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Radiation dose quantities and risk in neonates in a special care baby unit

Medical Physics Department, Royal Free Hampstead NHS Trust, Pond Street, London NW3 2QG, UK

	Abstract

Top Abstract Introduction Materials and methods Results and discussion Conclusions References

 
Radiographs are taken in the neonatal period most commonly to assist in the diagnosis and management of respiratory difficulties. Frequent accurate radiographic assessment is required and a knowledge of the radiation dose is necessary to justify such exposures. A survey of radiation doses to neonates from diagnostic radiography (chest and abdomen) has been carried out in the special care baby unit of the Royal Free Hospital. Entrance surface dose (ESD) was calculated from quality control measurements on the X-ray unit itself. Direct measurement of radiation doses was also performed using highly sensitive thermoluminescent dosemeters (TLDs) (LiF:Mg,Cu,P), calibrated and tested for consistency in sensitivity. ESD, as calculated from exposure parameters, was found to range from 28 µGy to 58 µGy, with a mean ESD per radiograph of 36±6 µGy averaged over 95 examinations. ESDs as derived from TLD crystals ranged from 18 µGy to 58 µGy for 30 radiographic examinations. The mean energy imparted, the mean whole body dose per radiograph and the mean effective dose were estimated to be 14±8 µJ, 10±4 µGy and 8±2 µSv, respectively. Assuming that neonates and fetuses are equally susceptible to carcinogenic effects of radiation, which involve an overestimation of risk, the radiation risk of childhood cancer from a single radiograph was estimated to be of the order (0.3–1.3) x 10^-6. Radiation doses compared favourably with the reference values of 80 µGy ESD published by the Commission of the European Communities in 1996, and 50 µGy published by the National Radiological Protection Board in 2000.

	Introduction

Top Abstract Introduction Materials and methods Results and discussion Conclusions References

 
Diagnostic radiology plays an important role inthe assessment and treatment of neonates requiring intensive care. It is often necessary to perform a large number of radiographic examinations depending upon the infant's birth weight, gestational age and respiratory problems. Radiographic examination of children, especially neonates, attracts particular interest because of the increased opportunity for expression of delayed radiogenic cancers as a consequence of relative longer life expectancy. The yield of certain forms of radiation-induced cancer, particularly leukaemia, appears to be some five times higher in children than in adults [1]. Moreover, the small sizes of newborn infants brings all organs within or closer to the useful beam, resulting in a higher exposure to effective dose conversion factor per radiograph than may be the case with adults. It istherefore important to ensure that radiation doses from radiographic examinations carried out in neonatal units are kept to a minimum whilst maintaining the quality of radiographic images.

Wide variations have been found in techniques, equipment performance and radiation dose in different hospitals in a European survey of paediatric radiology [2]. The results have highlighted the need to develop dose standards for paediatric and neonatal examinations. Recommendations on equipment and techniques for radiographic examinations of newborn infants have been made by the European Commission (EC) with the aim of at least achieving a "reference entrance skin dose" of 80 µGy [3].

This paper describes a prospective study of radiation dosimetry performed in the special carebaby unit (SCBU) at the Royal Free Hospital. A variety of dosimetric quantities including entrance surface dose (ESD), energy imparted (EI), whole body dose and effective dosehave been measured and recorded. Finally, an attempt has been made to evaluate the applicability of the thermoluminescent dosemeter (TLD) LiF:Mg,Cu,P as a reliable dosimetry method used in a SCBU.

	Materials and methods

Top Abstract Introduction Materials and methods Results and discussion Conclusions References

 
All radiographic examinations were performed on a capacitor discharge mobile X-ray unit type 38S(GEC) (GE Medical Systems; Slough, UK) with a single phase generator, total filtration of 3.6 mm aluminium (Al) equivalent thickness and an X-ray tube target angle of 17°. Radiographs were acquired using Kodak T-Mat G/RA film andKodak Lanex Regular Screen combination (Kodak Health Imaging, Hemel Hempstead, UK) with a 400 relative film–screen combination speed. Most examinations were carried out with the baby in an incubator and placed directly on top of the cassette with the focus-to-film distance (FFD) set at 100 cm. Pieces of lead rubber were placed on the Perspex top of the incubator to reduce the size of the X-ray beam to the area of interest. Dose measurements were performed for the most frequent simple examinations, namely abdomen and chest.

Indirect method of measurement
Measurements of tube output were made using a 15 ml ionization chamber with calibration traceable to the national standards. ESD was estimated for each patient and for each exposure from knowledge of the technique factors, X-ray tube output and backscatter factors (BSFs), in accordance with the following formula:

where the ISL factor is an inverse-square law correction from the focus-to-chamber distance (100 cm) to the focus-to-skin distance (FSD), and (µ_en/)_tis and (µ_en/)_air are the mass energy absorption coefficient for tissue and air, respectively.

A BSF of 1.1±5% was employed, determined by Chapple et al [4] for a neonate with body thickness of 5 cm, with tube potentials in the range 50–70 kVp, for a field size of 70–300 cm², using Monte Carlo techniques.

The FSD was not measured directly but wasapproximated by the difference between the known FFD and the neonate equivalent diameter. Owing to difficulties in obtaining an accurate measurement of length or trunk diameter, an average equivalent patient diameter equal to 7.5±1.4 cm was used [5].

The mass energy absorption coefficient ratio averaged over the X-ray energy spectrum was evaluated for muscle as defined by the International Commission on Radiation Units and Measurements (ICRU) and is equal to 1.05 for the 50–58 kVp range used in this study, with an uncertainty of no more than ±1% [6].

The uncertainty in ESD was calculated as the quadrature sum of the estimated uncertainties in output measurement (±3.2%), the use of patient diameter in the ISL correction (±5%) and the BSF evaluation (±5%), to give a value of ±8%.

The EI to the neonate is derived from the ESD integrated over the irradiated area (dose–area product (DAP)). The irradiated body area from each radiograph was deduced from measurements made of the exposed area within the film. This area varied widely owing to different patient sizes but mainly owing to the varying degrees of collimation employed. The DAP can be approximated by the product of the ESD and the irradiated film area demagnified from the FFD to the FSD. This approximation results in a DAP evaluation including backscatter, since the ESD has been calculated after applying the BSFs.

EI is calculated from the estimated DAP using conversion factors for neonates exposed to X-rays with energies between 50 kVp and 70 kVp, determined by Chapple et al [4] for a single-phase generator, an anode angle of 17° and a net filtration of 2.5 mm Al equivalent. Estimates of radiation risk can be made from EI by assuming that all radiosensitive organs are uniformly distributed through the irradiated portion of the body [4]. A whole body dose is determined by dividing IE by the weight of the neonate.

Effective dose was estimated by using the conversion coefficients provided by the National Radiological Protection Board (NRPB) [7] to estimate effective dose from measurements of ESD for a baby less than 1-year-old, and for anteroposterior (AP) chest and AP abdomen radiographic examinations.

Direct method of measurement
In this study LiF:Mg,Cu,P TLDs (TLD-100H; Bicron, Solon, OH) were employed. These combine the benefits of the standard LiF:Mg,Ti TLDwith a lower detection limit, improved tissue equivalence and 20–40 times greater sensitivity. Only a few reports have studied the performance of LiF:Mg,Cu,P TLDs in neonatal X-ray dosimetry. An excellent linearity in the µGy–mGy range as well as reproducibility at calibration dose levels (mGy to cGy) of 3% has been reported by Duggan et al [8]. Annealing, irradiation and read-out experiments were performed to verify the credibility of the TLD chips in their sensitivity and usefulness in this study. This cycle was repeated seven times and the mean and SD of each chip has been used.

Annealing was performed in a microprocessor-controlled annealing oven (Carbolite Model TLD 3 Mk II; Carbolite, Sheffield, UK). The TLDs were annealed at 240°C over 18 min and then removed from the oven straight away using a supplementary Alblock to cool them down more quickly. Read-out was performed on a Harshaw Model 5500 automatic reader (Bicron, Solon, OH) based on heating with hot nitrogen gas. A ramp heating cycle (270°C for 12 s, 10°C s^-1) was set up in the reader(Biuon, Solon, OH) to perform TLD read-outs [9].

A dedicated calibration employing a Perspex jig (5 cm block of tissue-equivalent Perspex) and tissue substitute phantom was performed using the mobile X-ray unit on the SCBU. The jig held the TLD chips and the calibrated 15 ml ionization chamber to measure the exposure in µGy. The phantom supported the jig and consisted of 1 cm slabs of solid water that, when stacked, represented different neonate thicknesses. The unenclosed TLDs were irradiated with radiographic exposure factors 55 kV, 2 mAs. The distance from the X-ray focus to the Perspex jig was fixed at 85 cm and the field size at the level of the TLD chips was adjusted to 15 x 15 cm².

Data from the Harshaw TLD reader were used to determine an individual calibration factor for each TLD chip used, in order to convert the read-out, given in coulombs, into calculated dose to the chips. The individual computed exposures were divided by the exposure given by the ionization chamber to give a normalized computed exposure (nC µGy^-1). A mean calibration factor was determined individually for each chip by taking the average of the normalized computed exposures for the total of seven exposures.

To ensure that the packaged TLD chips (in thinpolyethylene radiolucent sachets) would not actually show up on the films and would not obscure the anatomical and pathological details, they were placed on different thicknesses of solid water and were irradiated with the X-ray mobile unit (2 mAs, 48–60 kV). The image quality test showed that the chips were seen in the radiographic image when 4 cm and 5 cm of solid water was used. Consequently, the most appropriate place to put the TLDs was considered to be in the X-ray beam, on the shoulder of the baby if a chest X-ray was being performed, and on the hip of the baby if an abdominal X-ray was being performed.

	Results and discussion

Top Abstract Introduction Materials and methods Results and discussion Conclusions References

 
A total of 30 neonates were included in this study. The mean number of examinations received by one neonate was 3.2, which compares with values of 3.8[10], 5.3 [4] and 4.7 [5] in other studies.

The mean ESD per radiograph was calculated to be 36±6 µGy, averaged over the 95 radiographs included in the study. Table 1 summarizes the radiographic data and calculated ESDs accrued in this study.

A more significant measure of risk is the EI to the neonate and the effective dose; effective dose has not been considered in previous studies and only a few studies [4, 5, 10] have considered EI in SCBU radiology. Table 2 summarizes the results from the DAP, EI and effective dose calculations.

Estimates of radiation risk can be made eitherfrom EI, by assuming that all radiosensitive organs are considered uniformly distributed through the irradiated portion of the body [4], or from effective dose [7]. It is difficult to determine the most appropriate risk factor for neonates. The alternatives are either to correlate our data with studies on fetuses in utero, or to assume that thesensitivity to ionizing radiation for newborn babies is more similar to that ascribed to young children. Since the majority of neonates in this study were pre-term, the appropriate risk factor was felt to be that for fetalirradiation. According to the International Commission on Radiation Protection (ICRP) Report 60 [12], the risk of fatal childhood cancer owing to pre-natal exposure has been estimated to vary from 2.8 x 10^-2 Sv^-1 to 13 x 10^-2 Sv^-1. The authors stress that the risk in the first trimester appears to be greater than that found in the second and third trimesters, but this is not established. If we accept that the cancer risk (meaning leukaemia) is the same for the second andthird trimesters, it should be fairly similar for radiographs taken shortly after birth. Therefore, using these factors, the risk of childhood cancer from a single radiograph would be of the order (0.3–1.3) x 10^-6. However, the assumption that the newborn and fetus are equally susceptible tocarcinogenic effects of radiation involves an overestimation of risk. First, irradiation in utero involveswhole body exposure of the fetus, whereas neonatal radiography involves only partial exposure. Second, it is not known whether babies in a higher oxygen tension than fetuses run a greater riskof carcinogenesis from radiation [13].

The results show the risk from neonatal radiation to be fairly low, and it is considered to be substantially outweighed by the clinical benefit of the radiograph in assessing the progress of a sick baby. However, the risk vs benefit of each radiograph is important and must be considered carefully, especially as radiation effects are cumulative.

Comparison with previously published data
Our results may be compared with previously published data to attempt to delineate mechanisms for dose reduction. Table 3 summarizes a comparison between examination techniques, mean ESD and mean EI per radiograph in this study and in those of others. Effective dose has not been considered in any of the previous studies.

Probably the most significant factor in radiological technique accounting for the EI to a neonate is careful collimation of the radiation field to the area of interest. Adequately trained staff should perform neonatal radiographic examinations so that the number of repeat radiographs is reduced to the absolute minimum, and the highest standards of radiation protection are achieved.

Comparison between the two dosimetry techniques
Table 4 gives the results from TLD measurements for each examination and gives a comparison between ESDs measured with the TLDs and ESDs calculated from technique factors. The uncertainty in the measurement from the TLD reader was taken as half of the least significant figure. The combination of this error as a percentage error, with the standard error (STERR) as a percentage error, determined the overall error in the reading according to the following formula:

Uncertainties in the measurement of doses involve the fact that TLD chips were not placed on the centre of the radiation beam during the X-ray examination so as not to affect image quality. Since they were placed on the edge of thebeam (shoulder/hip), they measured a somewhat lower dose. To quantify this difference we repeated the irradiation of dosemeters with 55 kV and 20 mAs. A number of TLD chips were placed on the centre of the beam and a few at the edge. From the differences in TLD read-outs a difference of the order of 7% in dose was found.

	Conclusions

Top Abstract Introduction Materials and methods Results and discussion Conclusions References

 
A variety of quantities (ESD, DAP, EI, whole body dose and effective dose) need to be measured and recorded so that an estimation of riskcan be derived. Although the radiation risk ofX-ray examinations is found to be low in comparison with the benefit to the infant, radiography of newborns should be performed with full knowledge of the possible harmful effects, considering that infants are particularly susceptible toradiation-induced cancer and that premature babies may require a large number of X-ray examinations during the early weeks of life.

The risk of radiation to the newborn is minimized by making sure that only essential radiographs are taken, that careful collimation confines radiation to the relevant part of the infant, that radiation shields over the lower abdomen are used unless this area is to be included on the radiograph and that adequately trained staff perform the examinations so that the number of repeat radiographs is reduced to the absolute minimum.

Comparison between different studies revealed a large range of doses in a SCBU, showing a continuing need for assessment of radiation dose in the neonatal nursery together with regular review and implementation of dose reduction procedures. Each SCBU should have a dosimetry study performed to achieve further dose optimization and to establish reference dose levels for neonatal X-ray examinations.

Received for publication June 4, 2001. Revision received November 12, 2001. Accepted for publication December 7, 2001.

	References

Top Abstract Introduction Materials and methods Results and discussion Conclusions References

 

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