Sunday, June 24, 2012

So Rad: Radiation in Medical Imaging, The Basics

The Gist:  Although the evidence is somewhat murky because CT scans are relatively new and the data is extrapolated from atomic bomb survivors, there is about a 1 in 1000 chance that an individual exposed to 10 mSv (1 routine abdominal CT scan) will develop cancer attributable to that CT.  How this is derived can be found below (to hear it from the experts: NEJM review article).
Medicine is a science of uncertainty, and an art of probability. 
-William Osler

Radiation in medical imaging is a hot topic.  CT scans are often necessary in the Emergency Department to identify critical issues quickly, but there is a risk associated with the ionizing radiation from this imaging so it's somewhat important to have an ability to discuss this risk (cheat sheet from Academic Life In EM).

I hear rad this, milliSievert that...what's the deal?  Some of these terms are historic in nature; most medical literature uses the Sievert (Sv).

Most useful in medicine:
  • Sievert (Sv) - effective dose of radiation on biological tissue.  It's specifically designed to reflect the radiation risk in tissue.  This is the primary difference between a Sv and a Gy. 
    • 1 Sv= 100 rem.  We use milliSieverts in most medical imaging.
    • Neoplasms have typically been associated with radiation in excess of 50 mSv
  • Gray - amount of ionizing radiation energy absorbed by an object. Does not describe any biological effects.
    • 1 gray (Gy) = 100 rad.
    • No real direct interchangeability between Sv and Gy because the former is only applicable to biological tissue.
Less useful/Historic:
  • 1 roentgen = unit of radiation dose.
  • REM = roentgen equivalent in man. Amount absorbed by human tissue, depending on the type of tissue.
  • RAD = radiation absorbed dose. Work of energy absorbed by 1 roentgen (amount absorbed by 1 roentgen as it affects biological tissue). Used in mostly in industry. 
Just how much radiation are we talking about in medicine?  On average, a person receives an average background of 3-3.5 mSv by just inhabiting Earth.  

Most dialogue on radiation risk in medical imaging centers on CT scans because they utilize far more radiation than plain x-rays.  The actual radiation from CT scans seems pretty variable based on scanned organ and the CT scanner/operator.  The Smith-Bindman, et al study published in the Archives of Int Medicine estimated the actual radiation exposure of CT scans at study institutions using the dose-length product (DLP), which is an approximation of the total energy a patient absorbs from the scan, when adjusted for organ sensitivity to radiation.  It's also important to note that radiation dose depends on the type of CT scan and the effects depend on the sensitivity of the tissue in a given anatomic area.
  • Routine abdominal CT scan estimated exposure quoted as 8-10 mSv
    • Actual calculated radiation: Average 11-20 mSv (Range 4-45 mSv) (little variation with regard to with or without contrast).  
  • Multiphase abdomen and pelvis CT scanning: median 31 mSv (Range 6-90 mSv)
  • Chest CT for suspected pulmonary embolus: median 10 mSv (Range 2-30 mSv)
  • Routine head CT median:  2-3 mSv (Range 0.3-6 mSv)

So, how much does it take to cause harm?  
BEIR (Biological Effects of Ionizing Radiaiton) VII Report holds that a dose dependent, linear no-threshold relationship exists between ionizing radiation and human cancers.  Basically, this means that all exposure carries a certain probability of harm with no zero-risk dose and that the effects of multiple small doses are additive.  

The model is based on studies of the survivors of the 1945 atomic bombs in Japan.  Brenner has probably published the most popular articles translating this data into meaningful risks from medical imaging, including this paper.  The data stems from a cohort of over 120,000 individuals from the Radiation Effects Research Foundation.  A good overview of this database.  The data demonstrate:
  • ~30,000 survivors had low dose radiation exposure (5–100 mSv), approximately the same dose as many CT scans, making this cohort relevant
  • The risk of all solid cancers in this group is consistent with a linear increase in radiation dose.
    • Survivors in the dose category from 5-125 mSv (mean =34 mSv) show a significant  increase in mortality related to solid cancers (p = 0.025) (3).
  • Children are more radiosensitive than adults. The risk from exposure during fetal life, childhood and adolescence is estimated to be about 2-3 times as large as the risk during adulthood.
  • Cancers other than leukemia typically start to appear 10 years following exposure (5 years for leukemia) and the increased risk remains for the lifetime of the exposed individuals.
Cardis et al published a 15 country retrospective cohort study of industrial employees exposed to radiation on a regular basis (mostly nuclear power, n= 407,391) echoes the findings of the atomic bomb studies, demonstrating that low-dose radiation is associated with an increased risk of neoplasms. 
    • Excess relative risk, 0.97 per Sv (95% Cl 0.14-1.97) for all cancers excluding leukemia. This estimate corresponds to a relative risk of 1.10 for a radiation dose of 100 mSv. The excess relative risk for solid cancers was higher than the estimate for the atomic bomb survivors (0.32 per Sv, not significant).  Weakness:  As a retrospective observational study, there are several confounders, including smoking and the CI crosses 1.
    • Average exposure per year = 19.7 mSv (1-2 routine abdominal CT scans)
One of the major issues with this data is that these cancers are superimposed on a background cancer rate.  Additionally, there may be some selection bias and survivors may be more likely to have cancer listed as a diagnosis.

Putting the pieces together:  Children and young people are at greatest risk.
  • The Academic Life In EM blog has a handy PV card of the table from an Archives of Int Medicine study that gives quick numbers to help estimate a patient's risk.  (Aka:  your risk is 1 in ____ from this single scan). 
  • This is also a great, interactive risk calculator

Other good resources:
Dr. Sean Fox's slides on mededmasters.com provide a great visual supplement/review of basic radiation risks from imaging.
Interesting article:  Radiologists and Emergency Physicians Often See Radiation Risks and Benefits Differently

Next up..why this actually matters to clinicians 

References:

1. Smith-Bindman RLipson JMarcus RKim KPMahesh MGould RBerrington de Gonz├ílez AMiglioretti DL.  Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer.  Arch Intern Med. 2009 Dec 14;169(22):2078-86.

2.
  • Baumann B, Chen E, Mills A, Glaspey L, et al.  
  • Patient Perceptions of Computed Tomographic Imaging and Their Understanding of Radiation Risk and Exposure.  Annals of Emergency Medicine  Volume 58, Issue 1 , Pages 1-7.e2, July 2011

    3.  Brenner et al.  Cancer risks attributable to low doses of ionizing radiation: Assessing what we really know.  PNASvol. 100 no. 24 13761-13766

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