities of the Bundesländer (German Federal States).
The Greek Radiation Protection Regulations require that the medical physicists employed as Radiation Protection Experts (RPE) in radiology and nuclear medicine departments are responsible for organising and running adequate programmes for the determination of local reference levels for each type of examination performed. These levels must be compared to the national DRL values and if required adequate measures must be taken for the further reduction of patient doses.
The DRLs were set in the Legislative Decree n. 187 of 26 May 2000, that implemented in the Italian law the European Directive 97/43/Euratom. According to this Decree, each Radiological or Nuclear Medicine Department must set up a suitable quality control programme, aimed at the optimisation of the procedures. Moreover, the doses delivered to patients in each procedure must be evaluated every two years, checking their compliance with the DRL. All the personnel engaged in the use of ionising radiation for medical purposes must participate every five years to a refresher course on radiation protection, with special regard to the exposure of the patient.
Diagnostic Reference Levels were implemented into the national regulations in 2002. The determination of standard doses and administered activities is mandatory according to these regulations and have to be determined for the first round within two years. The national authority can require the reporting of the determined standard doses at any time, and did so for the first round of measurements where detailed data on the level of the individual patient were asked for. Normally the determination of standard doses is also checked in connection with inspections.
The DRL system is also being taken into consideration in the current revision of radiological protection legislation. A special article is to be established, requiring users to review and optimize exposure levels in relation to the DRLs. The applicable DRLs are published in Directives of the Swiss Federal Office of Public Health.
The Ionising Radiation (Medical Exposure) Regulations 2000 require all hospitals, surgeries, etc that carry out medical exposures to develop written procedures for the establishment, use of and adherence to DRLs. Further guidance on how to do this is provided by IPEM Report 88, 2004, Guidance on the Establishment and Use of DRLs for Medical x-ray Examinations. There is also a requirement in the Ionising Radiations Regulations, 1999, for every hospital, surgery, etc to provide a suitable quality assurance programme for all equipment used for medical exposures, which should include periodic assessments of representative doses to patients (patient dose audits). Guidance on how to comply with this requirement, including the DRL-related levels of patient dose above which remedial action should be taken, is given in IPEM Report 91, 2005, - Recommended Standards for the Routine Performance Testing of Diagnostic X-ray Imaging Systems. These two sets of regulations and guidance provide the main framework for the implementation and enforcement of DRLs in the UK. The IPEM guidance documents were prepared by joint working parties comprising representatives of the Institute of Physics and Engineering in Medicine, the National Radiological Protection Board (now the Radiation Protection Division of the HPA), the College of Radiographers, the Royal College of Radiologists and the British Institute of Radiology.
TRAINING, INFORMATION AND PUBLICATIONS ON DRLs DEVELOPED FOR MEDICAL STAFF
Training courses were organized along with the guidance on how to determine the standard doses and administered activity for the medical personal to facilitate the application of the regulation. Dose data recording forms were produced to help collect data.
The DRLs were first published in August 2003. In October 2004, guidelines for the use of DRLs, especially in diagnostic radiology, were issued to the ÄS of the Bundesländer for further distribution to the various radiological installations in their region. A paper "Establishment and application of Diagnostic Reference Levels for nuclear medicine procedures in Germany" has been published in the journal Nuklearmedizin (2004; 43: 79-84) to inform medical staff. A similar publication is being prepared for diagnostic radiology. According to legislation (StrlSchV and RöV) it is the responsibility of BfS to publish the DRLs.
GAEC, as the competent authority on radiation protection issues, organises special courses on the establishment and the implementation of DRLs for personnel in radiology and nuclear medicine departments. Moreover, the RPEs in large hospitals are responsible for providing the required training on the use of DRLs to the medical staff. Also, the importance of the use of DRLs as a radiation protection optimisation tool is also underlined during the on-site inspections of GAEC.
Medical physicists provide local training for radiologists, technicians and every physician (with particular regard to cardiologists and surgeons) engaged in the different uses of ionising radiation for medical purposes.
The regulations are accompanied by guidance on how to determine the standard doses and administered activity. It also gives examples of good radiological practice for the various examinations. In the beginning the authority put a great deal of effort into informing personnel about the concept of DRLs at different national meetings and courses run for the diagnostic community. Personal communications also played an important role in the information process.
Implementation of the DRL concept is promoted by the Swiss Federal Office of Public Health in various ways: users receive training on the concept directly during audits, and information is provided at conferences held by the relevant professional associations; at the same time, training DVDs are made available to users, giving a detailed account of radiological protection for patients and staff. In addition, awareness of the concept is to be raised by the publication of a booklet on this subject.
Medical physicists in the UK provide local training for health service staff and IPEM and BIR have run a number of meetings on the use of DRLs. Training is primarily based on guidance on the establishment and use of DRLs for medical x-ray examinations in IPEM Report 88, 2004. Presentations on the use of DRLs have been given at the UK Radiology Congress and NRPB has published related articles in the British Journal of Radiology and specialist journals and magazines aimed at radiographers. NRPB/HPA also publishes regular reviews of its national patient dose databases which include recommended national reference doses for a wide range of diagnostic and interventional x-ray procedures. The Department of Healths Administration of Radioactive Substances Advisory Committee (ARSAC) publishes notes for guidance on nuclear medicine procedures that include DRLs and are updated at regular intervals.
PERIODICITY AND METHOD USED TO UPDATE THE DRLs
So far, no update of DRLs has been planned but it is expected that progress will be made with the improvement of x-ray machines with respect to their ability to give information on the dose delivered during examinations, and with the improving awareness of practitioners.
There is no definite period for the update of the DRLs. But it is agreed that the DRLs should be updated within about 3 5 years by BfS. It is planned that the BfS will be informed anonymously by the ÄS about the relevant mean patient doses of all radiological installations. After a complete review of patient doses by the ÄS, updated DRLs can be established by the BfS based on the third quartile values of the distributions of the relevant mean patient doses in diagnostic radiology and on the mean activity values in nuclear medicine.
Although the procedure of DRLs establishment has not been completed yet, it is expected that they will be updated on a five year basis, if of course there is a need for that. Their updating will be based on the analysis of the data collected during GAECs on-site inspections.
So far, the DRLs have not been updated. Concerning the DRLs in x-ray procedures, the Italian Association of Medical Physics (AIFM) has formed a few working groups, devoted respectively to conventional procedures and mammography (with special regard to a comparison between the doses in screen-film vs. digital imaging) and CT (with special regard to new MSCT equipment). These groups are collecting data from selected radiological departments, throughout the whole Italian territory.
The DRLs have not been updated yet, but new updated values for nuclear medicine are on their way. Next year the authority is planning to analyze the standard doses reported for x-ray examinations and use this analysis as an input for the revision of the regulation which will probably result in additional examinations included in the concept and in lower values of Diagnostic Reference Levels for the existing ones.
It is envisaged that the DRLs will be updated every 5 to 10 years. The Swiss Federal Office of Public Health also supports solutions involving modern information technologies and networks. At the Bern University Childrens Hospital, an application of this kind has been implemented. Here, all the patient data and radiation doses are stored on a dedicated server and can thus subsequently be used to determine the DRLs. Efforts are also being made to enable a direct graphic comparison dose administered vs reference level to be displayed for CT and interventional procedures. Future RIS and PACS systems should fully exploit these possibilities.
NRPB/HPA publishes five-yearly reviews of its National Patient Dose Database (NPDD) which includes recommended national reference doses for a wide range of diagnostic and interventional x-ray procedures apart from those using CT. A separate database (called PREDICT Patient Radiation Exposure and Dose in CT) is held by NRPB/HPA for CT examinations and is based on national surveys of UK CT practice conducted in 1999 and 2003. The periodic reviews of these two databases (NPDD & PREDICT) comprise the major source of proposed national DRLS for x-ray imaging procedures in the UK. Reviews of NPDD have been published for the five-year periods ending in 1995 and 2000 and the latest review for the 5 years ending in 2005 will be published soon. It is anticipated that the PREDICT database will be updated and reviewed with a similar frequency. National DRLs for diagnostic nuclear medicine procedures are regularly updated by the Department of Healths Administration of Radioactive Substances Advisory Committee (ARSAC). They were last updated in 2006.
ASN will support any initiative aiming at the international harmonisation of any radiation control practice, including the harmonisation of DRLs. However, this practice being rather new, efforts may be placed in priority on the harmonisation of other radiation protection practices that are more generally applied than DRLs.
In the near future another expert meeting concerning DRLs is planned by BfS. Besides the update of the existing DRLs, the possible inclusion of dental x-ray examinations and paediatric CT examinations in the concept will be discussed.
It is of great importance to ensure that the established DRL values are applied properly in all medical laboratories. As it concerns the completion of a full set of DRLs, priority is given to the definition of DRLs for pediatric examinations and interventional procedures.
Concerning the DRLs in x-ray procedures, the Italian Association of Medical Physics (AIFM) has formed a few working groups, devoted respectively to conventional procedures and mammography (with special regard to a comparison between the doses in screen-film vs. digital imaging) and CT (with special regard to new MSCT equipment). These groups are collecting data from selected radiological departments, throughout the whole Italian territory.
In May 2006, the Ministry has organized a meeting with a number of medical professional organizations, in order to exchange information about knowledge and experience with DRLs and to discuss how to start the development and implementation in the Netherlands. It was concluded that the Radiology and Nuclear Medicine Platform of the National Committee on Radiation Dosimetry should play a role in this process. At this moment, the Ministry and the Platform are discussing a project plan for the implementation of DRLs. It is expected that this plan will be finalized early 2007. From that time onwards, the implementation activities will start.
It is recognized that pediatric and interventional examinations should be included in the concept, although both have their difficulties due to varying body size and varying complexity, respectively.
In the near future, the DRLs should be routinely applied in Switzerland whenever ionizing radiation is used in medicine.
Regarding the standardisation of DRLs internationally, in the past the UK has provided a substantial amount of the patient dose data that was used to establish European reference doses for diagnostic radiographic images for adult and paediatric patients and for CT. The UK will continue to participate in the European Study Group developing quality criteria and European DRLs in CT. However, the UK sees little benefit for the optimisation of patient protection in the UK to justify any future attempts to standardise DRLs at a European or even wider international level. To be effective in the UK, DRLs need to be based on current UK radiology practice. In the future it is hoped that our regular five-yearly reviews will be extended to cover other high-dose imaging procedures, particularly in CT and interventional radiology.
Many developments and concepts to collect and use DRLs have already been introduced in France, Germany, Greece, Italy, Sweden, Switzerland and the United Kingdom. In Netherlands it is expected that a plan for the implementation of DRLs will be finalized in 2007 and from that time onwards, the implementation activities will start. The methods used to implement the diagnostic reference levels, to inform and train the medical staff are quite different for each country. The future outlook and the ways DRLs will be developed in these countries are not clearly defined but several projects are well under way. Diagnostic Reference Levels give a direct link to patient doses and are an important tool to perform efficient dose management and to optimize patient doses. Countries should therefore try to develop concepts in order to implement and use diagnostic reference level to ensure patient doses are reduced as much as possible. The directions shown by these countries for the DRLs are quite promising. Regulatory bodies, medical staff as well as patient organizations should invest time in this constantly developing concept to optimize dose to patient in the different fields using ionizing radiation.