1 Introduction

With ongoing technological developments, computed tomography (CT) has become essential to pediatric radiology and has been established as an important and increasingly common part of the diagnostic workup of patients [1, 2]. As the gold standard for detecting and managing multiple diseases, CT is fundamental in pediatric medicine [3]. However, pediatric CT imaging poses a particular concern since pediatric patients are highly sensitive to radiation [4]. Computed tomography uses more radiation than conventional X-ray imaging methods [5]. Furthermore, CT use increases cancer risk over a child's lifetime. Compared to adults, children's organs and tissues are inherently more prone to cellular damage [6, 7]. The radiation dose received by children during routine CT scans, in some instances, is higher than what is required, which can lead to cancer in children [8]. Therefore, the justification and optimization of pediatric CT examinations are important [4].

An essential part of optimization is developing diagnostic reference level (DRL) values [9]. The International Commission on Radiological Protection (ICRP) described DRL values as a form of investigation levels applied to an easily measured quantity [10]. In practice, DRL values are recommended levels that an imaging facility should use to examine its methods and determine if acceptable image quality can be attained at lower doses [11]. The ICRP [10] stated that DRL values are not dose limits based on individual patients but on a specific group of patients. Furthermore, the ICRP [10] recommends that the DRL values should be based on patient weight since weight is the most reliable factor associated with the DRL quantity. Therefore, weight should be used for categorizing, and age can be used if the age of the patient is the only available measure, as the ICRP [10] proposed.

Most CT consoles display the volume CT dose index (CTDI_vol) and dose-length product (DLP) for a particular patient's CT scan, based on the reference phantom selected by the scanner [12]. The reference phantom size does not take into account the size differences between children and adults [10]. The American Association of Physicists in Medicine (AAPM) [13] developed the size-specific dose estimates (SSDE) taking into consideration the patient's body size [14]. Dose estimates based on the patient's size are considered more precise and should be used when the size parameters are available [10]. Additionally, different clinical indications for an examination may need different image qualities and, consequently, different amounts of radiation. Therefore, setting DRL values without considering the clinical indication has little significance [10].

Nephroblastoma is a dense malignant tumor of the kidney originating from kidney cells and different developmental stages of various tissue types, such as muscle, cartilage, and epithelium [15]. Nephroblastoma is prevalent in children between 1 and 5 years of age and represents 5.9% of all childhood malignancies [16]. Local diagnostic reference level (LDRL) values for CT brain, temporal bones, cervical spine, trunk, chest and abdomen have been established at two university hospitals in South Africa, using the CTDI_vol and DLP for different age groups [17]. Other pediatric LDRL values for emergency pediatric head CT scans based on the patient's age have been developed at a South African tertiary-level hospital, using the CTDI_vol and DLP [18]. At the time of this study, no pediatric CT DRL values based on nephroblastoma have been established in South Africa. Internationally, DRL values for pediatric patients were compiled by several authors using the DLP and CTDI_vol based on the patient's weight [19, 20]. Hwang et al. [21] developed pediatric DRL values using the DLP, CTDI_vol, and SSDE based on weight. Clinical indication-based DRL values for tumor and infection in pediatric CT abdomen were developed by Almén et al. [20] using the DLP and CTDI_vol for weight groups.

A literature evaluation revealed that no documented pediatric DRL values were available for CT examinations to investigate and stage nephroblastoma in South Africa and internationally. Nephroblastoma was the most common clinical indication for pediatric patients presenting for abdominal CT examinations at the participating hospital [22]. The absence of established clinical indication-based pediatric DRL values in South Africa limits the ability to identify unusually high dose levels and ensure optimal radiation protection during contrast-enhanced abdominal CT examinations of pediatric patients with nephroblastoma. The aim of the study was to establish pediatric DRL values for contrast-enhanced abdominal CT examinations to diagnose and stage nephroblastoma cases based on patient weight group.

2 Materials and Methods

2.2 Data Collection

The study followed a descriptive research design that involved systematically investigating social phenomena through statistical or numerical data [24]. Data of pediatric patients who underwent CT abdominal examination to diagnose nephroblastoma were collected. Patients' data who underwent CT abdominal examination to diagnose nephroblastoma during the period from January 1, 2018 to October 9, 2022 were collected retrospectively. A structured data recording document was used to collect dose information from the dose reports on the hospital's picture archiving and communication system (PACS) or CT console and the CT register file. The structured data recording document was obtained from Public Health England [25] and adjusted according to the requirements and specifications of this study. The CTDI_vol, DLP, clinical indication, anterior-posterior (AP) dimensions, lateral (LAT) dimensions, effective diameter, age, and weight of the patients were documented. When DRL values are established for multiphase CT examinations, the average CTDI_vol should be used, while the total CTDI_vol for single-phase CT examinations is used to calculate CT DRL values [26]. Since single-phase CT examinations were obtained for all patients in this study, the total CTDI_vol (see Figure 1) and the respective conversion factor for the SSDE parameters (AP, LAT, SUM, and effective diameter) as published by the AAPM [13] was used to calculate the SSDE. The SSDE parameters were calculated using equation 1 from the AAPM report [13].

$$${\text{CTDI}}_{\text{vol}}\,\times \,\text{conversion}\,\text{factor}\,=\,\text{SSDE}$$$(1)

Figure 2 illustrates the measurement of the AP and LAT dimensions on a patient's CT axial image. The measurements were obtained on the widest part of the patient on a CT axial image. The AP measurements were made on the level of the vertebral bodies between thoracic vertebra 12 and lumbar vertebra 3, and the LAT measurements were made on the level of the center of both kidneys. Electronic callipers were used to measure the AP and LAT dimensions in millimeters (mm), which were converted into centimeters (cm).

The contrast-enhanced abdominal examinations of the pediatric patients were performed with the General Electric (GE) HealthCare CT unit Discovery model CT750HD (GE HealthCare Technologies Inc.; Chicago, IL, USA) at the participating hospital. The annual quality control (QC) tests were performed on the CT unit, and the QC test results adhered to the manufacturer's recommended guidelines. The results of the QC tests were satisfactory, and the same CT unit was used throughout the study period. The GE HealthCare CT unit used for the CT scanning of participants involved in this study used the automatic tube current modulation (ATCM) (SmartmA and AutomA) and the iterative reconstruction (IR) algorithm, namely adaptive statistical iterative reconstruction (ASIR).

The target population consisted of pediatric patients who presented with nephroblastoma and underwent contrast-enhanced abdominal CT examinations at the participating hospital. Out of 265 patients that formed part of the study population, only 121 patients adhered to the inclusion criteria of this study to form the sample size. Therefore, the sample size of pediatric patients forming part of this study was 121 patients. Stratified sampling was used to randomly select pediatric patients based on age, weight, and clinical indication [27]. Patients aged younger than 18 years who weighed between 0 and less than 80 kg were considered pediatric patients. Only patients who presented with nephroblastoma and underwent contrast-enhanced chest-abdomen-pelvis CT examinations between January 1, 2018 and October 9, 2022 were included in the study. The exclusion criteria were patients who were 18 years and older; patients not presenting with nephroblastoma for contrast-enhanced chest-abdomen-pelvis CT examinations during the study period; and patients booked for CT examinations of the brain-chest-abdomen, neck-chest-abdomen, neck-chest-abdomen-pelvis, pulmonary and/or abdominal angiogram.

3 Results

The data of 121 patients who adhered to the study's inclusion criteria were analyzed, where 66 patients (54.6%) were male. The patients were initially categorized into five weight groups. However, when developing pediatric DRL values, the ICRP [10] recommended collecting data from at least 20–30 patients per category with agreed weights for CT examinations. Therefore, three weight groups, group 1 (0 to < 5 kg) with one (0.83%) patient, 4 (30 to < 50 kg) with seven (5.76%) patients, and 5 (50 to < 80 kg) with two (1.65%) patients, did not have the required number of patients. Consequently, DRL values were established only for weight groups 2 (5 to < 15 kg) with 57 (47.11%) patients and 3 (15 to < 30 kg) with 54 (44.63%) patients.

The pediatric typical DRL values developed in this study were based on the median of the data distribution. The pediatric DRL values were different for various weight groups for all dose quantities, as depicted in Figure 3. The pediatric DRL values based on the CTDI_vol ranged from 2.4 to 2.7 mGy for weight groups. The DLP, pediatric DRL values ranged from 78.4 to 108 mGy.cm for weight groups. The DRL values based on the SSDE parameters (AP, LAT, SUM, and effective diameter) ranged from 4.9 to 5.6 mGy weight groups.

A weak positive correlation was observed between patient weight and dose quantities (CTDI_vol, DLP, and SSDE parameters) for weight group 2 with r = 0.04 for CTDI_vol, r = 0.01 for DLP, and r = 0.01–0.05 for SSDE parameters. All these dose quantities had a nonsignificant correlation (Spearman correlation test; p > 0.001) with weight for weight group 2. Weight group 3 showed a positive significant correlation (Spearman correlation test; p < 0.001) between the patient weight and all dose quantities with r = 0.54–0.62 for CTDI_vol, DLP, and SSDE parameters. The p values determined by the Kruskal-Wallis test indicated a significant (Kruskal-Wallis; p < 0.001) difference between both weight groups and among dose quantities. To investigate whether there was a significant difference between CTDI_vol and SSDE_sum, the median of the paired differences was calculated. The median difference between CTDI_vol and SSDE_sum was 2.87, with 95% confidence limits of 2.73–2.96. Since the confidence interval does not include zero, this indicates a statistically significant difference between the two measures.

The pediatric DRL values developed in this study were also compared with DRL values developed in international studies [19-21], with the comparison summarized in Table 3. For all indicated groups, the DRL values of this study were lower than the EC [28] DRL values and higher than those of other international studies.

4 Discussion

This study aimed to establish pediatric DRL values for contrast-enhanced chest-abdomen-pelvis CT examinations in patients with nephroblastoma. To our knowledge, there is a paucity of literature specifically addressing DRL development for CT contrast studies in this patient population. Most existing DRL values are based on anatomical regions rather than clinical indications. However, this approach (anatomical DRL values) presents limitations, as different clinical indications for the same anatomical region may require varying imaging qualities, scan phases, and, ultimately, different radiation doses [10]. Dalah and Bradley [30] observed that radiation doses can vary among studies involving patients with identical clinical indications. This variation may be attributed to several factors, including patient size and scan parameters. According to Dalah and Bradley [30], developing scan-acquisition-based DRLs per protocol involves comparing patients subjected to the same protocol, clinical indication, and scan acquisition series. Our study's findings support Dalah and Bradley's [30] suggestions, as all participants with a clinical diagnosis of nephroblastoma underwent a contrast-enhanced CT chest-abdomen-pelvis protocol performed with one scan phase, namely the portal venous phase.

Various scanning parameters were considered because of their known impact on radiation dose, including kilovoltage (kV), milliampere-seconds (mAs), gantry rotation time, section thickness, pitch, and scan length. The ATCM was used in all scans, allowing most parameters to remain consistent except for mAs, which varied according to tissue attenuation. While dose reports were retrieved from the PACS, the GE CT unit used, did not provide detailed imaging parameter documentation, limiting retrospective analysis. The kVp was set at 100–120, and the pitch ranged between 0.53 and 0.98. As reported by Raman et al. [31], ATCM increases mAs in areas with higher attenuation and decreases it in lower-density regions. Although this study did not investigate the specific impact of each parameter on dose, prior literature confirms that radiation dose generally increases with higher mAs, kVp, thinner slices, and extended scan ranges, while decreasing with higher pitch, faster rotation, and proper patient centering [8].

Pediatric DRL values in this study were calculated based on weight groups, as recommended by the ICRP [10], EC [28], and Hwang et al. [21], respectively. DRL values were reported using the median of dose quantities: CTDI_vol, DLP, and SSDE_SUM. Additionally, the 75th percentile values, as shown in Table 3, were calculated to allow for more direct comparison with international reference levels, which often report DRL values based on the 75th percentile. Since this study was conducted at a single healthcare facility performing a high volume of specialized scans for nephroblastoma, we report typical DRL values, which are especially useful in settings where national DRLs are unavailable [10]. Using median values to derive these typical DRLs aligns with methodologies used by other studies, such as Tan et al. [32].

Our study's DRL values could not be directly compared to international DRL values for CT chest-abdomen-pelvis, as no published data exist for nephroblastoma-specific DRLs by weight group. Nonetheless, comparisons were drawn with studies employing anatomical regions and weight-based classifications that most closely aligned with those used the present study. The DRL values we reported were lower than those published by the EC [28] but higher than those reported by Almén et al. [20] and Hwang et al. [21]. The lower values in Hwang et al. [21] may be attributed to their use of a single scan phase, ATCM, and IR algorithms, all of which contribute to dose reduction. Our study also used ATCM (SmartmA and AutomA) and IR (ASIR), which can reduce dose by 30%–40% [10]. However, ATCM may increase pelvic radiation dose when the patient is improperly centered in the gantry, possibly contributing to higher DRL values in our study compared to Hwang et al. [21, 33]. Sebelego et al. [34] noted that variations in CT units and their age can impact DLP and CTDI_vol values. The lower DRL values reported by Almén et al. [20] and Hwang et al. [21] may be attributed to differences in CT equipment, though they did not specify the units used. Almén et al. [20] and Hwang et al. [21] studies used CT abdomen-pelvis examinations, which have a shorter scan length, while our study included chest-abdomen-pelvis examinations, leading to longer scan lengths and higher doses. This explains their lower DRL values compared to ours. Almén et al. [20] did not specify the use of ATCM or IR and employed a different CT unit with a larger sample size and no contrast media administration, which could also explain their lower dose values. Importantly, Hwang et al. [21] is the only study besides ours to report SSDE values stratified by weight groups. This allowed for a more meaningful comparison between studies, as SSDE is a more patient-size–specific dose metric than CTDI_vol or DLP alone. The similarity in methodology strengthens the relevance of the comparative analysis and highlights the value of including SSDE when developing pediatric DRL values. A significant difference between the CTDI_vol and SSDE was observed, supported by a 95% confidence interval at 50% median, indicating that these quantities impact radiation exposure differently. Since the CTDI_vol reflects only the scanner's output and not patient-specific doses [13], the AAPM [13] recommends using SSDE, which adjusts for patient size and provides a more accurate dose estimation.

Statistical analyses further validated the use of weight-based groupings. The Kruskal-Wallis test showed significant differences in dose quantities across weight groups, confirming that patient weight significantly influences radiation dose. Additionally, Spearman's correlation revealed a significant positive relationship between all dose quantities (CTDI_vol, DLP, SSDE_SUM) in both weight groups. According to the Spearman's correlation analysis, the parameters DLP, CTDI_vol, and SSDE_SUM did not exhibit a statistically significant correlation with weight in weight group 2. The lack of correlation in group 2 may be due to variations in ATCM responsiveness, mispositioning within the gantry, or patient anatomy, which affects surface-based dose modulation more than weight [8]. Despite this, patient weight strongly correlated with body measurements and age, as found in studies by Imai et al. [14] and Hwang et al. [21]. This supports ICRP [10] recommendations to categorize pediatric patients by size, particularly weight, when developing DRL values. Our study followed this approach, incorporating SSDE as a patient-size-specific dose quantity.

A major limitation in both our study and previous literature was the difficulty in collecting sufficient pediatric data. Almén et al. [20] and Hwang et al. [21] both reported challenges in establishing DRL values across all weight and age groups due to limited data. Similarly, our DRL values were only calculated for weight groups 2 (5 to < 15 kg) and weight group 3 (15 to < 30 kg). This limitation is reflective of the epidemiology of nephroblastoma, which is most prevalent in children aged 1–5 [16]. Additionally, during the time of data collection, the data available on the CT unit were of patients who underwent CT chest-abdomen-pelvis examinations to diagnose nephroblastoma during the period from January 1, 2018 to October 9, 2022. The COVID-19 pandemic may have further impacted patient attendance, as concerns over virus exposure likely caused delays or cancellations in scheduled imaging appointments. These pandemic-related disruptions may have contributed to the reduced sample size, particularly in older or heavier-weight groups. Moreover, the participating hospital does not have DRL values, and the current practice has not changed since; therefore, the authors are of the opinion that the DRL values can be used as a baseline.

Ultimately, patient weight remains the most dependable factor linked to a DRL quantity [10, 28] because the radiation dose used for pediatric examination varies greatly due to the great variation in patient weight [10]. However, further research should examine how ATCM adjusts exposures in pediatric populations across varying body sizes. To improve future DRL accuracy, we recommend that weight be measured directly rather than estimated from contrast dose. Additionally, SSDE should be prioritized when calculating DRL values due to its sensitivity to patient size [13]. Future studies should aim for a minimum of 20–30 patients per weight group and consider clinical indication in DRL development, as exposure levels may vary significantly for the same anatomical region depending on the diagnostic need.

5 Conclusion

Clinical indication-based pediatric typical DRL values in chest-abdomen-pelvis CT examinations for body weight groups using SSDE were established. For efficient dose optimization, clinical indication, body weight, and SSDE should be considered when developing pediatric DRL values. The pediatric DRL values (based on the median) developed in this study were lower than the DRL values developed by EC [28] and higher than the DRL values developed in other studies [20, 21]. The pediatric DRL values developed in this study can be used as a baseline for future pediatric DRL values for the CT chest-abdomen-pelvis of nephroblastoma patients.

Author Contributions

Tebello Pitso: conceptualization, writing – original draft, investigation, data curation, methodology, formal analysis, and visualization. Ida-Keshia Sebelego: writing – review and editing, supervision, data curation, and validation. Henra Muller: writing – review and editing, supervision, and validation.

Conflicts of Interest

The authors declare no conflicts of interest.

Transparency Statement

The lead author, Ida-Keshia Sebelego, affirms that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted; and that any discrepancies from the study as planned (and, if relevant, registered) have been explained.