OBJECTIVE: Our aim was to estimate the cumulative effective doses (CEDs) from radiologic procedures for a cohort of pediatric oncology patients.
METHODS: A retrospective cohort study of the imaging histories of 150 pediatric oncology patients (30 each in 5 subgroups, that is, leukemia, lymphomas, brain tumors, neuroblastomas, and assorted solid tumors) for 5 years after diagnosis was performed. All procedures involving ionizing radiation were recorded, including radiography, computed tomography (CT), nuclear medicine (NM) studies, fluoroscopy, and interventional procedures. CED estimates were calculated.
RESULTS: Individual CED estimates ranged from <1 mSv to 642 mSv, with a median of 61 mSv. CT and NM were the greatest contributors; CT constituted 30% of procedures but 52% of the total CED, and NM constituted 20% and 46%, respectively. There was considerable variability between tumor subgroups. CED estimates were highest in the neuroblastoma (median: 213 mSv [range: 36–489 mSv]) and lymphoma (median: 191 mSv [range: 10–642 mSv]) groups and lowest in the leukemia group (median: 5 mSv [range: 0.2–57 mSv]).
CONCLUSIONS: CEDs from diagnostic and interventional imaging for pediatric oncology patients vary considerably according to diagnoses, individual clinical courses, and imaging modalities used. Increased awareness may promote strategies to reduce the radiation burden to this population.
WHAT'S KNOWN ON THIS SUBJECT:
Risks associated with ionizing radiation for children are higher than those for adults, and the use of medical imaging continues to increase. There have been several high-profile publications on CEDs for adult patients but no similar pediatric studies.
WHAT THIS STUDY ADDS:
This study is important in highlighting the significant exposure that can be accumulated by children through diagnostic and interventional radiologic procedures, and it discusses the issues of potential future malignancy risk and approaches to help minimize this risk.
In recent years, there have been growing concerns regarding the potential risks associated with ionizing radiation from radiologic procedures.1 The population radiation burden from medical imaging has increased sevenfold over the past 2 decades, with the greatest contributions and increases being primarily from computed tomography (CT) and nuclear medicine (NM).2
Children are particularly vulnerable, because they not only are more radiosensitive than adults but also have a longer life expectancy over which the risk can become realized.3,–,5 Imaging procedures involving ionizing radiation have a central role in the diagnosis and management of many pediatric conditions. However, there is increasing awareness of the potential risks associated with cumulative exposure for patients who undergo frequent or repetitive studies.
To assess cumulative doses for individuals who are exposed to multiple radiologic modalities involving ionizing radiation (radiography, fluoroscopy, CT, and NM), a common dose metric is required. Effective dose is currently the most practical measure available to clinicians for this purpose. Several studies have investigated cumulative effective doses (CEDs) in adults,6,–,13 but published data for children are limited and are confined largely to studies involving plain radiography or head CT.14,–,16
Imaging is used extensively for diagnosis and monitoring for pediatric oncology patients, and much of the imaging involves ionizing radiation. Although accurate, timely imaging is central to the care of these patients and contributes to their survival, the use of serial, higher-dose, radiologic procedures in the first few years after diagnosis may lead to significant cumulative radiation exposure. As survival rates for childhood malignancies continue to improve, with 80% of patients expected to reach adulthood,17 the long-term sequelae related to all aspects of diagnosis and management are of increasing importance. The purpose of this study was to document the cumulative radiation burden associated with diagnostic, interventional, and follow-up imaging in a cohort of pediatric oncology patients.
A retrospective review of the imaging history for the 5 years after diagnosis was performed for 150 children who presented to our institution in 2001 with new malignancies. The study cohort consisted of 30 consecutively presenting patients in each of 5 diagnostic subgroups, namely, leukemia, lymphomas, brain tumors, neuroblastomas, and assorted solid tumors. All patients received their core and follow-up imaging at this institution. Patients who received joint care, with follow-up imaging performed in part at another institution, were excluded. Institutional research ethics board approval was obtained.
Basic demographic data for each patient were noted. Departmental picture archiving and communication system, NM, and interventional databases were consulted to record all procedures involving ionizing radiation. For radiography, the number of radiographic views was recorded; for CT, the anatomic region of coverage and the number of scanning phases were recorded. Individual radioisotope doses were recorded in megabequerels. Interventional fluoroscopy time and number of digital subtraction angiographic images were noted. Patient age at the time of each procedure was recorded. Diagnostic images supplied by a referring hospital were included. Data were entered by 2 authors (Dr Ahmed and Mr Shroff) and were verified on a separate occasion by a second investigator (Drs Thomas, Connolly, or Ahmed).
Age-specific effective dose estimates were tabulated for all procedures as outlined in Table 1. These estimates were derived from a combination of previously published institutional data18 (CT), patient-specific radioisotope doses and published conversion tables19 (NM), and data from the published pediatric radiology literature20,–,27 (radiography, gastrointestinal/genitourinary fluoroscopy, and NM). Interventional procedure doses were calculated by summing fluoroscopic doses (in-house measurements with a conversion coefficient28) and by considering each digital subtraction angiographic frame as equivalent to a regional radiograph, on the basis of phantom studies.29 CED estimates were calculated by summing effective doses over each patient's imaging history, by using descriptive statistics.
Demographic features of the study cohort (N = 150) and of each diagnostic subgroup (N = 30) are presented in Tables 2 and 3, with summary tumor staging data. The median age was 7.6 years (range: 0.1–17.3 years). Forty-one percent of the subjects were female. Twenty-six children (17%) died during the 5-year follow-up period.
A total of 4338 procedures involving ionizing radiation were performed in the study cohort in the first 5 years after diagnosis, with a median of 19.5 procedures per patient (range: 2–109 procedures per patient). The distribution of procedures according to imaging modality is presented in Table 4.
Individual estimated CEDs ranged from 0.0015 to 642 mSv (median: 61 mSv; mean: 113 mSv) (Fig 1); 41% (61 patients) received estimated CEDs of >100 mSv, 22% (33 patients) received >200 mSv, and 1.3% (2 patients) received >500 mSv. Table 5 illustrates the median and range of estimated CEDs and the contribution of each imaging modality according to diagnostic subgroup. The median CED was lowest in the leukemia subgroup (5 mSv) and highest in the neuroblastoma subgroup (213 mSv). Larger proportions of patients with lymphomas and neuroblastomas received >100 mSv, compared with the other subgroups (lymphomas, 83%; neuroblastomas, 80%; solid tumors, 40%; leukemia, 0%; brain tumors, 0%). The highest individual estimated CED (642 mSv) occurred in the lymphoma subgroup.
Figure 2 presents the relative contribution of each imaging modality, expressed as a proportion of the total study cohort CED and as a proportion of the total number of procedures. CT and NM were the greatest contributors toward the total CED; CT constituted 30% of procedures but 52% of the cohort CED, whereas NM constituted 20% of procedures and 46% of the CED. Plain radiographs represented 44% of studies but accounted for <1% of the CED.
There is increasing awareness of the potential risks associated with ionizing radiation in diagnostic imaging procedures and, in particular, the greater radiosensitivity of children. A growing acceptance of the linear/no-threshold hypothesis of radiation exposure and of the likely cumulative nature of exposure30 has led to concerns regarding the potential long-term effects of repeated imaging in childhood and young adulthood.
Readers may encounter numerous dose parameters used to assess the radiation dose associated with radiologic procedures. These parameters include surface and organ doses (in milligrays), radionuclide activity measures (in megabecquerels), and formulated descriptors designed to reflect modality-specific technology, such as the volume CT dose index (in milligrays) and dose-length product (in milligray-centimeters) for CT. It is not possible to compare these different dose metrics easily. A common measure is required for quantification and comparison of diverse radiologic procedures. Effective dose is defined by the International Commission on Radiological Protection as the sum of the absorbed doses in all tissues and organs of the body, each weighted according to its radiation sensitivity, and is measured in millisieverts. Effective dose allows the conversion of exposure to a localized region of the body into a whole-body equivalent with respect to radiation detriment. A variety of models are used to derive effective dose estimates from the various dose parameters available for each imaging modality. The effective dose of a pediatric chest radiograph is the order of 0.01 to 0.02 mSv, that of a voiding cystourethrogram 0.5 to 1 mSv, that of a chest CT scan 2 to 4 mSv, that of a bone scan 4 to 7 mSv, and that of a gallium scan 25 to 50 mSv. Knowledge of the effective doses of radiologic procedures enables comparison with other imaging modalities or with annual background radiation (∼3 mSv) and estimation of CEDs from multiple sources.
We have demonstrated significant variability in the CEDs received by pediatric oncology patients, both between individuals within a tumor subgroup and between subgroups. Radiation exposure varies on the basis of diagnoses, individual clinical courses, and imaging modalities used most frequently. CT and NM accounted for >95% of the total cohort CED. The number of plain radiographs was higher, but they contributed only minimally to CEDs. Fluoroscopy and interventional procedures were also relatively low contributors.
The leukemia and brain tumor subgroups underwent the fewest procedures (median CED: <12 mSv). Routine use of MRI in the follow-up monitoring of brain tumors, with less use of CT, is an important factor. Disease surveillance in leukemia is centered on clinical symptoms and laboratory parameters, typically with infrequent need for high-dose modalities.
Patients with neuroblastomas had the highest overall median CED (213 mSv). Regular follow-up CT was the largest contributor, followed by NM procedures. Patients with lymphomas had the next highest median CED (191 mSv). This subgroup underwent fewer CT and NM procedures than did the solid-tumor group, but the high dose derived from serial gallium scans (median scan dose: 38 mSv) led to overall higher CEDs. The individual with the highest CED estimate (642 mSv) belonged to the lymphoma group and is in clinical remission to date. This patient underwent 6 radiographs, 14 CT scans (10 with neck, chest, abdomen, and pelvis coverage), and 13 NM procedures (all gallium scans) during the 5-year follow-up period. In the interval since this study, positron emission tomography has become available at our institution and has replaced the use of gallium for patients with lymphomas. Although positron emission tomography doses are still moderately high (5–8 mSv),31 this change in practice can be expected to result in dose reductions for future patients.
There is limited published pediatric literature for comparison. Holmedal et al16 demonstrated CEDs of up to 63 mSv (mean: 19 mSv) for children undergoing serial head CT scans related to ventriculoperitoneal shunts. Other pediatric studies confined themselves to plain radiography.14,15 Our study represents the first pediatric cohort study to include all ionizing imaging modalities.
There are more data for adults. In the largest study, Sodickson et al6 reviewed the CT histories of adult patients at a tertiary academic center. They found a mean CED of 54 mSv (median: 24 mSv; maximum: 1375 mSv). Other studies investigated specific patient diagnostic groups or clinical settings, including Crohn disease,7 renal colic,8,9 cardiac disease,10 cystic fibrosis,11 trauma,12 and the emergency department.13 Some studies were confined to estimation of cumulative CT doses,6,8,9,11,–,13 but others attempted to include all ionizing modalities.7,10 Patient numbers, selection criteria, follow-up periods, imaging modalities included, and data presented varied, but maximal CEDs of 153 to 579 mSv, means of 11 to 122 mSv, and medians of 9 to 91 mSv were reported. These compare with our cohort mean CED of 113 mSv, median of 61 mSv, and maximum of 642 mSv. Therefore, the level of exposure of some children is of the same order as that of many adult patients.
Sodickson et al6 found that 15% of adult patients had estimated CEDs above 100 mSv, the level at which the Biological Effects of Ionizing Radiation VII report32 from the National Academy of Science considered there to be good evidence for risk of significant harm. The larger proportion of our patients (41%) with CEDs above this level likely reflects the increased imaging requirements of oncology patients, compared with a nonselected patient group with both benign and malignant conditions.
Our study has several limitations. Because this was a retrospective study, CEDs might have been underestimated if patients underwent imaging procedures at other institutions. Efforts were made to minimize this. Any available initial diagnostic procedures that had been performed elsewhere were included in the study, as were any subsequent follow-up scans forwarded to the patients' oncologists. A small number of outside procedures might be unaccounted for but we would anticipate their contribution to be small, because we included only patients whose care was centralized at our institution. An advantage of a retrospective study is the potential to avoid any bias in imaging requisition behavior induced by awareness of an ongoing study.
Determination of representative effective doses for a large number of different procedures for children of varying ages presents many challenges. Doses for each procedure vary with patient age and size, technical parameters in use at that date (in general, doses have decreased in our department over the past 10 years), and the equipment used (for example, 3 CT scanners were in service during the study period). Whenever possible, efforts were made to use individual and/or departmental dose data, rather than published data from other institutions. CT doses were based on our institutional data18 and, unlike in most previous studies, included assessment of the number of contrast phases used, which may affect doses by a factor of ≥2. With the exception of iodine-131-meta-iodobenzylguanidine scans, NM doses were based on the individual recorded activity (in megabequerels) administered. Interventional radiology doses were calculated by using patient-specific fluoroscopy times and numbers of digital subtraction angiography exposures but were simplified to estimates of posteroanterior and lateral projections, because we did not have individual data on projection geometric features or magnification. Published literature data were used for doses for plain radiography and fluoroscopy, procedures that are at the lower end of the dose range or were performed relatively infrequently.
Another limitation that affects the precision of dose estimates is the use of 5-year age categories; effective dose data were not available for narrower age brackets. This could result in some overestimation or underestimation of doses for patients at the limits of their age group. Given the complexities of pediatric dosimetry, however, we think that our study represents a reasonably accurate estimation of CEDs and one that is more institution-specific than many previous pediatric and adult studies.
Our study findings reflect our institutional imaging practices. Other practices may use imaging modalities in different proportions or frequencies, depending on availability, local expertise, preferences, and perhaps varying attitudes toward the potential risks of ionizing radiation.
Exploration of the relationship between CEDs and tumor stages or therapy regimens was beyond the scope of this work but is a key area for future study. We have since completed further investigation into the role and contribution of diagnostic and surveillance imaging in the lymphoma subgroup.33
It is important to attempt to interpret these data with reference to risk assessment and clinical context. It is difficult to put the potential long-term risks of diagnostic imaging related to the care of patients with acknowledged life-threatening conditions in meaningful context. The benefits of accurate, timely imaging are immense and must not be underestimated. Survival rates for childhood cancer have increased considerably over the past 3 decades, however, with a current overall 5-year survival rate of 81% and survival rates of >85% for Hodgkin's disease, acute lymphoblastic leukemia, and Wilms tumor.17 Therefore, it is incumbent on us to consider all possible sources of potentially harmful, long-term effects.
The Biological Effects of Ionizing Radiation VII report32 estimated an excess lifetime cancer risk of 1 case in 1000 population for a standardized population receiving 10-mSv exposure. Approximately one-half of those cancers may be expected to be fatal. There is an inverse exponential relationship between estimated risk and age of exposure, with children being at greater risk than adults and the youngest children being at the greatest risk. The excess lifetime risks are estimated at 1.4 case (boys) and 2.6 cases (girls) per 1000 population for exposure at the age of 10 and 1.8 and 3.3 cases per 1000 population, respectively, at the age of 5.6,32 If we consider a mixed-gender risk factor of 2 cases per 1000 population per 10 mSv, then exposure to the median CED of our study cohort (61 mSv) at the age of 10 might result in an excess lifetime cancer risk of 1.2%. Individuals receiving >100 mSv (41% of our cohort) might have an excess risk of >2%, and those receiving >200 mSv (22%) might have an excess risk of >4%.
Some caution is necessary in the use of this model, which assumes a normal population without any predisposing conditions that might themselves reduce life expectancy further than expected for age- and gender-matched control subjects.6 Nearly two-thirds of pediatric cancer survivors experience some late effects resulting in significant long-term morbidity and, in some cases, death34; these include cardiac, pulmonary, and endocrine disorders, infertility, neurocognitive defects, and secondary neoplasms.34,35 Survivors are already known to be at greater risk of future malignancies than the background population, with a relative risk of 14.8 of developing a second malignant neoplasm being demonstrated in a population treated between 1970 and 1986, before contemporary levels of diagnostic imaging-derived radiation exposure.34 Both chemotherapy and radiotherapy regimens are thought to be significant contributing factors.35 Any additional risk associated with diagnostic radiology must be considered concomitantly.
Approaches to ensure that exposure of this vulnerable population to diagnostic imaging radiation is appropriate and is minimized where possible center around the 2 principles of radiation protection, that is, justification for a procedure and optimization of its performance. Increased awareness among referring physicians of the relative magnitude of radiation doses associated with imaging procedures and of current risk estimates is an essential first step.36,37 With the advent of the recent “Image Gently” campaign led by the Alliance of Radiation Safety in Pediatric Imaging38 and an improving literature base on effective doses in medical imaging,39 knowledge is becoming more widely disseminated.
Whenever an imaging request is considered, an assessment of the risk/benefit balance for the individual patient should be made, with due consideration of the expected impact on clinical management.40 The use of alternative nonionizing modalities (ultrasonography and MRI) should be promoted wherever appropriate and available. When CT is required, age-related CT protocols and new technologic advances in dose reduction should be used.41,42 Ultra-low-dose protocols for specific indications should be developed (eg, low-dose sinus CT for exclusion of fungal disease for patients with neutropenia).18 Follow-up intervals for high-dose procedures must be appropriate, and careful consideration should be given to the radiation burden when surveillance protocols are determined by national pediatric oncology organizations.
Finally, our study has demonstrated significant cumulative ionizing radiation exposure from medical imaging for a subgroup of pediatric patients. This may raise awareness of other patient groups that may be undergoing regular surveillance imaging for nonmalignant conditions, including inflammatory bowel disease, cystic fibrosis, congenital cardiac defects, renal calculi, and ventriculoperitoneal shunt management. We must take this opportunity to audit and to reexamine our pediatric imaging practices to ensure that any potential risks are minimized.
The range of CEDs received by pediatric oncology patients from diagnostic and follow-up imaging is wide. This study provides the first such data in a pediatric patient population undergoing serial imaging involving multiple ionizing modalities. Some patients underwent reassuringly few imaging procedures, but others received high cumulative exposure. With current improvements in patient survival rates and increasing focus on minimizing the long-term effects of therapeutic regimens, it is incumbent on us to consider the potential long-term risks associated with ionizing radiation in medical imaging. It is hoped that increasing awareness of the cumulative diagnostic radiation exposure to which pediatric oncology patients are exposed will encourage strategies to reduce radiation burdens wherever possible.
We are grateful for the assistance of Maria Green in extracting data from the departmental NM database.
- Accepted June 25, 2010.
- Address correspondence to Karen E. Thomas, BM, BCh, Hospital for Sick Children, Department of Diagnostic Imaging, 555 University Ave, Toronto, Ontario M5G 1X8, Canada. E-mail:
FINANCIAL DISCLOSURE: The authors have indicated they have no financial relationships relevant to this article to disclose.
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- cumulative effective dose •
- CT =
- computed tomography •
- NM =
- nuclear medicine
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- Copyright © 2010 by the American Academy of Pediatrics