Currently the DRLs cover CT examinations (adult and paediatric), nuclear medicine, cone-beam computed tomography of the head and neck region, cardiology and conventional x-ray examinations including dental examinations (adult and paediatric). When applicable, the DRLs are given based on an indication of the examination or a specific procedure and can be classified as clinical DRLs. Currently the oldest DRLs are for the adult CT examinations, which are from 2013.
Traditionally the data for establishing the DRLs has been collected by sending the facilities a spreadsheet to be filled with a specified number of patient doses. A weight limit to represent a normal-sized patient has been used for examinations where the weight has a substantial effect on the dose. Depending on the patient flow and examination types in the facility, this kind of manual data collection can be slow and it might be hard to find enough patients to fit the weight limits. Especially since, for some examinations, the weight distribution of patients differs greatly from the weight distribution of the people in general.
The question of a normal-sized patient is highlighted in paediatric examinations, because the size of the patients varies significantly. Usually DRLs are given for different age or weight groups. For establishing the DRLs, a large number of doses must be collected for every group, which can be very slow as the total number of patients is usually quite low. The facilities have the same problem when comparing their average doses to the DRLs.
There are some possible ways to tackle these problems. Automatic dose management systems decrease the amount of work required for dose collection and increase substantially the possible number of data points to be used. To utilise these systems effectively, an emphasis must be given to the completeness of the data. For example, if a weight limit is used, the weight of every patient should be stored to have access to far greater amount of data. Also, as in many cases the DRLs are and should be based on the indication of an examination, the information on the indication should not be lost when the dose information is transferred to the system from the imaging device. Hence, the imaging protocol should be named according to the indication. Some dose management systems use mapping tools to relate protocol names from different devices to the indication in question. In Finland, dose management systems have not been extensively used in establishing DRLs. Some imaging facilities use automatic dose management systems for determining their average doses to be compared with DRLs. In Finland, a national PACS is in development and dose data from the examinations will be stored in the system. It is planned for the national competent authority to have access to anonymised dose data, which should open new possibilities for establishing DRLs.
Since the size of a paediatric patient varies significantly, some paediatric DRLs in Finland are given for different age groups and for some, a DRL curve is established. In a DRL curve, the reference level is a function of a patient weight. When comparing their doses to the curve, the facility should mark the doses in the chart and visually determine if the majority of the doses are below or above the curve. Alternatively, they can fit an exponential curve to their data points and see if it lies beneath the DRL curve. An example of a DRL curve and its use can be seen in figure 1.
The DRLs are usually set as 75th percentile value of the distribution of the median doses from different devices. So, in principle, after a DRL is issued, median dose from every fourth device would be more than the DRL. Having a median dose from an examination higher than the related DRL, is not necessarily a sign for inadequate optimisation. For example, with older equipment, it might not be possible to have doses less than DRLs, without compromising the image quality too much. On the other hand, having median doses that are less than DRLs, does not guarantee that the device is well optimised. To encourage further optimisation, achievable dose levels are presented for selected examinations. In Finland they are generally set to the median (50th percentile) value of the distribution and are said to correspond to dose levels that should be able to be achieved with modern equipment.