Investigation of pediatric backscatter factor with an anthropomorphic phantom

doi:10.21203/rs.3.rs-3398679/v1

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Investigation of pediatric backscatter factor with an anthropomorphic phantom

https://doi.org/10.21203/rs.3.rs-3398679/v1

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Background

Entrance surface dose (ESD) is used to establish diagnostic reference levels (DRLs), which require a backscatter factor (BSF) for calculation. However, as no studies on pediatric BSF exist, adult BSF is currently used in pediatric ESD calculations.

Objective

The aim of this study was to derive a BSF using a pediatric phantom and conduct a comparison with BSFs derived from adult phantoms and existing data in general radiography.

Materials and methods

Incident air-kerma at the surface of a pediatric anthropomorphic phantom was measured using a set of optically stimulated luminescent dosimeters (OSLDs). Measurements were obtained using nanoDot OSLDs under different beam qualities and field sizes in three body regions, at a focal–surface distance of 100 cm. Air-kerma was measured using a set of nanoDots in the same positions in the absence of the phantom, and BSF was calculated using the two sets of data. The pediatric BSF values were compared with those obtained from adult phantom and existing data.

Results

BSF values were significantly lower for the pediatric phantom than for the adult phantom and were lowest in the chest (16%) under specific conditions and the greatest discrepancies of BSF values was found in PMMA tissue (19%) on every tube voltage when compared with the existing data.

Conclusion

Based on data obtained for the three body parts investigated, pediatric ESD should be calculated using human shaped pediatric anthropomorphic phantom rather than adult using adult and existing data.

pediatric dosimetry

backscatter factor

nanoDot

phantom

Radiation dose in pediatrics is a constant concern because children are more affected by stochastic and deterministic effects than adults due to their high tissue sensitivity and long-life expectancyIn response to concerns regarding radiation dose, several organizations have published guidelines for its measurement in pediatric patients, such as the IAEA Human Health Series No. 24 [1] and ICRP publications 121 and 135 [2, 3]. These publications suggest the use of entrance surface dose (ESD) in diagnostic reference levels (DRLs) for general radiography. According to ICRP publication 135, the DRLs should be revised every 3–5 years to ensure their optimization and that they correspond to developments in technology and techniques. In Japan, the latest DRLs were derived in 2020 for seven modalities [4]. ESD is calculated using a backscatter factor (BSF), which is the ratio between the dose on the surface of the object and the dose in air at the same point when the object is absent.

From the past, BSF measurements has been published. Various types of radiation dosimeter were used to measure the entrance air kerma at the surface of the irradiation object and kerma in air in the same position when the object was absent. Additionally, Monte Carlo simulations also used to calculate the BSF with simple geometry irradiation object. These studies include those of Grosswendt [5, 6], Chica [7], and Petoussi-Henss [8], of which the latter was cited in IAEA TRS-457 with regard to the reference BSF for adults [9]. However, these data were calculated with an adult phantom and for field sizes ranging from 10 to 20 cm² While, a pediatric phantom is smaller and has a greater irradiation area than an adult phantom, and differences in thickness and diameter therefore result in different BSF measurements. Direct measurements of BSF have been performed using radiochromic film [10], ionization chambers [11, 12] and TLDs (thermoluminescence dosimeters) [13, 14] and various other established dosimeters. Optically stimulated luminescent dosimeters (OSLDs) have been produced since 2015. They have many advantages over other dosimeters (minimal fading effect, ability to obtain a wide range of measurements and energies in a small detection volume, and they do not interfere with the interpretation of diagnostic images) [15] that suggest their suitability as alternative tools for BSF measurement.

The received radiation dose depends on the shape and region of the body, as the human body has an uneven surface and consists of non-homogeneous tissue types. Arimoto and Asada investigated BSF in different regions using an anthropomorphic phantom with non-flat surfaces, using OSLDs and various beam qualities and field sizes. They derived BSF at 50, 80, and 120 kVp for field sizes of 10 cm x 10 cm, 20 cm x 20 cm, 30 cm x 30 cm, and 40 cm x 40 cm [16]. However, the BSFs derived from these experiments were all calculated using real or computer-model adult phantoms, and to the best of our knowledge there are no reports of BSFs calculated for pediatric use, although various BSFs have been defined for use in other fields (including radiotherapy, nuclear medicine, and diagnostic radiology) and modalities [17, 18].

The aim of this study was to derive a BSF using a pediatric phantom and conduct a comparison with BSF derived from adult phantoms [16] and existing data [8] in general X-ray radiography.

Materials

The nanoDot OSLD is composed of Al₂O₃:C crystals enclosed in a light-tight plastic holder. When these crystals are exposed to ionizing radiation, they store energy that is later emitted as luminescence at 420 nm when stimulated with light at 540 nm. The luminescence intensity depends on the absorbed dose and the stimulation light's intensity. This information was established by Jursinic [19]. The nanoDot OSLD has a wide energy range of usability, spanning from 5 keV to 20 MeV. It boasts a lower detection limit of 0.1 mGy and an accuracy of 10% in energy dependence within the diagnostic energy range of 70 kVp to 140 kVp, and standard accuracy within 10%. In addition, experimental data have revealed 40–50 times greater sensitivity for OSLDs compared with TLDs [20–22].

All experiments were performed using KXO-50SS (Toshiba Corporation, Otawara, Japan) X-ray generator (total filtration 3.0 mmAl). To compare with the experiment of Arimoto and Asada [16] the set of parameters were utilized with a total filtration of 3.0 mmAl. Half-Value layers (HVL) were determined using ThinX RAD (RaySafe, Hova, Sweden) at 2.0, 3.1 and 4.6 mmAl, 100 cm of focal- surface distance, the examination areas including the skull, chest and pelvis with field sizes of 10 x 10, 20 x 20, 30 x 30 and 40 x 40 cm². NanoDot OSLD dosimeters (Nagase-Landauer, Tsukuba, Japan) were used to measure air-kerma and entrance surface air-kerma. OSLD measured values were read using a microStar reader (Nagase-Landauer). An Olivia 1-year pediatric anthropomorphic phantom (height 75 cm; weight,10 kg; physical density, 1.45 g / cc; electron density, 4.606 × 10²³; Computerized Imaging Reference Systems [CIRS] Inc., Norfolk, VA) and a RANDO adult female phantom (The Phantom Laboratory, Salem, NY) which comprises a human skeleton encased in tissue-equivalent plastic (with a mass density of 0.985 g/cm³ and an effective atomic number of 7.30), replicating the human body's shape and including simulated lung tissue (with a mass density of 0.32 g/cm³ and an effective atomic number of 7.30) were used [23]. SPSS Ver.26 software (IBM, Tokyo, Japan) was used in statistical analysis.

We performed a physical comparison between the pediatric and adult phantoms. Table 1 lists the Sagittal diameter (thickness) and diameter of various parts of the 1-year pediatric anthropomorphic phantom and transverse view of the thoracic section of the pediatric phantom which is shown in Fig. 1. The phantom is made from CIRS proprietary tissue-equivalent materials, with linear attenuations closely matching real soft tissue and bone (within 1%) and lung tissue (within 3%) across a range from 50 keV to 15 MeV. Figure 1 illustrates the low-density lung tissue in each phantom, equivalent to 0.2 g/cc [24].

Table 1

Sagittal diameter (Thickness) and diameter of pediatric and adult phantom
Phantom	Sagittal diameter [cm]			Transverse Diameter [cm]
Phantom	Skull	Chest	Pelvis	Skull	Chest	Pelvis
Pediatric	15.0	12.0	12.0	13.0	13.5	14.5
Adult	18.0	19.0	19.7	16.5	20.7	30.0

BSF measurements with different body regions

To enable comparison of BSFs calculated using the present pediatric phantom with those obtained in an adult phantom by Arimoto and Asada, we used the same experimental setup described in their study [16]. Figure 2(a) shows the geometry of the experiment for measurement of radiation dose in air. Several threads were stretched through the air to support placement of a sheet of tissue paper. Four nanoDots were positioned on the paper, close to the center of the field, as shown in Fig. 3(a). All measurements were obtained at a focal-source distance (FSD) of 100 cm and were exposed 5 times to 50, 80, and 120 kV x-rays, with varying mA and irradiation time. A total of 20 measurement values (4 dosimeters x 5 measurements) were recorded for each condition and considered as one group. The dose at the surface of the phantom was measured in three body regions (skull, chest, and pelvis) under the same conditions as above, in each of four field sizes (10 cm x 10 cm, 20 cm x 20 cm, 30 cm x 30 cm, and 40 cm x 40 cm, as shown in Fig. 2(b)). Figure 3(b), (c), and (d) shows nanoDot placement for measurements corresponding to the skull, chest, and pelvis, respectively.

Calculation and assessment of BSF

Incident air kerma was measured by each nanoDot in each set (five times for each nanoDot, four nanoDots for each set, five sets for each group). The values were read five times in each dosimeter by microStar, and the average of three values (excluding the maximum and minimum values) was used in the analysis. BSF was calculated after multiplying the average values by the calibration correction factor for each element at each of 50, 80, and 120 kV, using the following equation:

$$BSF=\frac{{DK}_{s}}{{DK}_{a}}$$

where ${K}_{s}$ [mGy] is the air kerma at the surface of the phantom and ${K}_{a}$ [mGy] is the air kerma that in air at the same position.

The obtained BSF values were compared among the three regions and were also compared with those that were obtained using the RANDO adult female phantom and reported by Arimoto and Asada [16]. To enable comparison with the results of Arimoto and Asada, we derived BSF values using the parameters of half-value layer (HVL), field size, tube voltage, mA, and irradiation time used with the RANDO phantom. These were then compared with the BSFs values obtained using the present Olivia pediatric phantom, in three regions (skull, chest, and pelvis).

BSF values were compared by paired Student-t test using SPSS Ver.26 (IBM, Tokyo, Japan). Statistical significance was assumed at a 95% level of confidence (P-value = 0.05).

Table 2 BSF values for each field size according to tube voltage between pediatric and adult phantom

Average ± 1SD; () = half-value layer, *difference between adult and pediatric values significant at P < 0.05

BSF in different body regions

Table 2 shows the obtained BSF values according to tube voltage for each field size, for all three body regions in pediatric and adult phantom. Naturally, BSF was affected by tube voltage and by field size. In the data from the study by Arimoto and Asada [16], tube voltage had a greater effect on BSF than did field size in adults, as BSF increased with increasing field size until becoming saturated at a field size of 30 cm x 30 cm and larger. BSF will show no change or decrease with increasing field size under some conditions (e.g., in the pelvis at 80 and 120 kV, and in the chest at 80 kV). In contrast, our pediatric BSF values increased with increasing field size, as shown in Fig. 4. Comparison of the BSF values derived from each phantom showed an increase in BSF values with increasing tube voltage for both the pediatric and adult phantoms, with good agreement. The difference in BSF between the pediatric and adult values was < 16% (maximum difference, 16.2%) (Fig. 5).

Comparison in pediatric and adult BSFs

Comparison by paired Student-t test showed significant differences in BSF between pediatrics and adults in all regions except for 10 cm x 10 cm and 30 cm x 30 cm at 50 kV in the skull region ( 0.38 and 0.44, respectively; Table 3), in which the pediatric values were all significantly lower than the adult values.

Comparison with existing data

Comparison of the present anthropomorphic data and the existing data [8] revealed that the discrepancy in all BSF values increased as tube voltage and field size increased (Table 3). The largest discrepancies were observed in PMMA tissue for each tube voltage in the skull region at each field size: 11.0% for a field size of 20 cm × 20 cm at 50 kV, 18.0% for a field size of 10 cm × 10 cm at 80 kV, and 19.6% for a field size of 20 cm × 20 cm at 120 kV. These differences demonstrate the effect of phantom shape on the measured BSF values.

We determined backscatter factor (BSF) in a pediatric phantom using nanoDots OSLDs. NanoDots exhibit good batch homogeneity (< 5%) and reproducibility (3.3%), along with a linear dose response, but have been reported to have high angular dependence (up to 70%) in low kVp techniques such as mammography [25]. According to the study by Al-Senan [25] energy dependence was approximately 60% and recommendations were made for applying correction factors across the investigated energy range. Taking into consideration the angular and energy dependences and avoiding uncertainties associated with accumulated dose, OSLDs (nanoDots) are suitable for use as point dosimeters in diagnostic settings [25]. In the present study, we calibrated the nanoDots with respect to tube voltage to account for energy dependence (air kerma was calculated by multiplying the reading by a calibration constant for tube voltages of 50, 80 and 120 kV). Energy dependence was mitigated by calibrating each element with the energy used, but was possibly underestimated due to the nature of angular dependence. Nonetheless, we believe that underestimation may not be a significant issue for flat surfaces.

In the present study, increasing tube voltage and field size both increased the BSF values for the pediatric phantom, as shown in Table 2. The effect of tube voltage was more pronounced than that of field size, as the BSF consistently increased with tube voltage until reaching a saturated value. On the other hand, the BSF was converge to a saturation value when increasing field size as shown in Fig. 4. These findings align with existing BSF data [8], which demonstrated the highest BSF in lung tissue at 120 kVp, 3.0 mm Al, 0.1 mm Cu filter, and 6.62 mm Al HVL. This finding underscores the impact of HVL and filtering on BSF when tube voltages are fixed. However, differences emerged when these pediatric results were compared with adult BSF values [16]. In adults, the BSF increased until reaching a particular field size and then either remained unchanged or decreased with further increases in field size, which was presumably due to the 10% standard accuracy of nanoDots [22].

Discrepancies in BSF values can arise from various factors, and are not limited to tube voltage and field size. Beam quality also affects the BSF factor. Existing data [8], derived from a cuboidal mathematical phantom with three materials (water, ICRU, and PMMA), demonstrated greater BSF values than pediatric values across tube voltages and field sizes. This difference is attributed to variations in filtration and HVL, with increased filtration and HVL leading to higher BSF values. These findings corroborate the observations of Johns et al., who found that at a given HVL, kilovoltage and filtration influence the measured backscatter, with increasing HVL inducing higher BSF due to stronger beam qualities [26].

As shown in Table 1, the BSF is influenced by variations in phantom characteristics such as tissue density, thickness, and diameter, which is particularly relevant when considering phantoms of various compositions. For instance, the adult female RANDO phantom was designed with a focus on radiotherapy uses rather than being specifically tailored for diagnostic radiology. Consequently, its composition slightly differs from that of the human body and pediatric anthropomorphic phantoms, which were designed specifically for use in diagnostic radiology. The pediatric phantom includes materials that closely imitate human tissue in relation to factors like linear attenuation coefficient, physical characteristics, and electron density [24]. In the present study, we assumed that differences in diameter and thickness of the materials would have a more significant impact on the BSF in the pediatric and adult phantoms compared to the effects of their respective compositions. To validate this assumption, we measured the diameter and thickness of a pediatric Olivia phantom and an adult female RANDO phantom. The values for thickness were also less in the pediatric phantom: 15 cm vs. 18 cm for the skull, 12 cm vs. 19 cm for the chest, and 12 cm vs. 19.7 cm for the pelvis. Furthermore, with regard to accuracy, Benmakhlouf highlighted the necessity of performing material and thickness corrections of the irradiated objects when dealing with large field sizes [27]. These factors should be taken into consideration when interpreting the results of the present study and their implications for radiation dose measurements.

The shape and surface characteristics of a phantom also exert an influence on BSF. In adults, the breast region possesses intricate surface geometries that feature a valley-like configuration that is affected by transverse scattering of photons, thereby impacting the BSF [16]. However, the pediatric breast region has a smooth and flat surface. Accordingly, there is little to no effect of transverse scattering of photons in BSF calculated for a pediatric phantom in comparison to that for an adult phantom. Conversely, the pelvis does not have the valley-shaped expression of the chest, thus reducing the lateral scatter. However, when utilizing the nanoDots on the surface of the pelvis, there is a tendency for the dose to be underestimated due to the directional attributes of the dose reduction at 90° caused by transverse scattering, as reported by Al-Senan [25]. Whereas the prior BSF data [8] were derived for adults via the Monte Carlo method using a flat-surfaced cuboid phantom measuring 30 cm × 30 cm × 15 cm, we employed OSLDs and a direct measurement technique in the present study. Our study aimed to ascertain entrance air kerma at the surface and incident air kerma in the air at the same position without the pediatric anthropomorphic phantom present, and found that distinct BSF values emerged between the two groups. These findings underscore the importance of employing pediatric-specific BSF values when calculating ESD for pediatric cases.

The BSF values determined by Petoussi-Henss [8] may not align with BSF data derived from an anthropomorphic phantom, as evidenced in the comparison with BSF values acquired by Arimoto and Asada [16]. The present study undertook measurements and comparisons of BSF values between the pediatric anthropomorphic phantom (Olivia) and an adult phantom (RANDO), examining trends and correlations among factors including phantom type (pediatric vs. adult), tube voltage, and field size. BSF displayed an increase as tube voltage increased across all tested body regions, and exhibited increases for specific field sizes at certain tube voltages (Table 2, Fig. 3). In the pediatric phantom, BSF was lowest in the chest at 50 kV, indicating a minor backscattering effect in lung tissue compared to soft tissue, and peaked at 80 and 120 kV in the pelvis. Conversely, the adult phantom exhibited the highest BSF in the chest across all tube voltages due to its unique valley-like geometry, which is more susceptible to transverse scattering.

In addition, when comparing BSF values derived from the present pediatric data and existing data [8], the most differences were observed at 50 kV (11.0%), 80 kV (18.1%), and 120 kV (19.6%). These may arise from differences in HVL and other influence factors, which cause variation in beam quality and thus BSF values. It can be assumed that aluminum and copper filters enhance beam quality by compensating for low-energy components and reduce BSF. Investigating the influence of filters on BSF values is a subject of future inquiry. However, even though the differences have been found and showed that the BSF in children are slightly lower than in adults and as the difference between adult and pediatric values is in the range of some percent, so clinically, among the differences between two group the adult BSF is still possible for use to calculate ESD in pediatric.

There are some limitations of the present study. The results showed a difference in BSF values between the two groups in a limited scope of only three body regions, and thus the present findings cannot be applied to all regions of the body. Moreover, despite the presence of significant differences between the groups, it is challenging to draw firm conclusions about variations in trends because of the limited amount of data available. To accurately determine the trend of significant differences, it is necessary to conduct further research that includes other body regions, with an expanded amount of data.

However, BSFs have been calculated using a water phantom as the subject for more than 30 years. In DRLs for general radiography, ESD including a BSF is adopted as the DRL quantity. To estimate the ESD more accurately, it was necessary to use an anthropomorphic phantom instead of a water phantom, and to update the BSF, which was the original motivation for conducting the present experiment. In terms of the pediatric skull, we found that there was bone absorption and the BSF was considerably low, which are new findings. In addition, with the lack of available pediatric-specific BSFs, adult BSFs have been used to calculate pediatric ESD instead. We consider that the present finding of the importance of using a pediatric-specific BSF is of great clinical relevance.

Significant differences in backscatter factors (BSF) were identified between pediatric and adult phantoms in the three body parts evaluated and pediatric geometry should be used to calculate pediatric ESD. This suggests that pediatric backscatter factors should be used in those regions when calculating entrance surface dose in children. However, in the clinical process, it was found that the differences between the data groups are slightly small, and adult geometries can be used to calculate pediatric ESD.

Funding No funding was received for conducting this study

Conflict of interest The author declare that they have no conflicts of interest.

Ethical approval This article does not contain any studies with human participants performed by any of the authors.

Open access The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Version 1

posted

You are reading this latest preprint version

Investigation of pediatric backscatter factor with an anthropomorphic phantom

Status:

Version 1

Abstract

Background

Objective

Materials and methods

Results

Conclusion

Figures

Introduction

Materials and Methods

Materials

BSF measurements with different body regions

Calculation and assessment of BSF

Results

BSF in different body regions

Comparison in pediatric and adult BSFs

Comparison with existing data

Discussion

Conclusion

Declarations

References

Status:

Version 1