As Low As Reasonably Achievable
Your patient is a 40-something female who has never been to
your facility before but reports a history of chronic abdominal pain of
undetermined etiology. She has had an appendectomy and a cholecystectomy. She
is now presenting with two days of right-sided cramping abdominal pain
associated with nausea without vomiting and lightheadedness. Screening labs
protocoled by the triage nurse are unremarkable, and a bedside RUQ ultrasound
is negative for significant pathology. After two doses of morphine she is still
visibly uncomfortable in the stretcher. The team is reticent to pursue further
diagnostics, but given the fact that she currently carries no diagnosis for her
symptoms and is still in considerable distress, and the lack of any prior
imaging in your EMR, the decision is made to order a CT abdomen/pelvis with
contrast.
Clinical Question:
Given the fact that the pre-test probability for finding
significant pathology in this patient is low, how worried should you be about
exposing your patient to further ionizing radiation?
Literature Review:
Several groups of experts, including the American Association
of Physicists in Medicine, the National Council on Radiation Protection
& Measurements, and the US FDA, all report the median effective dose of
ionizing radiation (IR) from an abdomen/pelvis CT (the most common CT ordered
in the US) as 8 - 10 millisieverts (mSv) [1]. This is compared to 0.065 mSv for
a two-view chest X-ray, and 0.42 mSv for a screening mammography series [12].
At the risk of turning this into a physics lecture, the
Sievert is the SI unit of measurement for “dose equivalent” and for "effective
dose" of radiation. “Dose equivalent” includes a weighing factor that
accounts for the relative biological impact of different types of radiation. “Effective
dose” takes this a step further, by utilizing weighing factors to account for
the relative effect of radiation on different types of tissues. It represents
the probability of cancer induction, and is commonly used in medical research.
Conventionally, “effective dose” is used to measure low doses of IR which carry
the stochastic (i.e., somewhat random and not entirely predictable) risk of
inducing malignancy. This is in contrast to high doses of IR that cause
deterministic effects -- what we would consider radiation poisoning -- that are
certain to occur with exposure to high levels of IR. These exposures are
typically measured in Grays, the SI unit of absorbed dose (simply the mean
energy imparted to each unit of mass). For more detailed information on
radiation quantities and exposures, click here.
So when should we be concerned? There is no "safe"
dose of radiation. We are all exposed to a certain level of
"background" radiation which probably leads to a certain rate of
malignancy all on its own; any exposure to IR beyond this can only increase
this risk. Thus the radiation safety doctrine of "As Low As Reasonably
Achievable" governing IR exposure.
The National Academy of Sciences' National Research Council
published phase 2 of its Biological Effects of Ionizing Radiation (BEIR) VII
report in 2006. In this report, the group gathered available biological and
epidemiological data related to human IR exposures. This included survivors of
the atomic bomb blasts in Japan during WWII, people who lived close to nuclear
power accident sites, workers with occupational exposures, and patients who
underwent medical radiologic studies [2]. The breadth of available data suggest
that increased cancer risk is associated with acute exposures of 10-50 mSv, and
protracted exposures of 50-100 mSv [3]. Data also suggest a "linear, no
threshold" dose-response relationship -- again, no "safe" level
of IR exposure exists and any exposure, no matter how small, carries a nonzero
risk of developing malignancy [2].
It doesn't take a particle physicist to see that the high end
of the commonly reported dose of a CT scan overlaps with the low end of the
dose range leading to increased malignancy. While the absolute risk of
developing malignancy from a single CT scan may be quite low, the sheer number
of scans performed would suggest that, from a population standpoint, many new
malignancies are being induced by medical imaging.
Indeed, a group of authors used radiation risk models from the
BEIR report to estimate incidence of future cancers resulting from the 72
million CT scans performed in the US in 2007 (excluding those scans performed
on patients already diagnosed with cancer or those performed in the last five
years of a patient's life) [4]. They concluded that approximately 29,000 new
cancers (95% CI 15,000 - 45,000) could result from these scans, with the
largest contributor being CT scans of the abdomen & pelvis with 14,000
new cancers (95% CI 6,900 - 25,000). About a third of these projected cancers
resulted from scans performed on patients aged 35 - 54, and 15% from scans
performed on pediatric patients less than 18 years old. Lung cancers were
projected to be most common, followed by colon cancer and leukemia. Two thirds
of cancers were projected to occur in females.
 |
| Projected number of future cancers (mean and 95% uncertainty limits) that could be related to computed tomographic scan use in the United States in 2007, according to age at exposure. From Reference [4] |
| ||
| These were not the only authors to reach such concerning conclusions. Again using BEIR VII data & models, one group estimated that annual chest CT screening for lung cancer in adults starting at age 40 would result in a 1/10,000 estimated excess lifetime risk of radiation-induced lung cancer mortality for men and 3/10,000 for women, which would likely negate any mortality benefit gained by early identification of lung malignancy from such screening [5]. Another group examined risk from CT coronary angiogram, using simulated exposure doses and BEIR VII models, and found a 1/284 lifetime attributable risk for a 40-year-old woman, with risk inversely proportional to age [6]. Several other groups have used calculated doses of common scans and BEIR VII models to conclude that exposure to CT scans likely significantly increases lifetime risk of cancer [7,8,9]. |
Two chief
limitations affect all of these studies. First, they all rely on BEIR VII
models of lifetime attributable risk. Unfortunately, accurately quantifying
risks directly would require long-term follow-up of very large patient
populations. The BEIR VII models are based on direct evidence of the
carcinogenic behavior of IR in other populations, such as nuclear accident
& atomic bomb survivors. As such, they likely represent the best models
we have for estimating long-term risk of IR in medical imaging. In fact, in
vitro studies suggest that the form of IR utilized in medical imaging (x-rays)
may have even greater potential to damage cellular DNA than gamma rays (the
primary form of IR released in atomic bomb blasts) [4].
The second
limitation to the prior studies is that they almost universally relied on
estimated doses of IR from the various scans, with the majority falling in the
previously-suggested 8 - 10 mSv range. Practically speaking, it is nearly
impossible to truly quantify the IR absorbed by a human body in a CT scanner.
Some degree of estimation & calculation will always be necessary, but
most of the aforementioned studies relied on previously-published estimates of
dose or dose estimates from "phantom studies," not exams performed on
actual patients.
In 2009, a group of authors in San Francisco attempted to more
directly quantify the radiation dose of common CT study types, compare intra-
and inter-site variability, and determine what impact this variability has on attributable
cancer risk [10].
They estimated this effective dose using the Dose-Length Product, which is
recorded as part of the CT scan metadata. The DLP is an approximation of the
total energy a patient absorbs from the scan,
determined by multiplying the energy absorbed from a single slice (the CT Dose
Index or CTDI) by the total length of the body scanned. The authors combined
the DLP with details of the area imaged and used conversion factors to
translate this into an effective dose. This approach is used elsewhere in the
literature, and is described in a report by the American Association of
Physicists in Medicine's report on The Measurement, Reporting, and Management
of Radiation Dose in CT [11]. They also utilized methods & risk
estimates published in the BIER report to calculate the lifetime attributable
risks of cancer above baseline based on the magnitude of a single exposure.
Based on the study group's data, doses of many scans were
significantly higher than the commonly-reported 8 - 10 mSV. Abdomen/pelvic CTs
had the highest radiation doses, ranging on average from 15 - 31 mSv. The
corresponding median adjusted lifetime attributable risk of cancer was 4
cancers per 1000 patients. For a routine abdomen/pelvis CT with contrast, one
attributable cancer would be expected for every 470 scans of 20-year-old
females, or for every 870 scans of 40-year-old females. These numbers are even
more concerning for multiphase scans (e.g., angiography studies), which impart
effective doses up to 90 mSv based on the authors' calculations. At this end of
the scale, a 20-year-old female undergoing multiphase abdomen/pelvis CT could
face as high as a 1/250 risk of cancer attributable to the scan. One of the
takeaways from this study was that the same CT scan type could yield effective
dosages with over 13-fold variability between sites, or even between different
scans on the same equipment.
 |
| From Reference [10] |
 |
| From Reference [10] |
Overall, data suggest that anywhere between 1 - 3% of cancer
cases in the US may be due to exposure to IR from medical imaging [4,13]. The
ACR White Paper states that the massive increased use of CT scanning in the
past decade "may likely
result in an increase in the incidence of imaging-related cancer in the US
population in the not-too-distant future."
Faculty commentary:
Dr. Richard Griffey, Director of Quality and Safety for Washington University Emergency Medicine and evidence-based diagnostics advocate, had this to add:
“One of the things people often ask/wonder about…is what strategy makes sense on an individual patient level?
- What is the incremental harm of an additional study?
- Does the benefit of the study far outweigh the risks?
- What are the other options – ultrasound, watchful waiting, serial exams, MRI, etc.?
- If someone has already undergone multiple scans, how much additional risk does one more scan actually incur?
- Who should we be focusing on in avoiding ionizing radiation?"
Take-home:
- While it
is highly impractical to conduct a longitudinal study of cancer incidence based
on exposure to medical imaging, based on the best data we have it is highly
likely that such exposure will lead to tens of thousands of new malignancies in
the future.
- The risk
of malignancy is most pronounced in females, and is inversely proportional to
age.
- Patients
undergoing repeat CT scans are almost certainly exposed to a level of ionizing
radiation that increases their risk of cancer above baseline.
- It is our
responsibility as patient caregivers to be responsible in our ordering of CT
scans and other studies that expose patients to ionizing radiation. They carry
a nonzero risk of harm which must be weighed against possible diagnostic yield.
Submitted by C. Sam Smith (@CSamSmithMD), PGY-3
Faculty Reviewed by Richard Griffey
Submitted by C. Sam Smith (@CSamSmithMD), PGY-3
Faculty Reviewed by Richard Griffey
References:
[1] National
Council on Radiation Protection and Measurements, Ionizing Radiation Exposure
of the Population of the United States. 2009;NCRP report 160 http://www.ncrponline.org/.
[2] BEIR
VII Phase 2. Washington, DC National
Academies Press 2006.
[3] Proc Natl Acad Sci U S A 2003;100(24);13761-13766.
[4] Arch Intern Med. 2009;169(22):2071-2077.
[5] J Med Screen 2008;15(31):153- 158.
[6] JAMA 2007;298(3):317-323.
[7] Radiology 2004;232(3):735- 738.
[8] Radiology 2009;251(1):175- 184.
[9] AJR Am J Roentgenol 2009;192(4):887-
892.
[10] Arch Intern Med. 2009;169(22):2078-2086.
[11]
American Association of Physicists in Medicine, The Measurement, Reporting and
Management of Radiation Dose in CT: Report of AAPM Task Group 23 of the
Diagnostic Imaging Council CT Committee.
College Park, MD American Association of Physicists in Medicine2008;AAPM
report 96.
[12]
National Cancer Institute, Radiation risks and pediatric computed tomography
(CT): a guide for health care providers. 2009.
[13] J Am Coll Radiol 2007;4(5):272- 284.