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1 Medical Physics Department, 2 Radiology Department, University Hospital of Larissa, PO Box 1425, Larissa 41110, Greece
Correspondence: Kiki Theodorou, PhD, Medical Physics Department, University Hospital of Larissa, PO Box 1425, Larissa 41110, Hellas, Greece. E-mail: ktheodor{at}med.uth.gr
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Introduction |
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However, the potential risk from radiation exposure of the patients must be balanced against the diagnostic information. Attention to radiation risk for children has increased in recent years and several studies have been performed in the field of dose calculation and the related risk [4–14].
Urinary tract infection (UTI) is a common paediatric problem. UTI incidence during childhood has been estimated to be 8% for girls and 2% for boys [15]. Vesicoureteric reflux (VUR), the retrograde flow of urine from the bladder to the ureters and renal pelvis, has been identified as the most important risk factor for the development of UTI. The prevalence of VUR has been estimated to be 18–40% of the child population investigated for their first episode of UTI [10, 15].
Micturition cystourethrography (MCU) is the most frequent examination and represents about 30–50% of all fluoroscopic examinations performed in children [4, 14]. MCU is considered to be the gold-standard method used to detect and grade VUR and show urethral and bladder abnormalities [4–15].
The irradiated field during the investigation contains many of the most radiosensitive organs and tissues. It depends on the patient's size and involves the abdomen below the diaphragm to the symphysis pubis. Since medical exposure has been justified, owing to its potential benefit to the patient, there are no prescribed dose limits, but practitioners should apply the principle of optimization to ensure that patient dose is as low as reasonably achievable while obtaining the necessary diagnostic information [2–4]. Optimization could be achieved by selection of modern equipment, technique, well-trained personnel and well-defined diagnostic reference levels (DRLs) consistent with the intended diagnostic purpose [10]. For paediatric dose assessment there are a number of particular challenges, including the accuracy of low-dose measurements, the variation in patient size and the variation in field size.
DRLs are recommended by national and international organizations [1–4] for standard X-ray examinations to monitor and optimize the radiation dose and to improve the techniques without affecting the diagnostic value. Several quantities can be used, such as dose area product (DAP – Gy cm2) and entrance surface dose (ESD – Gy). DRL is not a dose limit, but should not be exceeded in normal practice and is a good indication of what is commonly termed "best practice" [2, 16].
Finally, during paediatric MCU one person (co-patient) is necessary to support the child, so as to minimize the risk of movement leading to unnecessary repeat exposures. Co-patients often have to stand close to the X-ray beam in order to support and comfort the patient. As a result, their neck, hands and other parts that will not be protected by a conventional lead apron may receive significant scatter radiation.
The aims of this study were (i) to quantify and evaluate the radiation doses for children from 0 to 2 years undergoing MCU according to the protocol used at the Radiology Department, University Hospital of Larissa (UHL), Greece, (ii) to compare the doses based on available data obtained by other researchers and reference levels recommended by international organizations, (iii) to evaluate the technique applied in order to reduce patient and co-patient doses, (iv) to estimate the radiation risk to the sensitive organs, the thyroid and testes/ovaries, and finally (v) to estimate the radiation risk to the co-patient (in 76% of the cases the mother), who was present during the examination in order to immobilize and calm the child. The dosimetry protocol has been approved by the hospital's scientific committee. Since the measuring techniques were not invasive and do not obstruct in any way the diagnostic procedure, no ethical approval was necessary.
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Methods and materials |
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TLD calibration was according to international protocols for the range of energies used in the study [18–20]. The TLDs were calibrated under reproducible reference conditions using the same X-ray machine (Philips Diagnost 93) against an ionization chamber model 9060/10X5-60 connected to a Radiation Monitor Controller model 9010 (Radcal Corporation, Monrovia, CA). Both the chamber and the electrometer were calibrated for the energy range 30–120 kV at the National Standard Laboratory. For the TLD and chamber irradiation, a polymethylmethacrylate (PMMA) calibration test bed has been constructed having dimensions 30x30x6 cm3, which simulates the patient's lateral and backscatter conditions (Figure 1
). The first PMMA slab was used to accommodate the TLD chips in an array of slots 10x10. Each TLD was identified by its position in the array (Figure 1a). Individual calibration factors were obtained by irradiating the entire group to the same dose. The measured signal of each TLD was divided by the mean signal of the group. This process was repeated three times to reduce the effect of statistical variations and to determine the stability and reproducibility of the signal. TLDs with sensitivity within 4% were used in this study. All the TLD chips had the same thermal history. The calibration cycle was carried out every month.
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Four TLDs were irradiated to a known dose (standard dose) of the same radiation quality under the standard conditions (Figure 1b
). A second group of four TLDs was also irradiated to another known dose (test dose) to check the accuracy of the standard dose. The radiation dose received by the patient was calculated using the individual calibration factors of the chips and relating that reading to the reading of the standard TLDs, which had received a known dose. The mean background signal for unirradiated TLDs was subtracted before any calculation. The minimum detection limit was determined to be 15 µGy. The linearity of the TLD's response for the range of dose used in this study has been verified. The uncertainty of TLD reading was estimated to be not more than ±8% according to the test dose results.
X-ray machine
A Philips Diagnost 93 constructed in 1996, with an overcouch X-ray tube and undercouch image intensifier, was used. Philips Diagnost 93 is a remote-controlled universal X-ray system with a 90°/–90° tilt table and is equipped with a serial changer and 23 cm image intensifier. The resolution of the image intensifier is 1.8 lp mm–1 and the geometrical distortion is 6% at the centre and 11% at the periphery, resulting in very good image quality. The kVp and mAs ranges are 40–125 and 1–850, respectively; the total filtration is 4 mm aluminium. Last image hold capability is available. Focus to image receptor distance was fixed at 110 cm. Quality assurance tests were performed regularly for the X-ray machine.
Patient dosimetry
A total of 52 children (35 boys and 17 girls) were examined at the University Hospital of Larissa, with mean age of 0.36 years (0.02–2 years), and weight in the range 2.28 kg to 16.4 kg. The children were divided into two reference groups: newborn (0–6 months) and up to 2 years old (>6 months to 2 years). For each patient all the following parameters were recorded: radiographic data (kV, mAs and exposure time), fluoroscopic data (minimum and maximum kV and mA and total screening time) and patient data (name, sex, age, weight, height, date of birth, clinical indication, radiologist, start and end time). The mean and the range of the patients' physical data and the number of radiographic and fluoroscopic images are presented in Table 1. The radiographic and fluoroscopic exposure factors for each age group are shown in Table 2.
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Radiolucent containers were designed from a thin plastic foil to accommodate four chips of TLDs, which could not obscure diagnostic findings. Radiation doses were assessed by directly placing TLDs on the patient's skin in four locations: over the skin at the centre of the X-ray beam and over the organ sites for testes, ovaries and thyroid.
TLDs remained in the same position throughout the examination. The radiation field was collimated tightly to the area of interest.
Co-patient dosimetry
TLDs were placed on the outside of the lead apron (0.5 mm thick lead equivalent) at the level of left breast towards the child's head. The absorbed dose by the co-patient is calculated to be 10% of the dose recorded by the TLDs outside the lead apron. Attenuation of 90% was measured by placing TLDs inside and outside the apron and comparing the readings. For the unprotected parts of the body, the absorbed dose was assumed to be the same as the dose recorded on the TLDs at chest level. The equivalent dose has been taken equal to the absorbed dose (applicable for low LET radiation). The effective dose to the organs and tissues has been calculated using the methodology and tissue weighting factors reported in ICRP 60 and NCRP 122 [2, 21]. A computer program was developed, allowing calculation of the dose to 14 organs and tissues.
Investigation procedure
A standard protocol for MCU was established in order to ensure consistency of performance and apply radiation protection principles. Ultrasound was usually performed to the urinary system before the investigation.
200 ml of contrast medium (50 ml of ionized contrast medium (Ultravist) diluted in 150 ml of normal saline solution (0.9%)) was administered via a urethral catheter using a gravity drip. Catheterization was performed under strict aseptic conditions: the skin was carefully cleaned with antiseptic solution (Bethadine) and then a 6-F feeding tube was inserted in the urethra with the help of a sterile anaesthetic gel (Xylocine gel 2%). After urine egression the catheter was advanced a few centimetres more and was secured to the skin surface with tape. The catheter was taped in the left inner thigh of the child in order to avoid its projection over the male urethra in the lateral views.
Intermittent fluoroscopy was performed with automatically selected kV and mAs exposure parameters to detect VUR or other abnormality. Radiographic images were taken in cases of presence of reflux or of difficulty in evaluating a finding such as air-filled intestinal loops obscuring the area of interest.
One fluoroscopic image was obtained before the administration of contrast to ensure the correct position of the catheter (scout view). After contrast administration, the examination has two phases: filling of the bladder and voiding. Fluoroscopic images were taken during early filling of the bladder (valuable in case of ureteroceles and Grade 1 reflux that can be obscured by a fully filled bladder) and with full bladder.
During voiding, fluoroscopic images of the urethra were taken (in the lateral position for boys and supine position for girls); fluoroscopic images of the renal area and the bladder view were taken following voiding (for neurogenic bladder). Most of the small children (<1 year) do not empty their bladder completely. Occasionally right or left oblique views were performed. Since VUR is an intermittent phenomenon, filling and voiding of the bladder is repeated at least three times.
Effective dose and risk estimation
The measured ESDs for the thyroid, ovaries and testes were used to calculate organ equivalent doses HT using CHILDOSE (NRPB-SR279) [22], which is based on Monte Carlo simulations in paediatric phantoms for the same projections used in the clinical cases.
Effective dose E, has been calculated from the corresponding equivalent doses using the expression:
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where HT is the equivalent dose to tissue T and WT is the weighting factor representing the relative radiation sensitivity of tissue T.
The risk of developing cancer in a particular organ following MCU, or genetic effects in future generations after irradiation, was estimated by multiplying the mean organ equivalent dose with the risk coefficients obtained from ICRP 60 Table 3 [2]. It is well known that radiation-induced cancers cannot be distinguished from those produced by other possible carcinogenic agents because of the high natural incidence and the long latent period. Therefore, cancer risk estimation depends on the observation of a number of cancers of different kinds that arise in irradiated groups [1–3].
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Results |
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. The ESD and surface-scattered doses to the thyroid, testes, ovaries and the co-patient's radiation dose values are also presented in the same table. Radiation dose values varied widely with X-ray tube potential, patient size, source to image receptor distance and filtration applied. Table 4 gives the radiation dose to the patients with positive and negative VUR independent of the age.
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. The risk coefficients and organ equivalent dose (mSv) values for previously mentioned organs are also presented in the same table. The risks are very small especially when compared with the benefits that accurate diagnosis and treatment can provide.
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presents a comparison of the ESD values for the different age groups obtained from various studies reported in literature.
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Discussion |
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).
Radiographic exposure factors used in this study ranged from 65 to 80 kVp and from 4 to 21 mAs during the examination. Fluoroscopic applied voltage ranged from 40 to 78 kVp and tube current from 0.1 to 1.6 mA during image acquisition (Table 2). The applied voltages of the current study are comparable with the values reported in literature [7, 9].
In this study, the mean ESD was 1.13 mGy for all the patient populations. The mean radiation dose for the newborn group was found to be 1.15 mGy, whereas the mean measured value for the 2 year group was 1.05 mGy (Table 3). ESD was 50% lower than in other studies (Table 6). The difference could be due to imaging protocols, equipment and the number of radiographic images per MCU examination. This result indicates that a high degree of patient dose optimization was achieved in this study. The mean ESD per anteroposterior radiographic image ranged between 0.54 and 0.73 mGy per exposure, which is slightly lower than the corresponding values reported in the literature [9, 24]. In most non-reflux cases (negative) (45%) the patients were examined using only fluoroscopic images.
The mean ESD for patients with positive VUR is 1.45 mGy and for patients with negative VUR is 1.05 mGy (Table 4). ESD is higher for patients with positive VUR because the mean number of radiographic images is 2.0, whereas the mean number of radiographic images is 0.5 for patients with negative VUR, for both sexes.
The estimation of ovarian dose is very important because the ovaries contribute 20% of the effective dose based on the weighting factor from ICRP 60 [3].
The mean organ equivalent dose assessed from ESD using NRPB software [22], thyroid, ovaries, and testes was 0.006 mSv, 0.44 mSv and 0.33 mSv, respectively. The surface dose for ovaries (0.7 mGy) and testes (0.47 mGy) is similar to the equivalent dose, but the difference is significant for thyroid (0.15 mGy) because the thyroid is away from the radiation field.
For male patients, the European guidelines for paediatric radiology [4] recommend the use of testicle protective shielding during MCU. The ESD to the gonads is high because most of time they were inside the radiation field.
The mean effective dose per procedure assessed using the same software from NRPB [22] is 0.20 mSv, while in the literature, Shultz et al [13] report a mean effective dose of 0.2 mSv for newborn and 0.4 mSv for a 5-year-old child. Fotakis et al [9] reported a mean effective dose of 0.91 mSv for boys and 0.71 mSv for girls in the newborn group whereas the effective doses for a 1-year-old child were 0.89 and 0.83 mSv for boys and girls, respectively.
The mean co-patient dose was 0.14 mGy, range 0.03–0.5 mGy. The equivalent doses for thyroid and skin outside the lead apron are estimated to be 0.72 µSv and 0.072 µSv respectively. Mantovani and Giroletti [7] reported a mean effective dose to the co-patient of 4±7 µSv, and the equivalent doses to the thyroid and eye lens were estimated to be 20±19 µGy.
The overcouch X-ray tube is associated with a higher exposure to the hands and eyes of the co-patient than an undercouch X-ray tube, which produces higher scatter beneath the couch arising from primary beam interactions from the bottom of the couch and patient [25]. Overcouch X-ray machines are primarily intended for remote operation from the protected control area or from behind a mobile protective screen. However, overcouch systems generally provide better access to the patient. In our study, this was the only type of equipment available.
The risk of malignancy of thyroid, ovaries and testes and of hereditary effects to the children were less than 1.0 per million (Table 5). The radiation risk per examination was estimated from the effective dose to be 26 per million. The risk of radiation-induced cancer can be considered as negligible.
The radiation dose results of this study are appropriate for adoption as the local initial DRL value for this technique, following the recommendations of the European guide [4] and ICRP [2]. We proposed a DRL of 1.7 mGy for ESD based on the third quartile for newborn and children up to 2 years old.
In this study, the MCU radiation dose was optimized using mainly fluoroscopic images. The main advantage of this is radiation dose reduction while maintaining the same level of VUR detection. The number of diagnosed cases with VUR is 21.15% and is similar to those reported by Mantovani and Giroletti (20%) [7]. The MCU examination technique depends on the cooperation of the co-patient, resulting in an increased examination time (20–24 min). The increase in examination time is also related to the difficulty in obtaining prompt micturation for children and because of the repetition of bladder filling (this disadvantage is avoided in adults). It is very important that the radiologist who performs this technique has adequate training in paediatric fluoroscopy and the MCU procedure.
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Acknowledgments |
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Received for publication June 28, 2006. Revision received February 2, 2007. Accepted for publication February 20, 2007.
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