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Correspondence |
The European Medical Exposures Directive (97/43/EURATOM) states that "member states shall promote the establishment and use of diagnostic reference levels for radiodiagnostic examinations" [1], and this recommendation was transposed in to Irish Legislation in 2002 [2]. To date, there exists a deficit in the published literature on the establishment of paediatric diagnostic reference levels (DRLs) for nuclear medicine examinations. We conducted a review of paediatric nuclear medicine services within Ireland, with a view to establishing paediatric DRLs for common nuclear medicine examinations.
In nuclear medicine imaging, DRLs are levels of activity, for typical examinations, for groups of standard sized patients [1, 2]. However, paediatric nuclear medicine imaging does not deal with standard sized patients. In administering radiopharmaceuticals to children, it is important to remember that the biodistribution and kinetics of radiopharmaceuticals are different from adults, particularly in neonates [3, 4]. The administered radioactive dose is distributed over smaller volumes in infants, giving rise to a higher absorbed dose. Consequently, the activity administered must be reduced [3]. It is common practice to express radiopharmaceutical schedules for children as fractions of adult administered amounts [5], and formulae based on the child's age, height, weight and body surface area have been used [5, 6]. In order to establish DRLs, current practices in paediatric nuclear medicine imaging in Ireland have been reviewed.
11 hospitals within Ireland perform nuclear medicine imaging on children. Three of the hospitals are dedicated paediatric hospitals. A questionnaire was sent to the 11 centres requesting information on the types of diagnostic scans performed on children, the standard adult activity per procedure and the method used for calculating the paediatric administered activity. Information was also sought as to the application of a minimum activity and the typical workloads for each procedure type.
All 11 centres completed the survey. Bone scans are the most common procedure type, being performed in 10 of the 11 centres. Renal imaging with 99mTc dimercaptosuccinic acid (DMSA) is the most frequently performed procedure in those centres that perform the investigation. There was variation in adult administered activities for a given examination. For example, for 99mTc mercapto acetyl triglycine (Mag)-3 renograms, the adult administered activity varied from 80 MBq to 185 MBq.
In calculating paediatric administered activity from the standard adult activity, seven centres scale by the child's weight according to Clark's formula. Two centres scale the adult administered activity using factors published by the Paediatric Task Group of the European Association of Nuclear Medicine (EANM) [7]. One dedicated paediatric hospital uses the EANM factors for all examinations except lung perfusion studies, in which case Clark's formula is used, as it places a greater restriction upon the number of particles that may be administered. Only one centre calculated the activity to be administered by scaling the standard adult activity by the age of the child according to (age+1)/(age+7).
For neonates and infants, the use of a fraction of the adult dose would result in too small an administered dose and an entirely unsatisfactory study [3]. Hence, a minimum dose is normally applied [7–10]. 9 of the 11 centres reported the application of a minimum administered activity. One dedicated paediatric hospital reported two minimum administered activities per examination, depending on the age of the child, with minimum activities for infants and for children aged 16 years.
In this work, the third quartile value of the adult administered activity is recommended as a basis for calculating paediatric DRLs for six procedure types: 99mTc phosphonates for bone scan, 99mTc DMSA renal imaging, 99mTc diethylene triamine pentaacetic acid (DTPA) renogram, 99mTc Mag-3 renogram, 99mTc-pertechnetate thyroid and 99mTc-pertechnetate meckles.
It was found that the majority of centres adopt radiopharmaceutical schedules that utilize weight, based on Clark's formula, in order to scale the adult administered activity for paediatric patients. The adult DRLs were scaled accordingly to yield paediatric DRLs using growth charts [11] to predict typical weights for newborns, 1 year olds, 5 year olds, 10 year olds and 15 year olds. These age groups were considered to be representative of the paediatric age range, and typical weights were obtained by averaging the weights for boys and girls. Paediatric DRLs for the six commonly performed procedures are presented in Table 1
. In the case of newborn children, it was necessary to apply the minimum dose for 99mTc DMSA renal imaging, 99mTc DTPA renogram, 99mTc Mag-3 renogram and 99mTc-pertechnetate thyroid procedures, as the activity calculated according to weight was less than the recommended minimum activity. The median of the minimum activity was applied in this case.
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Yours etc.,
Funding for this research was provided by the Meath Foundation, Adelaide and Meath Hospital Dublin Incorporating the National Children's Hospital, Tallaght, Dublin 24, Ireland.
1 Department of Medical Physics and Bioengineering, St. James's Hospital, James's Street, Dublin 8, Ireland, 2 Department of Radiology, Adelaide and Meath Hospital Dublin Incorporating the National Children's Hospital, Tallaght, Dublin 24, Ireland
References
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