National Diagnostic Reference Levels for Nuclear Medicine in Kuwait

DISCUSSION

The ICRP introduced 3 principles that became a cornerstone in radiation protection. These principles evolved into 3 key words: justification, optimization, and limitation. Optimization aims to ensure that every exposure is performed with the lowest ionizing radiation needed to execute the procedure, following the “as low as reasonably achievable” principle. DRLs are considered an effective optimization tool for improving radiation protection in diagnostic medical imaging (4) and are not in any way dose limits or constraints, nor do they serve regulatory purposes. However, they aim to identify whether some common procedures present unusually high values, alerting the department to act accordingly by, for instance, reviewing procedures, protocols, or equipment.

The first national DRLs for commonly performed nuclear medicine imaging procedures in Kuwait, including hybrid imaging procedures such as PET/CT and SPECT/CT, were established in this study. Table 1 shows that there is a large variation in the reported administered activities. For instance, there is a dispersion (as variance) between the activities used for ^99mTc-methyl diphosphonate bone scanning, as indicated by the large SD and the 3-fold difference between the maximum and minimum values. Thus, there is a need for reference levels and standardization of activities. The effective radiation doses for average-weight adults can be categorized according to Towson and Smart (18) into the following: high-dose (>10 mSv) procedures (⁶⁷Ga for infection and ¹³¹I for thyroid cancer), moderate-dose (<10 mSv) procedures (¹⁸F-NaF for bone imaging; ¹⁸F-FDG for tumor and brain imaging; and ^99mTc for bone, cardiac, brain, renal, lung, hepatobiliary, salivary, thyroid, and parathyroid scans), and low-dose (<1 mSv) procedures (the rest of the procedures).

It is noteworthy that the average reduction in effective doses when using DRLs in routine work is up to 25%. The impact of dose reduction is estimated by comparing the maximum to the DRL-recommended activities and is stronger when applied to high-dose procedures involving ⁶⁷Ga and ¹³¹I radioisotopes. Additionally, the DRLs can also have a lower value, that is, the 25th percentile, as shown in Table 1, indicating that below a certain dose, the resulting image quality could be diagnostically insufficient. Thus, the 25th percentile is an indicator of the minimum dose that can be used to achieve acceptable image quality. As described by Korpela et al. (19) the first step in optimizing medical exposure is the establishment of national DRLs, which allow identification of unusually high or low activities compared with the national distribution.

Table 2 shows that the DRLs in Kuwait are comparable to, and agree well with, those reported from other countries. The DRL in Kuwait for ¹⁸F-FDG tumor imaging is generally lower than those in the United States, the United Kingdom, Australia. The large differences between the ¹⁸F-FDG DRL in Kuwait and those reported for other countries could reflect the fact that the data in this study were gathered recently (many years later than other reported data), at a time when scanners with more advanced imaging technology and sophisticated dose-saving technologies have been used; since that time, a greater awareness of the need for optimization may have come about.

For myocardial perfusion scans, especially rest studies, the DRLs tend to be higher than other values presented in Table 2. The higher DRLs could be due to the fact that the stress and rest parts of the study are performed on 2 different days and that the 1-d protocol is not routinely performed in Kuwait. Optimization of radiopharmaceutical activities for myocardial perfusion scans has been widely promoted by the establishment of the national DRLs.

Recently, there have been several reviews of child and adolescent administered activities that led to development of pediatric guidelines in nuclear medicine (20,21). Table 3 shows the minimum recommended pediatric administered activities that can be used to minimize variations in the practice of pediatric nuclear medicine in Kuwait. These calculated activities are weight- and age-based. Table 4 shows the recommended conversion factors for administered activities based on patient weight. These factors can be used for children, adolescents, and adults who weigh more than average.

CT scanning in hybrid imaging procedures is performed for different purposes, ranging from obtaining diagnostic-quality high-dose images to ultra-low-dose images for attenuation-correction protocols (22). The variations between CT radiation doses delivered to the patient in hybrid imaging examinations is due mainly to the varied types of equipment settings and acquisition protocols. A detailed analysis of current practice in Kuwait for CT in hybrid imaging studies was demonstrated in a reported national dose audit (23). The DRLs of the CT portion associated with hybrid imaging procedures performed for attenuation correction and localization purposes in Kuwait are presented in Tables 5 and 6. CT dose can be optimized for PET/CT examinations by further investigating the CT protocol parameters that contribute to the dose received by patients.

The DRLs in Kuwait are consistent with those presented in the literature for nuclear medicine centers around the world. It is recommended that DRLs be reviewed periodically—for example, every 5 years. Periodic review of DRLs is required because imaging technologies and radiopharmaceuticals are rapidly advancing, and these advances can result in reducing the radiation doses to patients. Comparison with reference values such as DRLs is an effective tool to alert professionals in some departments that have not fully implemented the “as low as reasonably achievable” principle of dose optimization (24).

Because a DRL is supposed to be the activity needed for good, diagnosable image quality, it is not enough to evaluate patient doses and set DRLs on the basis of only administrated activity regardless of image quality. Introduction of reference doses, including image quality criteria and the acceptable-quality dose (AQD principle), has been proposed (25). Thus, efforts are needed to develop reliable patient-specific methods to objectively analyze image quality in relation to dose. There are some large variations in the subjective analysis of image quality due to differences in physician preferences on what constitutes an image of diagnosable quality. Some of the approaches used to evaluate image quality need further evaluation, as demonstrated by the Japanese Society of Nuclear Medicine Technology (26).