Use of weight-based vs age-based groupings in the study of typical values of air kerma area product (P_KA) for paediatric radiographs of chest and abdomen

Abstract

Objective:

To compare age groupings versus weight groupings in the calculation of typical air kerma area product (P_KA) values in paediatric X-ray exams of chest and abdomen in our hospital.

Methods:

Data were analysed from 687 abdominal and 1374 chest X-ray examinations. The P_KA of exams was extracted with Radimetrics, and patient weights were collected from electronic records. Data were organised in different age groups and typical P_KA values were estimated. The process was repeated by organising data in different weight groups.

Results:

Typical P_KA values for the four younger age groups (<1m, 1m - < 4y, 4y - < 10y and 10y - < 14y) were comparable to typical values for their equivalent weight groups (<5 kg, 5–15 kg, 15–30 kg and 30–50 kg, respectively). However, typical P_KA values at the late adolescent age group (14y - < 18y) were much lower than its equivalent weight group (>50 kg).

Conclusions:

Age and weight groupings were found at our site to be interchangeable for the calculation of typical paediatric P_KA values. The only exception was the late adolescent group, whose weight distribution can account for the difference in typical P_KA results within its equivalent weight group.

Advances in knowledge:

In calculating typical P_KA values for radiological paediatric body examinations, departments must ascertain if using age groups, which is typical practice, is equivalent to using weight groups. Otherwise, results may misrepresent local practice.

Introduction

The clinical use of ionising radiation is governed by the principles of justification and optimisation. With regard to optimisation in diagnostic imaging, an invaluable and effective tool is the adoption of diagnostic reference levels (DRLs), against which typical values of DRL quantities surveyed in a radiology department can be compared. DRLs are not limits but “a form of investigation level used as a tool to aid in optimisation of protection in the medical exposure of patients for diagnostic and interventional procedures. It is used in medical imaging with ionising radiation to indicate whether, in routine conditions, the amount of radiation used for a specified procedure is unusually high or low for that procedure”¹. Apart from good practice, comparing local, typical values of DRL quantities with recommended national or international DRLs is also part of legislative requirements in the European Union and the United Kingdom^2,3.

In terms of paediatric radiological imaging, recent European guidelines recommend that weight groupings are used for the development of DRLs of general X-ray and CT examinations of the body, as patient weight, rather than age, should be the governing factor of the dose delivered.⁴ However, this may be difficult to implement because patient weight is not always monitored directly in radiology departments as it is not a field required to proceed with a general X-ray or CT study, as it is for example in MRI examinations. For this reason, even recent work on the development of national DRLs in France still uses age rather than weight groupings for paediatric DRLs.⁵ Similarly, the new national DRLs for Germany recognised that only minimal data were available for weight groupings; therefore, correlations between age and weight were used for paediatric DRLs.⁶

Our hospital started clinical operations in 2019, becoming the only tertiary referral paediatric hospital in the country. One of the priorities of the radiology department has been to ensure that imaging protocols are optimised, by means of a favourable comparison of typical values of DRL quantities in our hospital with international published DRLs, as national DRLs have not been available. The use of age rather than weight groups for the calculation of typical values is more practicable at our site and previous work on the establishment of DRLs in paediatric computed tomography showed that using age groups is a pragmatic approach as long as sufficient data (>30) in a particular age group are collected.⁷ However, there was a need to validate that the use of age groups was appropriate for the paediatric patient population seen in our hospital. Therefore, the aim of this work has been to compare age groupings vs weight groupings in the calculation of typical values of DRL quantities in general radiology X-ray exams of the body (chest and abdomen) and to analyse any differences identified.

Methods and materials

The hospital is equipped with seven digital radiology general X-ray rooms of the same vendor and model (Digital Diagnost, Philips Medical Systems, Netherlands), of which two are used for outpatients, four for inpatients and one for emergencies. The hospital also has two portable digital radiology X-ray systems (Mobillet Plus, Siemens Healthineers, Germany) dedicated for imaging in the neonatal intensive care unit. All systems are equipped with P_KA meters, which are checked for calibration as part of recommended quality control procedures.⁸ The protocols for chest and abdomen X-ray examinations have been prepared based on previous published recommendations applicable to the technology used, and the parameters are shown in Table 1⁹. Note that the chest supine protocol parameters are shown for all ages for completeness; however, it is the standard only for all patients in the 0–6 months age group and applicable in the majority of the 6–18 months age group. The parameters of the examination protocol are selected automatically by the systems, based on the age of the patient. In case a patient with an increased body habitus presented, radiology technologists have been instructed to select the older age group protocol rather than modify parameters of the automatically selected protocol. Upon completion of the exam, both patient age (date of birth) and P_KA are automatically pushed to the dose management system of our hospital (Radimetrics, Bayer Healthcare, Germany).

Table 1.

Exposure parameters for different ages/weights and the three protocols included in the study. Exposures shown in kVp/mAs format; kVp, kilovoltage peak; mAs, milli-ampere-seconds. For all exposures tube additional beam filtration: 0.1 mm copper +1 mm aluminium

Age	Weight	Chest PA/AP 180 cm	Chest supine 110 cm	Abdomen AP/PA 110 cm
0–6 months	<5 kg	Not applicable	63/1.6	63/2
6–18 months	5 - < 10 kg	73/2	66/1.6	66/2
18–36 months	10 - < 15 kg	73/2	66/1.6	66/2.5
3–7 years	15 - < 30 kg	85/1	70/1.2	70/4
8–12 years	30 - < 50 kg	90/1	73/1.6	73/5
13–17 years	50 - < 80 kg	90/1.2	77/2	77/8
Adult size	>80 kg	125/1.2a G	82/2.5 G	81/10a G

AP, anterior–posterior; G, grid exposure 8:1 ratio; PA, posterior–anterior.

^aUse automatic exposure control (500 speed) with both lateral chambers. The mAs shown is indicative.

The P_KA is the DRL quantity used in our hospital to calculate typical values of exposure and compare with international DRLs. The P_KA values of chest and abdomen X-ray examinations for paediatric patients (under the age of 18) over a one-year period were extracted using the dose management system. For each exam, the weight of the patient on the day of the X-ray examination was identified in the hospital electronic patient medical records (PowerChart, Corporation Inc., USA). Data were carefully reviewed on the picture archiving and communications system (Intellispace, Philips Medical Systems, Netherlands) to remove any exams that may have been incorrectly selected by radiology technologists (e.g., selecting the standard abdomen protocol for a study of a patient with barium and vice versa) or studies where a non-standard protocol may have been implemented (e.g., chest studies of late-adolescent patients with scoliosis rods where the use of Automatic Exposure Control was not indicated). This resulted in 687 abdominal and 1374 chest X-ray examinations.

For each type of X-ray examination, data were divided into the age groups recommended in the European guidelines and shown in the third column of Table 2⁴. The median and mean age within each group was calculated in order to verify that our study population provides a good sample for that age range. In addition, the median and mean weight for each age group was calculated in order to ascertain if the weight of our study population is aligned with the recommended weight range given in the European guidelines, given in the second column of Table 2. Lastly, the age-based typical values of the P_KA were calculated as the median of P_KA values for each exam and age group. The process was repeated by dividing the data for each exam into the weight groups recommended in the European guidelines, given in the second column of Table 2, and calculating the weight-based typical DRL quantity as the median of P_KA values. Median values of P_KA were used for the calculation of both age-based and weight-based typical values, following recommendations by the International Commission on Radiological Protection.¹

Table 2.

Approximate equivalence of weight and age groups for the purpose of comparing weight-based DRLs with age-based DRLs⁴

Description	Weight group	Age group based on weight for age charts	Most common age groups used for national DRLs
Neonate	<5 kg	<1 m	0 y
Infant, toddler and early childhood	5 - < 15 kg	1 m - < 4 y	1 y
Middle childhood	15 - < 30 kg	4 - < 10 y	5 y
Early adolescence	30 - < 50 kg	10 - < 14 y	10 y
Late adolescence	50 - < 80 kg	14 - < 18 y	15 y

Results

The typical values of P_KA based on age-based groupings of chest and abdomen paediatric X-ray examinations are shown in Table 3. The same table also demonstrates the population sample in each age group, along with the metrics (median and mean) of the sample‘s age and weight within that age group. Figure 1 and Figure 2 show the distribution of our sample population’s weight in different age groups for chest and abdomen examinations, respectively. The numbers on the horizontal axes of the histograms indicate the maximum value of the bin intervals used. Table 4 presents the typical values of P_KA based on weight-based groupings of chest and abdomen paediatric X-ray examinations, along with the median and mean weights for the samples.

Table 3.

Age-based typical values of P_KA for paediatric chest and abdomen examinations. The table also lists the sample size from our study population for each age group, along with the median and mean of the age and weight for each population sample. The age and weight groups from the European guidelines, along with the European DRLs are also provided in this table for the readers’ easy reference.

Chest examinations
European Guidelines		European DRL PKA (mGy cm2)	Sample size	Sample age (years)		Sample weight (kg)				Median PKA (mGy cm2)
Age group	Weight group			Median	Mean	Median	Mean	Min	Max
<1m	<5 kg	15	379	0.006	0.010	2.5	2.4	1.0	4.4	5.8
1m - < 4y	5 - < 15 kg	22	615	2.1	2.2	8.5	8.2	2.0	20.3	13.5
4y - < 10y	15 - < 30 kg	50	189	7.9	7.6	22.7	23.0	12.0	49.9	25.3
10y - < 14y	30 - < 50 kg	70	98	11.9	12.1	39.8	38.6	13.0	85.0	37.2
14y - < 18y	50 - < 80 kg	87	93	16.0	15.9	43.0	50.6	19.3	104.0	49.0

Abdomen examinations
European Guidelines		European DRL PKA (mGy cm2)	Sample size	Sample age (years)		Sample weight (kg)				Median PKA (mGy cm2)
Age group	Weight group			Median	Mean	Median	Mean	Min	Max
<1m	<5 kg	45	115	0.012	0.011	2.2	2.1	0.9	4.0	6.7
1m - < 4y	5 - < 15 kg	150	172	2.0	2.1	10.6	10.3	2.5	18.9	25.6
4y - < 10y	15 - < 30 kg	250	187	6.9	7.1	23.7	24.1	9.4	52.0	45.8
10y - < 14y	30 - < 50 kg	475	109	11.6	11.8	42.0	42.4	12.7	89.5	125.6
14y - < 18y	50 - < 80 kg	700	104	15.5	15.5	47.0	55.4	19.0	107.6	169.0

Figure 1.

Weight distribution of patients in different age groups that undertook chest x-ray exams at our site.

Figure 2.

Weight distribution of patients in different age groups that undertook abdominal x-ray exams at our site

Table 4.

Weight-based typical values of P_KA for paediatric chest and abdomen examinations calculated as median of P_KA. The table also lists the sample size from our study population for each weight group, along with the median and mean of the age and weight for each population group. All patients aged less than 18y.

Chest examinations
Weight range	Number of exams	Sample weight (kg)		Median PKA (mGy cm2)
		Median	Mean
<5 kg	536	2.8	2.8	6.5
5 - < 15 kg	515	9.1	8.8	15.8
15 - < 30 kg	154	20.6	20.3	23.1
30 - < 50 kg	98	39.0	43.0	38.6
>50 kg	71	67.4	70.4	100.3

Abdomen examinations
Weight range	Number of exams	Sample weight (kg)		Median PKA (mGy cm2)
		Median	Mean
<5 kg	152	2.8	2.6	7.9
5 - < 15 kg	159	11.9	11.2	25.1
15 - < 30 kg	182	21.3	21.4	42.8
30 - < 50 kg	116	41.0	40.0	131.4
>50 kg	78	64.0	62.8	245.6

Discussion

Typical P_KA values of chest and abdomen X-ray examinations in our hospital for all paediatric age groups are lower than the European DRLs as presented in Table 3 and Table 4. The same can be noted for typical P_KA values for all weight groups, as presented in Table 4, with the exception of the typical P_KA value for chest X-ray examinations at the paediatric weight group of over 50 kg, where the typical value of 100.3 mGycm² is higher than the European DRL of 87 mGycm²; although excluding patients with weight over 80 kg, which is the upper weight limit of the European DRLs, would decrease the typical P_KA value. It is worth noting that the more recently published recommended paediatric DRLs for France and Germany do not make reference to the late-adolescence weight or age-equivalent group^5,6. A limitation of our work is that the population sample in the late adolescent age group was smaller compared to other, younger groups; however, in all cases, it was larger than the minimum number of data (>30) recommended in previous work.⁷

The difference seen between typical P_KA values in our hospital and European DRLs, especially in abdominal examinations, could be attributed to the paediatric expertise and specialisation of our radiology team, when compared to a large cohort of European imaging centres, including general hospitals. This is a known limitation of DRLs, and the reason why the concept of Acceptable Quality Dose (AQD) has been introduced.¹⁰ The AQD allows sites to set dose levels that can provide acceptable image quality for the imaging task and patient habitus presenting, therefore shifting focus from DRL dose to image quality.

As seen in Table 3, the median and mean age for our patient cohorts within each age group was close to the mid-point of the European guidelines’ age ranges, demonstrating that our patient samples were a good representation of corresponding age groups. In addition, the median and mean weights within each age group were also consistent with the mid-point of the equivalent weight ranges of the European guidelines for all age groups apart from the late adolescent one (14y - < 18y), where both mean and median values were near the lower end of the equivalent weight range, and approximately half the population weighed less than the minimum of the equivalent weight group (50 kg). Figures 1e and 2e show the distribution of our population’s weight for the late adolescent age group for both chest and abdomen X-ray examinations, which demonstrates two peaks, one at approximately 40–50 kg and another one at approximately 80 kg.

The two peaks could be due to different prevalent health conditions requiring X-ray examinations, resulting in differing patient weights. For instance, within the same age group, patients with conditions associated with hypotonia and multiple chronic conditions are likely to have lower weight compared to patients with community acquired chest infections. In this case, this differentiation could be linked to the different clinical indications for imaging referral. Recent initiatives have highlighted the importance of establishing clinical DRLs, whereby DRLs are based not only on the body area imaged but also on clinical indication.^11,12 Our work demonstrates how the clinical indication may also have an effect on establishing typical values of DRL quantities on account of the dependence of dose delivered on patient weight, if patient weight is dependent on healthcare condition. As the peaks appear at approximately the same weights for both types of examinations, they may also be attributed to different ethnic populations presenting in our hospital; data have shown that paediatric obesity is more prevalent in nationals of the Arabic Gulf compared to other nationalities resident in the region, such as Southeast Asian and European.¹³ Further work is required to study the potential correlation of ethnicity or clinical indication with differences in patient weight for the late adolescent age group at our site.

When comparing typical P_KA values for different age groups with typical P_KA values for equivalent weight groups in our hospital, it is noticeable that values are comparable (difference on average less than 10%) for all ages and weights in both chest and abdomen X-ray examinations with the exception of the late adolescent age group (14y - < 18y). For that group, the typical P_KA value is lower than the typical P_KA value of the corresponding weight group for both chest and abdomen X-ray examinations. This difference can be explained by the weight distribution in the age group (Figures 1e and 2e), showing a higher peak at lower weights where P_KA would typically have lower values. This is also confirmed by comparing the mean to the median of weight for our patients’ age groups, showing that they are very close for all but the late adolescent group.

Conclusion

At our hospital, and for our paediatric patient population, typical P_KA values of both chest and abdominal general X-ray examinations were comparable whether age-based or equivalent weight-based groups were used in their calculation, in line with previous recommendations from paediatric computed tomorgraphy.⁷ The only exception was the late adolescent age group (14y - < 18y), where the weight distribution demonstrated two peaks for the patient populations of both X-ray examinations, at approximately 40–50 kg and at 80 kg. On account of this weight distribution, using age groups instead of weight groups underestimated the typical P_KA values used for that age group. Therefore, age and weight groups can be used interchangeably only at younger patient ages at our site.

In calculating typical P_KA values for radiological paediatric body examinations, departments must ascertain if using age groups, which is typical practice, is equivalent to using weight groups. Where that is not possible, or if weight recording is not feasible for a large number of patients, calculating the typical value of the P_KA for a representative but statistically significant number of patients within each weight group is recommended. The study of paediatric DRLs and exposure optimisation would be facilitated by vendors of radiological equipment if they allowed for exam protocol design on the basis of patient weight rather than age.