UKRG Handbook PAEDIATRIC ACTIVITIES OF RADIOPHARMACEUTICALS
Introduction
Although radiopharmaceuticals are routinely used in paediatrics, few are licensed for this group of patients. Without the appropriate licensing the onus is on the clinician performing the scan to take clinical responsibility for the administration of an un-licensed product.
Children have rapidly growing and dividing tissue which may be more sensitive to ionising radiation and also their organ size to body ratio may be different from the older child or the adult. This, along with the possibility of somatic and hereditary effects from even small amounts of administered radioactivity must be balanced against the possible benefits from the investigation (1).
Before the administration of any radiopharmaceutical to a paediatric patient due consideration must be given to the following general principles (2)
Consequently the activity administered must be reduced by any of the various methods available. The degree of reduction must take into account the smallest activity required to give the desired images and statistics, the test to be performed, the size and weight of the patient, sensitivity to detection equipment, type of examination and acquisition time.
The method employed for the activity reduction must be agreed by all clinicians and become a departmental policy or protocol. For most departments a simple chart of the activities to be administered per body weight reduces the chances of maladministration due to calculation errors. Any such chart requires the most up to date information and needs to be updated and signed by the responsible clinician.
Activity estimation
Scaling down the adult administered activity in simple proportion to body weight will generally result in the child receiving an effective dose equivalent similar to that an adult would receive.
However consideration must be given to the procedure in question as infants have a higher uptake in bones due to increased growth and brain imaging due to the organ reaching a high proportion of adult size in young childhood (2)
The method of administering activities to children in individual departments will also vary and so this should also be recognised as a potential complication.
Drawing up a small activity for a child involves the use of a needle and syringe and the administration is via a venflon or similar device. Is an allowance made for the residue left in the two devices ?
Paediatric activities can be calculated by the following methods :
| 1) Fraction of adult administered activity based on child body weight (2,3) | |||||
|
This is the method currently approved by ARSAC and has the following proviso in that no less than 10% of the adult activity is administered. Based upon administering to children by weight, generally assumes a standard adult weight as 70 kg the calculation is |
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| Administered activity = adult amount of activity x (child weight in kg/70) | |||||
Body weight (kg) |
Fraction of |
Body weight (kg) |
Fraction of |
Body weight (kg) |
Fraction of |
3 |
(0.10) |
16 |
0.23 |
45 |
0.64 |
4 |
(0.10) |
18 |
0.26 |
50 |
0.71 |
6 |
(0.10) |
20 |
0.28 |
55 |
0.79 |
8 |
0.10 |
25 |
0.36 |
60 |
0.86 |
10 |
0.14 |
30 |
0.43 |
65 |
0.93 |
12 |
0.17 |
35 |
0.5 |
70 |
1.00 |
14 |
0.20 |
40 |
0.57 |
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| 2) Surface area alone (4) | |||||
| Surface area can be calculated from the following formulae or table | |||||
SA = (wt) 0.425 x (ht) 0.725 x 0.007184 |
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Surface area (m2) |
Divide Adult |
Approximate Age |
|||
0.24 |
7.2 |
new born |
|||
0.27 |
6.4 |
||||
0.31 |
5.6 |
||||
0.35 |
4.9 |
||||
0.43 |
4.0 |
||||
0.58 |
3.0 |
1 year |
|||
0.75 |
2.3 |
5 years |
|||
1.00 |
1.7 |
10 years |
|||
1.57 |
1.1 |
15 years |
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| This method is least favourable due to the complex calculations involved and the need for the patient to have their correct height and weight measured. | |||||
| A number of other rules are used, primarily in the USA and these can be summarised as follows :- | |||||
| Clarkes Rule :- | Activity = ((Weight in Lbs) x (Adult Activity)) / 150 | ||||
| Youngs Rule :- | Activity = ((Age in Years) x (Adult Activity)) / (Age + 12) | ||||
| Websters Rule:- (Modified Youngs) |
Activity = ((Age in Years +1) X (Adult Activity)) / (Age in Years + 7) | ||||
Complications
Whichever method is chosen, there will be associated complications and a rule for each department should be drawn up using a particular method.
It is generally recognised that there is a minimum amount of activity that should be administered to a child. This enable the required image to be acquired in a suitable time scale and if undertaking statistical analysis, sufficient counts to be acquired.
The amount administered is a clinical decision and consequently any recommendations published here are only a guide for the user.
References