Review of the effectiveness of internal dosimetry monitoring regimes.

Routine monitoring is an important element of any occupational radiation protection programme to be able to determine how effective this protection is in practice. As well as providing information on normal operational conditions and routine worker dose uptakes, these programmes are also required to be able to detect the occurrence of abnormal or unexpected exposures to radionuclides, where these risks are deemed to be present in the workplace. Various monitoring techniques and methods are available and can be applied to the direct monitoring of workers or of the workplace. For many of the less radio-toxic radionuclides simple monitoring programmes are often more than sufficient to demonstrate compliance with operational and regulatory controls; however, multiple programmes, operated in parallel, are often required for the more radio-toxic radionuclides-e.g. Plutonium and americium-to be able to provide assurance that the potential risks of exposure are reliably and adequately controlled. When a potential exposure event is detected then further investigations are instigated to confirm whether an intake has occurred and to estimate the resultant dose. This paper presents an empirical review of the records of all such investigations over an eighteen-year period at the Harwell site, Oxfordshire, UK. The purposes of this review were to determine the relative effectiveness of different monitoring methods in being able to detect potential exposure events; and how efficient each method was in detecting potential exposures which, following investigation, were confirmed as real intakes. The analyses revealed that routine faecal sampling provided the better performance characteristics in terms of combined effectiveness and efficiency; and that the ability to detect potential exposures (at levels of up to 6 mSv) in the absence of any routine monitoring programme was limited. There was a very low incidence of potential exposures being detected by more than one monitoring technique, which emphasises the importance of operating multiple monitoring methods in order to optimise the probability and confidence of detecting potential exposures.

MWDS-2016: THE SLOW DISSOLUTION RATE FOR PLUTONIUM NITRATE INTAKES AT THE MAYAK FACILITY.

The slow dissolution rate of material deposited in the lung plays a key role in determining the eventual radiation dose received by the lung. It is therefore of great importance to establish a reliable value for this parameter, to incorporate into the latest Mayak Worker Dosimetry System (MWDS-2016). Disparate values have been obtained for the slow dissolution rate of plutonium nitrate. A volunteer study performed by Public Health England (PHE) and an analysis of United States Transuranium and Uranium Registries (USTUR) case 0269 have yielded slow dissolution rates in the region of 10-40 × 10-4 d-1. However, autopsies performed on 20 Mayak workers, exposed predominantly to nitrates, have resulted in estimates of slow dissolution rates of around 2.4 × 10-4 d-1. Three hypotheses have been proposed to explain this discrepancy: (1) a slower dissolution rate in the interstitium, (2) a third exponential component in the dissolution function and (3) a small component of oxide in the aerosol to which Mayak 'nitrate' workers were exposed. This paper describes tests of these competing hypotheses. Bayesian methods have been applied to the following datasets: PHE volunteer data; Beagle dog data; USTUR cases and Mayak worker data. It is concluded that a mixture of oxide and nitrate material, with the oxide forming ~14% of the intake, best describes the Mayak dissolution rate, without introducing values for other parameters which conflict with other studies.

The safe practice of CT coronary angiography in adult patients in UK imaging departments.

Computed tomography coronary angiography is increasingly used in imaging departments in the investigation of patients with chest pain and suspected coronary artery disease. Due to the routine use of heart rate controlling medication and the potential for very high radiation doses during these scans, there is a need for guidance on best practice for departments performing this examination, so the patient can be assured of a good quality scan and outcome in a safe environment. This article is a summary of the document on 'Standards of practice of computed tomography coronary angiography (CTCA) in adult patients' published by the Royal College of Radiologists (RCR) in December 2014.

An intake prior for the Bayesian analysis of plutonium and uranium exposures in an epidemiology study.

In Bayesian inference, the initial knowledge regarding the value of a parameter, before additional data are considered, is represented as a prior probability distribution. This paper describes the derivation of a prior distribution of intake that was used for the Bayesian analysis of plutonium and uranium worker doses in a recent epidemiology study. The chosen distribution is log-normal with a geometric standard deviation of 6 and a median value that is derived for each worker based on the duration of the work history and the number of reported acute intakes. The median value is a function of the work history and a constant related to activity in air concentration, M, which is derived separately for uranium and plutonium. The value of M is based primarily on measurements of plutonium and uranium in air derived from historical personal air sampler (PAS) data. However, there is significant uncertainty on the value of M that results from paucity of PAS data and from extrapolating these measurements to actual intakes. This paper compares posterior and prior distributions of intake and investigates the sensitivity of the Bayesian analyses to the assumed value of M. It is found that varying M by a factor of 10 results in a much smaller factor of 2 variation in mean intake and lung dose for both plutonium and uranium. It is concluded that if a log-normal distribution is considered to adequately represent worker intakes, then the Bayesian posterior distribution of dose is relatively insensitive to the value assumed of M.

Iterative reconstruction and individualized automatic tube current selection reduce radiation dose while maintaining image quality in 320-multidetector computed tomography coronary angiography.

AIM: To assess the effect of two iterative reconstruction algorithms (AIDR and AIDR3D) and individualized automatic tube current selection on radiation dose and image quality in computed tomography coronary angiography (CTCA). MATERIALS AND METHODS: In a single-centre cohort study, 942 patients underwent electrocardiogram-gated CTCA using a 320-multidetector CT system. Images from group 1 (n = 228) were reconstructed with a filtered back projection algorithm (Quantum Denoising Software, QDS+). Iterative reconstruction was used for group 2 (AIDR, n = 379) and group 3 (AIDR3D, n = 335). Tube current was selected based on body mass index (BMI) for groups 1 and 2, and selected automatically based on scout image attenuation for group 3. Subjective image quality was graded on a four-point scale (1 = excellent, 4 = non-diagnostic). RESULTS: There were no differences in age (p = 0.975), body mass index (p = 0.435), or heart rate (p = 0.746) between the groups. Image quality improved with iterative reconstruction and automatic tube current selection [1.3 (95% confidence intervals (CI): 1.2-1.4), 1.2 (1.1-1.2) and 1.1 (1-1.2) respectively; p < 0.001] and radiation dose decreased [274 (260-290), 242 (230-253) and 168 (156-180) mGy cm, respectively; p < 0.001]. CONCLUSION: The application of the latest iterative reconstruction algorithm and individualized automatic tube current selection can substantially reduce radiation dose whilst improving image quality in CTCA.

A Bayesian analysis of uncertainties on lung doses resulting from occupational exposures to uranium.

In a recent epidemiological study, Bayesian estimates of lung doses were calculated in order to determine a possible association between lung dose and lung cancer incidence resulting from occupational exposures to uranium. These calculations, which produce probability distributions of doses, used the human respiratory tract model (HRTM) published by the International Commission on Radiological Protection (ICRP) with a revised particle transport clearance model. In addition to the Bayesian analyses, point estimates (PEs) of doses were also provided for that study using the existing HRTM as it is described in ICRP Publication 66. The PEs are to be used in a preliminary analysis of risk. To explain the differences between the PEs and Bayesian analysis, in this paper the methodology was applied to former UK nuclear workers who constituted a subset of the study cohort. The resulting probability distributions of lung doses calculated using the Bayesian methodology were compared with the PEs obtained for each worker. Mean posterior lung doses were on average 8-fold higher than PEs and the uncertainties on doses varied over a wide range, being greater than two orders of magnitude for some lung tissues. It is shown that it is the prior distributions of the parameters describing absorption from the lungs to blood that are responsible for the large difference between posterior mean doses and PEs. Furthermore, it is the large prior uncertainties on these parameters that are mainly responsible for the large uncertainties on lung doses. It is concluded that accurate determination of the chemical form of inhaled uranium, as well as the absorption parameter values for these materials, is important for obtaining unbiased estimates of lung doses from occupational exposures to uranium for epidemiological studies. Finally, it should be noted that the inferences regarding the PEs described here apply only to the assessments of cases provided for the epidemiological study, where central estimates of dose were sought. Approved dosimetry service assessments of exposures are unlikely to yield significant underestimates, as pessimistic assumptions of lung solubility would almost always be used.

A method for calculating Bayesian uncertainties on internal doses resulting from complex occupational exposures.

Estimating uncertainties on doses from bioassay data is of interest in epidemiology studies that estimate cancer risk from occupational exposures to radionuclides. Bayesian methods provide a logical framework to calculate these uncertainties. However, occupational exposures often consist of many intakes, and this can make the Bayesian calculation computationally intractable. This paper describes a novel strategy for increasing the computational speed of the calculation by simplifying the intake pattern to a single composite intake, termed as complex intake regime (CIR). In order to assess whether this approximation is accurate and fast enough for practical purposes, the method is implemented by the Weighted Likelihood Monte Carlo Sampling (WeLMoS) method and evaluated by comparing its performance with a Markov Chain Monte Carlo (MCMC) method. The MCMC method gives the full solution (all intakes are independent), but is very computationally intensive to apply routinely. Posterior distributions of model parameter values, intakes and doses are calculated for a representative sample of plutonium workers from the United Kingdom Atomic Energy cohort using the WeLMoS method with the CIR and the MCMC method. The distributions are in good agreement: posterior means and Q(0.025) and Q(0.975) quantiles are typically within 20 %. Furthermore, the WeLMoS method using the CIR converges quickly: a typical case history takes around 10-20 min on a fast workstation, whereas the MCMC method took around 12-72 hr. The advantages and disadvantages of the method are discussed.

Simple weight-based contrast dosing for standardization of portal phase CT liver enhancement.

AIM: To investigate the use of a weight-based volume of contrast media to optimize portal enhancement in patients undergoing abdominal computed tomography (CT). MATERIALS AND METHODS: Thirty-one patients were assessed to establish whether a relationship existed between their weight and the portal liver enhancement achieved. Three methods of estimating weight were evaluated to establish which was the most appropriate to use in clinical practice. One hundred patients were then examined using 100 ml contrast media and 100 further patients using a weight-based contrast volume as dictated by a look-up table. The enhancement achieved by each technique was assessed. RESULTS: A good correlation was shown between patient weight and contrast enhancement when a fixed volume of contrast media was used (r=-0.825, p<0.0001). Asking the patient was shown to be the most appropriate method for estimating their weight. The mean portal liver enhancement using the fixed dose and weight-adjusted dose were 110 HU (SD=25.1) and 108 HU (SD=11.9), respectively. Weight-adjusted dose brought 37% more patients into the "ideal" enhancement range of 100-125 HU. CONCLUSION: The use of a simple, practical, weight-based look-up table to decide contrast media volumes during portal phase liver CT can greatly reduce inter-patient variability compared to a fixed-volume technique.

Empirical evaluation of lung solubilities of airborne contamination at Harwell facilities.

Lung solubility is the key parameter in determining intakes and doses from inhalation of airborne contamination. However, information on lung solubility can be difficult to acquire, particularly for the historical exposures that are of relevance to lifetime-dose reconstruction. In this study, an empirical approach has been made in which over 200 dose assessments, mainly for Pu and Am, from the period 1986 to 2005 were re-evaluated and the solubility mix required for the best fit to the data was determined. The average of these solubility mixtures for any building or facility can be used as the default solubility for retrospective dose assessments for that facility. Results are presented for a radiochemistry facility, a materials development facility and a waste-storage/handling building at Harwell. The latter two areas are characterised by aerosols that are predominantly insoluble (type S), whereas the radiochemistry facility has a heterogeneous mixture of insoluble and soluble aerosols. The implications of these results for dose reconstruction are discussed in the paper.

Characterisation of workplaces with risks of internal exposures: air sampling.

The measurement of air samples may be used to assess risks of internal exposures, either by the use of workplace static air samplers (SAS) or personal air samplers (PAS). These measurements need to incorporate information on the physical and chemical nature of the aerosol, and the relationship between the sampled and potentially breathed aerosol. This paper provides an overview of methods, which are typically used in occupational dosimetry to determine these characteristics. A specific practical example is provided to illustrate how SAS are used to determine that potential personal doses are less than 1 mSv per year, and therefore do not require individual monitoring. The paper also discusses the nature and potential impact of the uncertainties associated with PAS monitoring, and how this is managed in practice.

Uncertainties in estimation of intakes of actinides for dose reconstruction cases.

Intakes and doses arising from exposure to actinides must be reconstructed from historical bioassay data for the purposes of worker compensation and for epidemiology studies. The usual default assumption is that a series of urine activities is the result of a constant chronic intake. In reality, the urine activities will most likely arise from a random sequence of discrete intakes. In order to investigate the accuracy of the constant chronic assumption, we have created virtual urine datasets using Monte Carlo modelling and these were used as input to the code IMBA(1). Comparisons of estimated intakes with those used as input allow the uncertainties in the procedure to be estimated. The effects of incorrect assumptions about the scattering factors, activity median aerodynamic diameter (AMAD) and solubility can also be examined. The results show that the constant chronic assumptions leads to remarkably reliable estimates of intake, even for datasets generated by just a few intakes per year. The estimate of intake is fairly robust against mis-assignment of scattering factor and AMAD. However, as is well-known, the correct assignment of solubility is crucial in obtaining reliable estimates of intake and dose.

Unexpectedly high activity of 228Th in excretion samples following consumption of Brazil nuts.

A worker provided a routine faecal sample for plutonium and americium analysis. In the course of this analysis 500 mBq of (228)Th was discovered. There seemed no credible occupational route for intake of thorium. Further investigation revealed that the worker consumed approximately 25 g d(-1) of nuts, including Brazil nuts. A sample of these nuts was analysed and found to contain activities of (228)Th in sufficient quantity to account for the faecal activity. However, follow-up urine samples taken from the worker showed 0.6-0.7 mBq of (228)Th. The intake of (228)Th via nuts is insufficient to account for this activity in urine. However, it is likely that the intake of (228)Th was accompanied by similar activity of the parent (228)Ra, and biokinetic calculations show that decay of (228)Ra in vivo would produce sufficient (228)Th to account for the observed urine activity.

An intake of americium oxide powder: implications for biokinetic models for americium.

A worker inhaled 241AmO2 powder. Air sampling showed low activities but a nose blow revealed 92 Bq. Results from faecal sampling and lung and whole-body monitoring indicated an intake of about 200 Bq, but urine sampling, though commencing only 1 d after intake, showed below-threshold activities (< 0.2 mBq). This conflicts with predictions based on the ICRP Publication 67 biokinetic model for americium and the ICRP Publication 66 model for the human respiratory tract, if default lung parameters are used.

Evaluation of CT in identifying colorectal carcinoma in the frail and disabled patient.

Frail and physically or mentally disabled patients frequently have difficulty in tolerating formal colonic investigations. The aims of this study were to evaluate the accuracy of minimal-preparation CT in identifying colorectal carcinoma in this population and to determine the clinical indications and radiological signs with the highest yield for tumour. The CT technique involved helical acquisition (10-mm collimation, 1.5 pitch) following 2 days of preparation with oral contrast medium only. The outcome of 4 years of experience was retrospectively reviewed. The gold standards were pathological and cancer registration records, together with colonoscopy and barium enema when undertaken, with a minimum of 15 months follow-up. One thousand seventy-seven CT studies in 1031 patients (median age 80 years) were evaluated. CT correctly identified 83 of the 98 colorectal carcinomas in this group but missed 15 cases; sensitivity and specificity (with 95% confidence interval) 85% (78-92%) and 91% (90-93%), respectively. Multivariate analysis identified: (a) a palpable abdominal mass and anaemia to be the strongest clinical indications, particularly in combination (p<0.0025); and (b) lesion width and blurring of the serosal margin of lesions to be associated with tumours (p<0.0001). Computed tomography has a valuable role in the investigation of frail and otherwise disabled patients with symptoms suspicious for a colonic neoplasm. Although interpretation can be difficult, the technique is able to exclude malignancy with good accuracy.

Caecal carcinomas in the elderly: useful signs in minimal preparation CT.

OBJECTIVE: Frail, elderly and immobile patients frequently have difficulty in tolerating formal colonic investigations. Caecal tumours may account for up to 35% of colonic tumours. Barium enema and colonoscopy have limitations in assessing this region. The aims of this study were to evaluate the accuracy of a minimal preparation CT technique (merely with prolonged oral contrast medium) in identifying caecal carcinomas and to determine helpful radiological signs. MATERIALS AND METHODS: The CT technique involved helical acquisition following 2 days of preparation with oral contrast medium. The outcome of 4 years' experience (1995-1998) was reviewed. The gold-standards were pathological and cancer registration records, together with colonoscopy and barium enema where available, with a minimum of 15 months' follow-up. RESULTS: CT correctly identified 27 of 30 caecal carcinomas, and missed three, in a total of 1077 CT studies in 1031 patients (median age 80 years). There were also 21 false-positive cases in which CT incorrectly raised the possibility of a caecal tumour. The sensitivity, specificity, negative predictive value (NPV) and positive predictive value (PPV) were 90%, 98%, 99% and 56%, respectively. Serosal margin blurring, tumour length, presence of abnormal peri-colic fat and terminal ileal wall thickening were identified as useful radiological signs. CONCLUSIONS: Minimal preparation CT is able to identify caecal carcinomas with fair accuracy. Such evaluation may become important given the increasing population age and evidence of a 'proximal shift' in the site of colonic tumours in the elderly.

Spiral CT optimization for measurement of bronchial lumen diameter using an experimental model.

The aim of the study was to determine optimal parameters for demonstrating sublobar bronchi on spiral CT. Measurements were obtained from five parallel polyethylene tubes embedded in foam matrix with similar radiographic characteristics to segmental and subsegmental bronchi and to lung parenchyma, respectively. Collimation widths of 1.5, 2, 3 and 4 mm were used, with a pitch of 1 or 1.5 and a reconstruction interval of 1 mm or 2 mm. Various slice planes were used. Images acquired orthogonally were viewed normally. Images acquired in planes oblique or parallel to the long axes of the tubes were reformatted into a plane orthogonal to the long axes of the tubes to be comparable with the directly acquired orthogonal images. Tube diameters were measured at lung window settings (L, -400; W, 1300) and compared with known true inner and outer tube diameters. Measurements from images acquired orthogonal to the tube long axes were accurate regardless of slice thickness. Images acquired obliquely or parallel only produced accurate measurements at the lowest slice thickness (1.5 mm). Pitch and reconstruction interval had no effect on measurement error in any scan plane. It is concluded that a slice thickness of 1.5 mm or less, with a pitch of 1.5, should be used when acquiring images at angles other than orthogonal to the long axes of experimental tubes equivalent to the segmental and subsegmental bronchi. It is suggested that similar parameters should be used in vivo and that the examination should be targeted to the area of the bronchial tree in question to reduce patient dose and length of breath-hold.

Bone age assessment: a large scale comparison of the Greulich and Pyle, and Tanner and Whitehouse (TW2) methods.

PURPOSE: Comparison of bone age assessed using either the "atlas matching" method of Greulich and Pyle or the "point scoring system" of Tanner and Whitehouse (TW2). MATERIALS AND METHODS: 362 consecutive "bone age" radiographs of the left hand and distal radius performed in a large provincial teaching hospital. Data were analysed using the "method comparison" statistical technique. Ten per cent of the radiographs were re-analysed to assess intra-observer variation. RESULTS: The 95% confidence interval for the difference between the two methods was 2.28 to -1.52 years. Intra-observer variation was greater for the Greulich and Pyle method than for the TW2 method (95% confidence limit, -2.46 to 2.18 v -1.41 to 1.43). CONCLUSION: The two methods of bone age assessment as used in clinical practice do not give equivalent estimates of bone age and we suggest that one method only (preferably the TW2) should be used when performing serial measurements on an individual patient.

CT dimensions of the normal pericardium.

Previous studies have suggested that the upper limit of the thinnest portion of the pericardium is 3-4 mm using 10 mm CT slices. However, these studies suffered from small sample sizes, long data acquisition times and unconventional viewing parameters. We have measured the width of the thinnest portion of the normal pericardium using 10 mm (100 patients) and 1 mm (100 patients) high resolution CT (HRCT) slices with modern CT equipment and fixed mediastinal window settings (400/20). The pericardium was identified in all patients and was best seen anterior to the heart. The pericardium is exceptionally well seen using 1 mm HRCT slices and this may be the optimal technique for visualization of the pericardium. The upper limit of the thinnest portion of the normal pericardium (mean value + 2 SD) was 1.2 mm (10 mm CT slices) and 0.7 mm (1 mm HRCT slices). These values are substantially lower than those previously reported and in line with anatomical findings.