**5.4 Paediatric diagnostic reference level of examinations in nuclear medicine (PDRL)**

Establishing Dose Reference Level values for children is more challenging than for adults, due to the broad range of sizes of paediatric patients. Weight in children can

vary by a factor of more than 100 from a premature infant to an obese adolescent. The amounts of radiation used for examinations of children can vary extremely due to the great difference in children's size and weight.

Patient age groups have been used in the past in order to establish Paediatric DRL values. However, it has been recognized that age alone is not a representative parameter. Weight categories have to be included and should be used whenever possible. The difference in patient dose due to patient weight is expected and therefore weight ranges are recommended for establishing Paediatric DRL values.

Age groups around the ages of 0, 1, 5, 10 and 15 years can be used if age is the only available quantity. For examinations including the head, age grouping is recommended for establishing PDRL values. In paediatric imaging, sufficient data is an issue and therefore it has been suggested that the DRL quantity could be a function of patient's weight.

For nuclear medicine imaging, the DRL quantities and DRL values are set as administered activity per body weight (MBq/kg) as a practical and simple approach.

Activities for administration should be adjusted based on size or weight associated factors.

When regional or national DRL values, relevant for paediatrics, are not available, the local practice may be compared with appropriate available published data.

For CT used in a hybrid system SPECT/CT or PET/CT, the DRL quantities are Computed Tomography Dose Index (CTDIvol) and Dose Length Product **(**DLP), based on calibration with a 32-cm-diameter phantom for body examinations and a 16-cm-diameter phantom for head examinations.

The CTDIvol and the DLP are common methods to estimate a patient's radiation exposure from a CT procedure. The exposures are the same regardless of patient size, but the size of the patients is a factor in the overall patient's absorbed dose.

The unit of CTDIvol is the gray (Gy) and it can be used in conjunction with patient size to estimate the absorbed dose. The CTDIvol and absorbed dose may differ by more than a factor of two for small patients such as children. On the other hand, DLP measured in mGy.cm is a measure of CT tube's radiation output/exposure. It is related to volume CT Dose Index (CTDIvol). CTDIvol represents the dose through a slice of an appropriate phantom and DLP accounts for the length of radiation output along the long axis of the patient. DLP = (CTDIvol) [*in units: mGy.cm*]. DLP does not take into account the size of the patient and is not a measure of absorbed dose or the patient's effective dose.

The effective dose depends on factors including patient size and the region of the body being scanned. Values for these quantities should be obtained from patient examinations. Most CT scanners permit the determination of effective diameter or patient equivalent thickness. This is an additional improvement for setting Paediatric DRL values.

Size Specific Dose Estimate (SSDE) measured in mGy, is a method of estimating CT radiation dose that takes a patient's size into account. SSDE may be used in addition to the recommended DRL quantities as an extra source of information for the evaluation of the absorbed dose value.

Results from the largest international dose survey in paediatric computed tomography (CT) in 32 countries are included in ICRP Publication 135 where international DRL for Paediatric computed tomography were established [10]. Patient data were recorded among four age groups: <1 year, 1–5 years, <5–10 years and <10–15 years.

#### **5.5 Views related to paediatric DRLs**

The risk of harmful radiation effects is greater in children than in adults and optimisation of paediatric imaging is of particular importance because they have a longer life expectancy during which these effects may appear.

The amount of radiation used for examinations of children can vary greatly due to the excessive difference in patient size and weight from neonates to adult-sized adolescents.

Variation in patient radiation dose for two paediatric patients with the same size, same exposed area of anatomy should be the minimum. If not, this could be due to poor technique, or failure to adapt imaging protocols to account for both paediatric diseases and paediatric patient sizes. Weight or size-adjusted paediatric DRL values are therefore particularly important in optimization.

A number of factors need to be considered when communicating the development of DRL values for children. Some parameters are the same for adults and children. These include the choice of DRL quantities, the percentile of the distribution of the DRL quantity and whether to collect data from patient examinations or from measurements with phantoms.

DRL values for children, there cannot be as a single standard patient due to the large size range of paediatric patients [11].

Weight in children can vary by a factor of more than 100, from that of a premature infant.

Within the first 6 months of life, a typical baby's body weight doubles, and during the first year, it increases 3-fold. Ideally, five or more size ranges should be established between premature to infants (newborn, >1, >5, >10 and >15 years) [12].

It is preferable creation of groups based on paediatric patient body size and that body size be determined for individual patients before performing diagnostic imaging procedures by radiation sources.

In 1999, the European Commission issued Radiation Protection 109 (RP 109) with the title: 'Guidance on diagnostic reference levels (DRLs) for medical exposure'. This document indicates the critical need of establishing DRLs for high-dose medical examinations of patients sensitive to radiation, such as children. This work used average-sized adult phantom or standard size phantoms.

However, the same approach has not been considered appropriate for children due to the wide variation in body habitus.

DRL values for paediatric patients are only available for some common radiological examinations and there is a need to generate appropriately more.

The European Commission recognized this need and approved the 27-month tender project, European Diagnostic Reference Levels for Paediatric Imaging (PDRL) on the establishment of European DRLs for paediatric patients in December 2013. PDRL is coordinated by the European Society of Radiology (ESR, *Eurosafe Imaging)*, **Figure 5** [12].

The Japanese Society of Nuclear Medicine (JSNM) in 2014 has published the consensus guidelines for paediatric nuclear medicine. JSNM proposes dose optimization in paediatric nuclear medicine studies and widely discusses imaging techniques for the appropriate conduct of paediatric nuclear medicine procedures, considering the features of children imaging in order to produce harmonic PDRL [13].

Scientists in nuclear medicine departments must be familiar with



#### **Figure 5.**

*The calculated PDRLs may help in the standardization of the appropriate activity in paediatric nuclear medicine [18].*

• the patterns of dedicated clinical results when radiation activities in paediatric patients are minimized.

Regarding the reduction of radiation exposure to paediatric patients, continuous education and thoughtful application of techniques for radiation dose management may lead to the improvement of risk-benefit ratios when performing diagnostic imaging in children by radiopharmaceuticals.

Technology provides options such as new software and new hardware (collimators, computer components, etc.) for reducing radiation exposure while maintaining image quality driving to a minimum variation in PDRL values, globally.
