Radiation protection and dose monitoring in medical imaging: a journey from awareness, through accountability, ability and action…but where will we arrive?

Cumulative radiation exposure from multimodality recurrent imaging of CT, fluoroscopically guided intervention, and nuclear medicine

OBJECTIVES

To assess cumulative effective dose (CED) over a 4-year period in patients undergoing multimodality recurrent imaging at a major hospital in the USA.

METHODS

CED from CT, fluoroscopically guided intervention (FGI), and nuclear medicine was analyzed in consecutive exams in a tertiary care center in 2018-2021. Patients with CED ≥ 100 mSv were classified by age and body habitus (underweight, healthy weight, overweight, obese), as per body mass index percentiles < 5th, 5th to < 85th, 85th to < 95th, and ≥ 95th (age 2-19 years), and its ranges < 18.5, 18.5-24.9, 25-29.9, and ≥ 30 (≥ 20 years), respectively.

RESULTS

Among a total of 205,425 patients, 5.7% received CED ≥ 100 mSv (mean 184 mSv, maximum 1165 mSv) and their ages were mostly 50-64 years (34.1%), followed by 65-74 years (29.8%), ≥ 75 years (19.5%), 20-49 years (16.3%), and ≤ 19 years (0.29%). Body habitus in decreasing occurrence was obese (38.6%), overweight (31.9%), healthy weight (27.5%), and underweight (2.1%). Classification by dose indicated 172 patients (≥ 500 mSv) and 3 (≥ 1000 mSv). In comparison, 5.3% of 189,030 CT patients, 1.6% of 18,963 FGI patients, and 0.19% of 41,401 nuclear-medicine patients received CED ≥ 100 mSv from a single modality.

CONCLUSIONS

The study of total dose from CT, FGI, and nuclear medicine of patients with CED ≥ 100 mSv indicates major (89%) contribution of CT to CED with 70% of cohort being obese and overweight, and 64% of cohort aged 50-74 years.

CLINICAL RELEVANCE STATEMENT

Multimodality recurrent exams are common and there is a lack of information on patient cumulative radiation exposure. This study attempts to address this lacuna and has the potential to motivate actions to improve the justification process for enhancing patient safety.

KEY POINTS

• In total, 5.7% of patients undergoing multimodality recurrent imaging (CT, fluoroscopically guided intervention, nuclear medicine) incurred a dose of ≥ 100 mSv. • Mean dose was 184 mSv, with 15 to 18 times contribution from CT than that from fluoroscopically guided intervention or nuclear medicine. • In total, 70% of those who received ≥ 100mSv were either overweight or obese.

Making CT Dose Monitoring Meaningful: Augmenting Dose with Imaging Quality

Due to the concerns about radiation dose associated with medical imaging, radiation dose monitoring systems (RDMSs) are now utilized by many radiology providers to collect, process, analyze, and manage radiation dose-related information. Currently, most commercially available RDMSs focus only on radiation dose information and do not track any metrics related to image quality. However, to enable comprehensive patient-based imaging optimization, it is equally important to monitor image quality as well. This article describes how RDMS design can be extended beyond radiation dose to simultaneously monitor image quality. A newly designed interface was evaluated by different groups of radiology professionals (radiologists, technologists, and physicists) on a Likert scale. The results show that the new design is effective in assessing both image quality and safety in clinical practices, with an overall average score of 7.8 out of 10.0 and scores ranging from 5.5 to 10.0. Radiologists rated the interface highest at 8.4 out of 10.0, followed by technologists at 7.6 out of 10.0, and medical physicists at 7.5 out of 10.0. This work demonstrates how the assessment of the radiation dose can be performed in conjunction with the image quality using customizable user interfaces based on the clinical needs associated with different radiology professions.

Calculation of scan length and size-specific dose at longitudinal positions of body CT scans using dose equilibrium function

BACKGROUND

Dose evaluation at longitudinal positions of body computed tomography (CT) scans is useful for CT quality assurance programs and patient organ dose evaluation. Accurate estimates depend on both patient size and scan length.

PURPOSE

To propose practical evaluation of the average dose to the transverse slab of an axial image slice for adult body CT examinations, considering not only patient size but also scan length, and to compare the results with those of Monte Carlo (Geant4) simulation [D_sim (z)] and size-specific dose estimates at longitudinal positions of scans [SSDE(z)] from international standards (IEC publication no. 62985).

METHODS

In a scan series, the total dose at each z-axis location was calculated using the input information identical to the SSDE(z) evaluation. Each axial image slice (slice thickness, 2.5 or 5 mm) was first considered independently. Its z-axis coverage and CTDI_vol (from the DICOM headers) were used to directly calculate a z-axis dose profile for the average dose over the cross-section of a water phantom, using the approach to equilibrium function. The phantom diameter was taken to be equal to the patient water equivalent diameter at that slice. The above was repeated at all slices and the dose at each z-axis location was accumulated from all profiles, referred to as D_calc (z). For validation, we considered a cohort of 65 patients, who underwent chest and abdominopelvic examinations. The resultant D_calc (z) was compared with D_sim (z) and SSDE(z), both available in a previous paper.

RESULTS

D_calc (z) evaluation could be used to accurately assess the scan range average dose, with an accuracy of 7.1%-8.7% for 65 patients in two examinations. On individual image slices, the maximum difference in magnitude between D_calc (z) and D_sim (z) [and between SSDE(z) and D_sim (z) in parentheses] was 37.5% (85%) [two edges (2 × 5 cm) of chest scan range], 17.8% (35.2%) (the remaining central region of chest scan), 26.8% (74.1%) [two edges (2 × 5 cm) of abdominopelvic scan range], and 14.2% (22.5%) (the remaining central region of abdominopelvic scan).

CONCLUSIONS

Identical input data are used for D_calc (z) and SSDE(z) evaluations. The latter is limited to the z-axis locations within scan range. At each image slice, SSDE(z) is equivalent to the midpoint dose of a fixed-mA scan of 15-30 cm (scan length). In contrast, D_calc (z) considers dose accumulation from varying scan length (from sub-centimeter to about 1 m) and tube current, and dose profile is also computed outside scan range. Besides greatly improving dose evaluation for individual image slices, D_calc (z) allows for evaluating dose accumulation from multiple series, which typically span different scan ranges. Our proposal may assist CT manufacturers and dose index monitoring software in assessing dose at longitudinal positions of body CT scans.

Patient-level dose monitoring in computed tomography: tracking cumulative dose from multiple multi-sequence exams with tube current modulation in children

BACKGROUND

In children exposed to multiple computed tomography (CT) exams, performed with varying z-axis coverage and often with tube current modulation, it is inaccurate to add volume CT dose index (CTDI_vol) and size-specific dose estimate (SSDE) to obtain cumulative dose values.

OBJECTIVE

To introduce the patient-size-specific z-axis dose profile and its dose line integral (DLI) as new dose metrics, and to use them to compare cumulative dose calculations against conventional measures.

MATERIALS AND METHODS

In all children with 2 or more abdominal-pelvic CT scans performed from 2013 through 2019, we retrospectively recorded all series kV, z-axis tube current profile, CTDI_vol, dose-length product (DLP) and calculated SSDE. We constructed dose profiles as a function of z-axis location for each series. One author identified the z-axis location of the superior mesenteric artery origin on each series obtained to align the dose profiles for construction of each patient's cumulative profile. We performed pair-wise comparisons between the peak dose of the cumulative patient dose profile and ΣSSDE, and between ΣDLI and ΣDLP.

RESULTS

We recorded dose data in 143 series obtained in 48 children, ages 0-2 years (n=15) and 8-16 years (n=33): ΣSSDE 12.7±6.7 and peak dose 15.1±8.1 mGy, ΣDLP 278±194 and ΣDLI 550±292 mGy·cm. Peak dose exceeded ΣSSDE by 20.6% (interquartile range [IQR]: 9.9-26.4%, P<0.001), and ΣDLI exceeded ΣDLP by 114% (IQR: 86.5-147.0%, P<0.001).

CONCLUSION

Our methodology represents a novel approach for evaluating radiation exposure in recurring pediatric abdominal CT examinations, both at the individual and population levels. Under a wide range of patient variables and acquisition conditions, graphic depiction of the cumulative z-axis dose profile across and beyond scan ranges, including the peak dose of the profile, provides a better tool for cumulative dose documentation than simple summations of SSDE. ΣDLI is advantageous in characterizing overall energy absorption over ΣDLP, which significantly underestimated this in all children.

Assessment of Residual Radioactivity by a Comprehensive Wireless, Wearable Device in Thyroid Cancer Patients Undergoing Radionuclide Therapy and Comparison With the Results of a Home Device: A Feasibility Study

Objective: To investigate the feasibility of using a wireless wearable device (WD) in differentiated thyroid cancer (DTC) patients undergoing radionuclide therapy with I-131 (RAI) and protected hospitalization, this study compared the measurements of residual radioactivity obtained with those registered by a permanent environmental home device (HD). Methods: Twenty consecutive patients undergoing RAI hospitalized in restricted, controlled areas were enrolled. The patients underwent comprehensive monitoring of vital/nonvital parameters. We obtained 45580± 13 measurements from the WD, detecting the residual radioactivity for each patient during approximately 56 hours of hospitalization, collecting data 53 times per hour. The samples, collected during daily activities, were averaged every two hours, and the results correlated with those from the HD. Bland-Altman analysis was also used to evaluate the agreement between the two techniques. Results: A significant relationship between the WD and HD was observed (r = 0.96, p < 0.0001). Bland-Altman analysis recognized the agreement between measurements by the WD and HD. The mean value at the end of the first day of hospitalization was 80.81 microSv/h and 60.77 microSv/h (p = ns for WD and HD), whereas those at the end of the second day were 47.08 and 24.96 (p = ns). In the generalized linear model (GLM), a similar trend in performance across time was found with the two techniques. Conclusion: This study demonstrates good agreement between the residual radioactivity measures estimated by the WD and HD modalities, rendering them interchangeable. This approach will allow both the optimization of medical staff exposure and safer patient discharge. Abbreviations: wireless device (WD); differentiated thyroid cancer (DTC); radionuclide therapy with I-131 (RAI); home device (HD); generalized linear model (GLM).

LOCAL STUDY OF DIAGNOSTIC REFERENCE LEVELS FOR COMPUTED TOMOGRAPHY EXAMINATIONS OF ADULT PATIENTS IN İZMIR, TURKEY

PURPOSE

This study aims to develop local diagnostic reference levels (DRLs) for the most common computed tomography (CT) examinations carried out around Izmir, Turkey.

METHODS

Five common CT examinations (head, neck, chest, abdomen-pelvis (AP), chest-abdomen-pelvis (CAP)) from four different radiology centres have been included in the study. CT dose index-volume (CTDIvol) and dose length product (DLP) values were recorded for 50 patients per exam in each centre. Third quartiles of CTDIvol and DLP values were determined as DRLs and compared with international findings.

RESULTS

51.3% of the patients were male and 48.7% were female, with a mean age of 57 (between 18 and 93). DRLs for CTDIvol were recorded as 70, 16, 15, 23 and 16 for head, neck, chest, AP and CAP examinations, respectively, while the corresponding DLPs were 1385, 604, 567, 998 and 1180 mGy.cm.

CONCLUSION

Results are mostly comparable to the latest international data, except for the head examinations, which were observed to slightly exceed the DRLs established by other countries.

EFFECTIVE DOSE FOR RADIOLOGICAL PROCEDURES IN AN EMERGENCY DEPARTMENT: A CROSS-SECTIONAL STUDY

The extent of radiation exposure in emergency settings is not well documented; here, the corresponding effective dose (ED) is provided. In 500 patients admitted in row to the emergency department, ED was compared in patients according to complaints and their visiting physicians. Out of all, 220 patients aged 43.5 ± 22.2 years (admission: 2.0 ± 1.6 days) had at least an imaging. The main reasons for admission were trauma (10.5%) and then orthopedic problems (8.6%). EDs from CT and radiography were 1.66 ± 3.59 and 0.71 ± 1.67 mSv, respectively (from all 2.29 ± 4.12). Patients with abdominal (5.8 ± 5.2 mSv; p < 0.002) and pelvic (12.0 ± 6.3 mSv; p < 0.007) complaints received higher ED from CT and radiography and, also, patients visited by surgeons (7.94 ± 6.9 mSv). CT scan was the main source for ED to patients. Irrespective of the final diagnosis, patients with abdominopelvic complaints and those visited by surgeons are at higher exposure risk.

Doctors' knowledge of the doses and risks of radiological investigations performed in the emergency department

Objectives:

To assess emergency doctors’ knowledge of radiation exposure doses and risks, as the increasing use of radiological investigations in emergency medicine practice is very concerning because of the associated risk of cancer.

Methods:

Doctors from different specialties and with different levels of training working in emergency departments of 8 hospitals in Jeddah, Kingdom of Saudi Arabia, filled out a questionnaire. Participants estimated the radiation doses of different imaging modalities and answered questions regarding possible associated risks.

Results:

One hundred seventy-one doctors returned completed questionnaires. The overall correct dose estimation rate was 20.8%. Doses were more correctly estimated by consultants versus specialists and residents (p=0.007), and by emergency physicians versus doctors from other specialties (p=0.05). The correct answer rate was insignificantly higher among doctors with formal training on radiation protection (p=0.065). The overall correct answer rate was unsatisfactory for 4 questions assessing physicians’ knowledge of risks. Questions about the lifetime risk of cancer due to ionizing radiation were more correctly answered by consultants versus residents and specialists (p=0.05). Specialists were more knowledgeable about the risk of imaging on fetuses (p=0.05). Doctors with formal training answered 3 out of 4 questions more correctly than doctors without formal training, but no difference existed between them regarding imaging modalities, that they selected for pregnant patients (p=0.297).

Conclusion:

Doctors working in emergency departments had poor knowledge about radiation doses and risks. This issue warrants urgent attention.

Medical imaging dose optimisation from ground up: expert opinion of an international summit

As in any medical intervention, there is either a known or an anticipated benefit to the patient from undergoing a medical imaging procedure. This benefit is generally significant, as demonstrated by the manner in which medical imaging has transformed clinical medicine. At the same time, when it comes to imaging that deploys ionising radiation, there is a potential associated risk from radiation. Radiation risk has been recognised as a key liability in the practice of medical imaging, creating a motivation for radiation dose optimisation. The level of radiation dose and risk in imaging varies but is generally low. Thus, from the epidemiological perspective, this makes the estimation of the precise level of associated risk highly uncertain. However, in spite of the low magnitude and high uncertainty of this risk, its possibility cannot easily be refuted. Therefore, given the moral obligation of healthcare providers, 'first, do no harm,' there is an ethical obligation to mitigate this risk. Precisely how to achieve this goal scientifically and practically within a coherent system has been an open question. To address this need, in 2016, the International Atomic Energy Agency (IAEA) organised a summit to clarify the role of Diagnostic Reference Levels to optimise imaging dose, summarised into an initial report (Järvinen et al 2017 Journal of Medical Imaging 4 031214). Through a consensus building exercise, the summit further concluded that the imaging optimisation goal goes beyond dose alone, and should include image quality as a means to include both the benefit and the safety of the exam. The present, second report details the deliberation of the summit on imaging optimisation.

Radiation Safety in Children With Congenital and Acquired Heart Disease: A Scientific Position Statement on Multimodality Dose Optimization From the Image Gently Alliance

There is a need for consensus recommendations for ionizing radiation dose optimization during multimodality medical imaging in children with congenital and acquired heart disease (CAHD). These children often have complex diseases and may be exposed to a relatively high cumulative burden of ionizing radiation from medical imaging procedures, including cardiac computed tomography, nuclear cardiology studies, and fluoroscopically guided diagnostic and interventional catheterization and electrophysiology procedures. Although these imaging procedures are all essential to the care of children with CAHD and have contributed to meaningfully improved outcomes in these patients, exposure to ionizing radiation is associated with potential risks, including an increased lifetime attributable risk of cancer. The goal of these recommendations is to encourage informed imaging to achieve appropriate study quality at the lowest achievable dose. Other strategies to improve care include a patient-centered approach to imaging, emphasizing education and informed decision making and programmatic approaches to ensure appropriate dose monitoring. Looking ahead, there is a need for standardization of dose metrics across imaging modalities, so as to encourage comparative effectiveness studies across the spectrum of CAHD in children.

Meeting the Needs for Radiation Protection: Diagnostic Imaging

Radiation and potential risk during medical imaging is one of the foremost issues for the imaging community. Because of this, there are growing demands for accountability, including appropriate use of ionizing radiation in diagnostic and image-guided procedures. Factors contributing to this include increasing use of medical imaging; increased scrutiny (from awareness to alarm) by patients/caregivers and the public over radiation risk; and mounting calls for accountability from regulatory, accrediting, healthcare coverage (e.g., Centers for Medicare and Medicaid Services), and advisory agencies and organizations as well as industry (e.g., NEMA XR-29, Standard Attributes on CT Equipment Related to Dose Optimization and Management). Current challenges include debates over uncertainty with risks with low-level radiation; lack of fully developed and targeted products for diagnostic imaging and radiation dose monitoring; lack of resources for and clarity surrounding dose monitoring programs; inconsistencies across and between practices for design, implementation and audit of dose monitoring programs; lack of interdisciplinary programs for radiation protection of patients; potential shortages in personnel for these and other consensus efforts; and training concerns as well as inconsistencies for competencies throughout medical providers' careers for radiation protection of patients. Medical care providers are currently in a purgatory between quality- and value-based imaging paradigms, a state that has yet to mature to reward this move to quality-based performance. There are also deficits in radiation expertise personnel in medicine. For example, health physics academic programs and graduates have recently declined, and medical physics residency openings are currently at a third of the number of graduates. However, leveraging solutions to the medical needs will require money and resources, beyond personnel alone. Energy and capital will need to be directed to:• innovative and cooperative cross-disciplinary institutional/practice oversight of and guidance for the use of diagnostic imaging (e.g., radiology, surgical specialties, cardiologists, and intensivists);• initiatives providing practical benchmarks (e.g., dose index registries);• comprehensive (consisting of access, integrity, metrology, analytics, informatics) and effective and efficient dose monitoring programs;• collaboration with industry;• improved use of imaging, such as through decision support combined with evidence-based appropriateness for imaging use;• integration with e-health such as medical records;• education, including information extending beyond the medical imaging community that is relevant to patients, public, and providers and administration;• identification of opportunities for alignment with salient media and advocacy organizations to deliver balanced information regarding medical radiation and risk;• open lines of communication between medical radiation experts and appropriate bodies such as the U.S. Environmental Protection Agency, the U.S. Food and Drug Administration, and the Joint Commission to assure appropriate guidance on documents and actions originating from these organizations; and• increased grant funding to foster translational work that advances understanding of low-level radiation and biological effects.

Early chest computed tomography in adult acute severe community-acquired pneumonia patients treated in the intensive care unit

BACKGROUND

The value of early chest computed tomography (CT) was evaluated among severe community-acquired pneumonia (SCAP) patients.

METHODS

The study population consisted of 65 of 457 SCAP patients with concomitant chest CT and radiograph performed within 48 h of ICU admission. Each image pair was re-evaluated by two radiologists. The type of pneumonia, the presence of pleural fluid and atelectasis were assessed. Therapeutic and diagnostic procedures induced by CT results were analysed together with clinical, microbiological and outcome data.

RESULTS

Alveolar pneumonia was observed in 72.3% of patients by radiograph and in 75.4% of patients by CT. Pleural fluid was detected via chest radiograph and CT in 17 (26.2%) and 41 cases (63.1%), (P < 0.001) and atelectasis in 10 (15.4%) and 22 cases (33.8%), (P = 0.002), respectively. In 34 patients (52.3%), the CT revealed 38 new findings (58.5%) not shown in plain chest radiograph. Out of these 34 patients, therapeutic interventions or procedures were performed in 26 (76.5%). The number of infected lobes correlated negatively with the lowest PaO2 /FiO2 ratio (ρ = -0.326, P = 0.008) for chest CT scans.

CONCLUSION

Compared with chest radiograph, chest CT generated new findings in nearly 60% of SCAP patients, leading to new procedures or changes in medical treatment in nearly 75% of those patients. Chest CT better describes the pulmonary involvement and severity of oxygenation disorder compared to a plain chest radiograph.

CT Radiation: Key Concepts for Gentle and Wise Use

Use of computed tomography (CT) in medicine comes with the responsibility of its appropriate (wise) and safe (gentle) application to obtain required diagnostic information with the lowest possible dose of radiation. CT provides useful information that may not be available with other imaging modalities in many clinical situations in children and adults. Inappropriate or excessive use of CT should be avoided, especially if required information can be obtained in an accurate and time-efficient manner with other modalities that require a lower radiation dose, or non-radiation-based imaging modalities such as ultrasonography and magnetic resonance imaging. In addition to appropriate use of CT, the radiology community also must monitor scanning practices and protocols. When appropriate, high-contrast regions and lesions should be scanned with reduced dose, but overly zealous dose reduction should be avoided for assessment of low-contrast lesions. Patients' cross-sectional body size should be taken into account to deliver lower radiation dose to smaller patients and children. Wise use of CT scanning with gentle application of radiation dose can help maximize the diagnostic value of CT, as well as address concerns about potential risks of radiation. In this article, key concepts in CT radiation dose are reviewed, including CT dose descriptors; radiation doses from CT procedures; and factors and technologies that affect radiation dose and image quality, including their use in creating dose-saving protocols. Also discussed are the contributions of radiation awareness campaigns such as the Image Gently and Image Wisely campaigns and the American College of Radiology Dose Index Registry initiatives.

The impact of pediatric-specific dose modulation curves on radiation dose and image quality in head computed tomography

BACKGROUND

The volume of CT examinations has increased with resultant increases in collective dose values over the last decade.

OBJECTIVE

To analyze the impact of the tube current and voltage modulation for dose values and image quality of pediatric head CT examinations.

MATERIALS AND METHODS

Head CT examinations were performed on anthropomorphic phantoms and four pediatric age categories before and after the introduction of dedicated pediatric curves for tube voltage and current modulation. Local diagnostic reference levels were calculated. Visual grading characteristic image quality evaluation was performed by four pediatric neuroradiologists and image noise comparisons were performed.

RESULTS

Pediatric-specific modulation curves demonstrated a 49% decrease in mean radiation dose for phantom examinations. The local diagnostic reference levels (CTDIvol) for clinical examinations decreased by 52%, 41%, 46% and 40% for newborn, 5-, 10- and 15-year-old patients, respectively. Visual grading characteristic image quality was maintained for the majority of age categorizations (area under the curve = 0.5) and image noise measurements did not change (P = 0.693).

CONCLUSION

Pediatric-specific dose modulation curves resulted in an overall mean dose reduction of 45% with no significant differences in subjective or objective image quality findings.

Estimates of diagnostic reference levels for pediatric peripheral and abdominal fluoroscopically guided procedures

OBJECTIVE

The objective of our study was to survey radiation dose indexes of pediatric peripheral and abdominal fluoroscopically guided procedures from which estimates of diagnostic reference levels (DRLs) can be proposed for both a standard fluoroscope and a novel fluoroscope with advanced image processing and lower radiation dose rates.

MATERIALS AND METHODS

Radiation dose structured reports were retrospectively collected for 408 clinical pediatric cases: Half of the procedures were performed with a standard imaging technology and half with a novel x-ray technology. Dose-area product (DAP), air Kerma (AK), fluoroscopy time, number of digital subtraction angiography images, and patient mass were collected to calculate and normalize radiation dose indexes for procedures completed with the standard and novel fluoroscopes.

RESULTS

The study population was composed of 180 and 175 patients who underwent procedures with the standard and novel technology, respectively. The 21 different types of pediatric peripheral and abdominal interventional procedures produced 408 total studies. Median ages, mass and body mass index, fluoroscopy time per procedure, and total number of recorded images for the standard and novel technologies were not statistically different. The area of the x-ray beams was square at the level of the patient with a dimension of 10-13 cm. The dose reduction achieved with the novel fluoroscope ranged from 18% to 51% of the dose required with the standard fluoroscope. The median DAP and AK patient dose indexes were 0.38 Gy · cm(2) and 4.00 mGy, respectively, for the novel fluoroscope.

CONCLUSION

Estimates of dose indexes of pediatric peripheral and abdominal fluoroscopically guided, clinical procedures should assist in the development of DRLs to foster management of radiation doses of pediatric patients.

Radiation epidemiology and recent paediatric computed tomography studies

Recent record-linkage studies of cancer risk following computed tomography (CT) procedures among children and adolescents under 21 years of age must be interpreted with caution. The reasons why the examinations were performed were not known, and the dosimetric approaches did not include individual dose reconstructions or account for the possibility for missed examinations. The recent report (2013) on children by the United Nations Scientific Committee on the Effects of Atomic Radiation concluded that the associations may have resulted from confounding by indication (also called 'reverse causation'), and not radiation exposure. The reported cancer associations may very well have been related to the patients' underlying health conditions that prompted the examinations. Reverse causation has been observed in other epidemiological investigations, such as a Swedish study of thyroid cancer risk following I-131 scintillation imaging scans, and in studies of brain cancer risk following Thorotrast for cerebral angiography. Epidemiological patterns reported in the CT studies were also inconsistent with the world's literature. For example, in a UK study, teenagers had a higher risk of brain tumour than young children; in an Australian study, cancers not previously linked to radiation were significantly elevated; and in a Taiwanese study, the risk of benign tumours decreased with age at the time of CT examination. In all studies, solid tumours appeared much earlier than previously reported. Remarkably, in the Australian study, brain cancer excesses were seen regardless of whether or not the CT was to the head, i.e. a significant excess was reported for CT examinations of the abdomen and extremities, which involved no radiation exposure to the brain. In the UK study, the significance of the 'leukaemia' finding was only because myelodysplastic syndrome was added to the category, and there was no significance for leukaemia alone. Without knowledge of why CT examinations were performed, any future studies will be equally difficult to interpret. It is noteworthy that two recent studies of children in France and Germany found no significant excess cancer risk from CT scans once adjustment was made for conditions that prompted the scan, family history, or other predisposing factors known to be associated with increased cancer risk. Nonetheless, such studies have heightened awareness of these relatively high-dose diagnostic procedures, and the need to reduce unnecessary examinations and lower the dose per examination commensurate with the desired image quality.