Assessment of radiation doses for common radiographic examinations: an institutional dose survey.
INTRODUCTION Over the years, the number of patients undergoing radiological examinations has increased tremendously, concomitant with the growing concern regarding the long-term risks of cancer arising from ionising radiation exposure. Consequently, more attention has been focused on the levels of radiation doses received by patients. Large-scale dose surveys have been performed to assess the radiation doses from radiological examinations. A survey carried out in the UK in the early 1980s showed that the average doses from similar radiographic examinations could vary by up to seven times between different hospitals and up to 100 times between hospitals and that radiation doses could vary by up to 100 times between patients.[1] In many instances, the doses were higher than what was necessary to produce an image quality sufficient enough to make a diagnosis. Since the findings of these dose surveys were publicised, more emphasis has been placed on the need to optimise imaging conditions to minimise the risk to patients from radiation exposure.[2] The results of these and similar surveys had formed the basis of recommendations for the levels of dose quantities surveyed. These recommendations were referred to in diverse terms such as guidance levels, exposure guides, guideline doses, reference doses, and in 1996, the International Commission on Radiological Protection (ICRP) introduced the term 'diagnostic reference levels (DRLs)' in their recommendations.[3] Since the publication of this document in 1996, many countries have established national DRLs for various imaging modalities. The latest guidance on DRLs from the ICRP was released in 2017.[4] In 1999, the European Commission issued guidance on the establishment of DRLs in its directive on radiation protection, and these recommendations have been updated regularly since then.[5] DRLs are dose levels for typical imaging examinations that are determined from groups of standard sized patients and these imaging examinations are performed on broadly defined types of equipment. Also, DRLs have been recognised to be an effective means to indicate whether, in routine conditions, the dose to a patient in a specified radiological procedure for medical imaging is unusually high or low. Currently, there are no national DRLs for Singapore or recommended DRLs at our institution. Therefore, we conducted an institutional dose study to determine the typical radiation doses and DRLs for selected imaging examinations or procedures performed in our institution. We also compared our typical radiation dose levels and DRLs with those published for similar imaging practices, which serves to provide a broad indication of our institution's relative performance in terms of radiation dose. Importantly, establishing our baseline dose levels and DRLs will also enable benchmarking with other institutions and the worldwide literature, so that dose improvement processes may be implemented where necessary. METHODS The survey on radiation doses from imaging examinations was reviewed and approved by the hospital's Institutional Review Board. The dose data was collected retrospectively from the examination databases of a picture archiving and communication system (PACS). A sample size of about 100 patients (on average) was used for each selected radiographic examination. As the radiation dose for an X-ray examination is dependent on the size of the patient, it is essential to have some criteria to control the patient size if the number of patients in a sample (from which data are collected) is limited. For this dose survey, we achieved this by using dose data from patients with weights restricted to a range of 50–80 kg. The radiographic examinations or procedures selected for the dose survey were: (a) abdomen anteroposterior (AP); (b) ankle two views; (c) chest posteroanterior (PA); (d) chest lateral (lat); (e) cervical spine AP; (f) cervical spine lat; (g) thoracic spine AP; (h) thoracic spine lat; (i) lumbar spine AP; (j) lumbar spine lat; (k) elbow two views; (l) knee two views; (m) pelvis AP; and (n) shoulder AP and lateral scapula view. After each radiographic examination, the X-ray images will be transferred to PACS in the Digital Imaging and Communications in Medicine (DICOM) format. The DICOM medical image consists of two components: a header and the actual image.[6] The header consists of data that describe the image, the critical data being patient information and the dose quantity recorded for radiation exposure. We have chosen to collect dose data from the selected DICOM studies to perform the dose survey. The dose quantity surveyed should be easily available and preferably the one that directly evaluates the radiation that the patient is exposed to during the examination.[4] In our institution, air kerma–area product (PKA) meters are installed in all digital radiography (DR) rooms, and thus, PKA was the dose quantity collected for this survey. An illustration of PKA measurement is presented in Figure 1. This quantity is quoted in units of Gray square centimetre (Gy cm²); it assesses the amount of radiation (measured in Gray) and the area to which it is delivered (measured in square centimetre).Figure 1: Air kerma–area product PKA measurement.RESULTS Based on the 1992 recommendations of the Institute of Physical Sciences in Medicine/National Radiological Protection Board/College of Radiographers, a survey for a radiographic examination in an X-ray facility should generally involve collection of data on the dose quantity for at least 20 patients to achieve sufficient accuracy in the results.[7] In this dose survey, the average sample size for the various examinations was 109. Age of the selected patients ranged from 17 to 94 years. The results of the dose survey and the 75th percentile values of the dose distributions are presented in Table 1. Data from a dose survey could be used to develop institution DRL for a group of examination rooms or imaging facilities. In surveys where several examination rooms or facilities are involved, having the institution DRL value fixed at the 75th percentile of the dose distribution will help us to consider if more attention is required for optimisation of the radiation dose and to identify which X-ray units require this optimisation.Table 1: Median and 75th percentile doses.DISCUSSION In the current study, PKA was the ideal dose quantity collected, especially for radiography and fluoroscopy, as all the X-rays incident on the patient can be assessed by the PKA meter. The assumption is that the X-ray beam to the patient will be collimated aptly. Since both the X-ray beam size and the air kerma will determine the values of PKA, all factors that could influence patient dose during an examination have been considered. In an X-ray system installed with a PKA meter or where PKA is calculated, this dose quantity is easily available. As the PKA values are based on actual radiographic projections, these data from imaging examinations will provide DRL quantities that are closely related to clinical practice. However, we should note that if the X-ray beam passes through the patient table before exposing the patient, the PKA values will be influenced by beam attenuation by the table. Although the PKA values (evaluated by the meter or calculated by the system) reported in the DICOM header are readily available, we should always verify the accuracy of these values before a survey. The values of PKA may be considerably higher than those appearing on the DICOM header unless these values have been verified periodically. The ICRP has recommended that a procedure should be set up to calibrate the PKA meters and verify the accuracy of PKA displayed by the imaging system, before the readings are recorded in the DICOM radiation dose structured report.[4] The report was developed to record and store all information on each irradiation event that are related to the radiation dose received by the patient. For irradiation-based imaging modalities, this information will include the radiation dose from projection X-ray, computed tomography and radiopharmaceutical administration in nuclear medicine. Before this dose survey, we checked the accuracy of the PKA meters in the DR rooms during quality assurance testing of these systems. There are no established national DRLs for imaging examinations in Singapore. Therefore, we compared our results with the DRLs of the UK and Republic of Ireland, and with the most common values of DRLs from the European Union (EU) member states in terms of PKA.[8,9] The most common values of DRLs are published in the European Union document, Radiation Protection No. 180.[10] Compared to these published figures, our median and 75th percentile values from the dose survey were all demonstrably lower. Figure 2 shows the comparison of DRLs (75th percentile values) from the UK, Republic of Ireland, European Union and our dose survey. We also compared the median DRLs from our dose survey with those from the Republic of Ireland [Figure 3].Figure 2: Graph shows outcome of 75th percentile of survey as referenced to published diagnostic reference levels. AP: anteroposterior, DRLs: diagnostic reference levels, EU: European Union, Lat: lateral, PA: posteroanterior, UK: United kingdom, Y: lateral scapulaFigure 3: Graph shows comparison of dose survey median vs. Irish national median. AP: anteroposterior, Lat: lateral, PA: posteroanterior, Y: lateral scapulaThe current UK DRL is based on their 2010 national dose review, where 65% of the observed PKA doses were acquired from the computed radiography (CR) systems and only 32% from the DR (flat panel detector) systems.[11] The remaining 3% came from the systems using film screen (FS). This review also showed that the doses for DR systems were lower compared to the CR systems and the FS systems. The most common values for DRLs from the European Union were released in 2014. Hence, these values were acquired from a mixture of DR, CR and FS systems. Even for the Irish national DRLs that were released in 2021, only 79% of the surveyed facilities used the DR systems, 8% of the facilities used a combination of DR and CR systems and the remaining 13% of the facilities were using the CR systems. Our results, which are based entirely on DR systems, are therefore, lower than these DRLs. Another factor contributing to our lower DRLs is the difference in patient weight. The weight range for these international DRLs was around 70 ± 15 kg compared to our weight range of 50–80 kg. Performing a dose survey required considerable time and effort to manually collect the dose data and determine the typical (median) doses and DRLs for selected imaging examinations. After conducting the dose survey, we realised that a patient radiation dose monitoring system will improve the collection, analysis and reporting of radiation dose data compared to the current manual method. Having a patient radiation dose monitoring system installed in our institution will enable us to collect, monitor and analyse the radiation dose received by to our patients, and to create DRLs for imaging examinations in a largely automated way. With the advantage of more data from the imaging examinations, it will render assessment of radiation doses a lot easier and more accurate. The established dose values will also be more precise. The detailed information provided by dose monitoring software will allow us to optimise the image quality while minimising radiation exposure to the patient. Overall, the radiation doses obtained in this study were lower compared to the published DRLs of other countries around the world. As the imaging procedures are being optimised and improved, and as advances are being made in the imaging technology, the radiation dose received by patients for radiological examinations is projected to drop progressively. Hence, the DRL values should be reviewed regularly.[4] Countries with established DRLs had conducted reviews of radiation doses every 3–5 years, but this interval between the dose surveys will depend on the availability of the required manpower to undertake these surveys, the introduction of new hardware or software for imaging procedures, consistency of the survey results and participation of the imaging facilities in these countries. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.
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