| UNITS • SIEVERTS • mSv • REMS • mrem • CONVERSIONS |
| • mSv / millisievert / mSievert: A scientific unit of measurement for radiation dose, referred to as effective dose, is the mSv or millisievert. There are other radiation dose measurements: rad, rem, Roentgen, Sievert, and Gray. However, most articles and reports utilize the millisievert - mSv. |
| • Sievert (Sv) - the unit measures the equivalent dose of radiation (H), supposed to have a damaging effect equivalent to the same dose of gamma rays. |
| • Gray (Gy) - measures the absorbed dose of radiation (D), absorbed by any material. |
| • SI - both the gray (Gy) and the sievert (Sv) are SI derived units, defined as a unit of energy (joule) per unit of mass (kilogram). |
| • In the United States, the rem still often used. |
| • The conventional unit for its time derivative is mSv/h. |
• Conversions
1 rem = 0.01 Sv = 10 mSv
1 mrem (millirem) = 0.01 mSv = 10 μSv
1 Sv (sievert) = 100 rem
1 mSv (millisievert) = 100 mrem = 0.1 rem
1 mSv = 0.001 Sv
1 μSv (microsievert) = 0.1 mrem
1 μSv = 0.000001 Sv
|
| ALL-SOURCE COMPARATIVE EXPOSURES IN MILLISIEVERTS (mSv) |
| Naturally-occurring annual background radiation |
3 |
| Round-trip coast-to-coast commercial air travel |
0.03 |
| Airport backscatter scanner |
0.05 |
| Chest X-ray (2 views) |
0.1 |
| Mammography |
0.04 |
| Annual Exposure of Airline crew flying a NY-Tokyo polar route |
9 |
| CT scan: full-body |
10 |
| CT scan: abdomen and pelvis |
15 |
| CT scan: heart |
16 |
| Maximum allowable exposure for U.S. radiation workers |
50 / yr |
| Radiation dose at boundary of Fukushima Daiichi on March 16, 2001 |
1.9 / hr |
| Recommended limit for radiation workers every five years |
100 |
| Lowest annual dose at which any increase in cancer is evident |
100 |
| Level for relocating people after Chernobyl accident |
350 / Lifetime |
| Peak radiation recorded at Fukushima Daiichi nuclear site, 15 March 2011 |
400 /hr |
| Accumulated dosage to cause fatal cancer many years later in 5% of people |
1,000 |
| Single non-fatal dose that will cause radiation sickness |
1,000 |
| Single dose that would kill half of those exposed to it within a month |
5,000 |
| Typical exposure to Chernobyl workers who died within a month |
6,000 |
| Single dose, fatal within a few weeks |
10,000 |
| BEIR (Biologic Effects of Ionizing Radiation) |
| • X-rays and gamma rays are sources of low linear energy transfer (low-LET) ionizing radiation. Humans are exposed to ionizing radiation from both natural and man-made sources. |
| • Most radiation sources are a mixture of high- and low LET radiation. Low-LET radiation deposits less energy in the cell along the radiation path and is considered less destructive per radiation track. BEIR defines low LET radiation as a dose between near zero to about 100 mSv (0.1 Sv). As points of comparison, annual background radiation in the U.S. is 3 mSv; a CXR exposure, about 0.1 mSv; and a whole body CT, about 10 mSv. |
• Worldwide background radiation of 2.4 mSv / year comes from:
| |
High-LET: inhalation exposure to radon |
52 % |
| |
High-LET: ingestion |
5 % |
| |
High-LET: neutron component of cosmic radiation |
4 % |
| |
Low-LET: directly ionizing and photon component of cosmic radiation |
12 % |
| |
Low-LET: radiation exposure from the earth |
20% |
| |
Low-LET: ingestion |
7 % |
|
• In the US population, 79% of man-made radiation exposure is from medical xrays and nuclear medicine; consumer products (tobacco, domestic water supply, building materials, and to a lesser extent, smoke detectors, TV, computer screens contribute about 16%; occupation exposures, fallout, and nuclear fuel cycle contribute about 5%.
|
COMPARATIVE RADIOLOGIC MEDICAL EXPOSURES |
| X-rays are forms of radiant energy, like light or radio waves; but unlike light, they can penetrate the body to allow the visualization of internal structures through film or monitors: plain xrays, fluoroscopies, CT scans, mammography, etc. |
| Source |
Effective Radiation
Dose |
Compared to Natural Background Exposure
of 3 mSv/year |
| Natural background exposure |
3 mSv / year |
|
| Airport Backscatter (Single scan) |
0.05 mSv |
30 mins |
Coast-to-coast round trip
commercial
flight |
about 0.03 mSV
of cosmic rays |
18 mins |
| Head and Neck |
| CT - Head |
2 mSv |
8 months |
| CT - sinuses |
0.6 mSv |
2 months |
| Chest / Heart / Lungs |
| CXR (chest x-ray, 2 views) |
0.1 mSv |
10 days |
| CT - chest |
7 mSv |
2 years |
| CT - chest / low-dose |
1.5 mSv |
10 days |
| Cardiac CT / Calcium Scoring |
2 mSv |
2 months |
| Coronary CT angiography |
8 - 20 mSv |
3 - 7 years |
| Coronary angiography |
5 mSv |
< 2 years |
| Coronary MSCT |
10 mSv |
> 3 years |
Resting scan plus
Technetium-99 sestamibi
cardiac stress scan |
11.3 msv |
< 4 years |
| Dipyridamole nuclear stress test (Thallium + technitium) |
25 msv |
> 8 years |
| Abdomen |
| Upper GI series |
5-6 mSv |
1.5 - 2 years |
| Lower GI series (Barium Enema) |
7 - 8 mSv |
3 years |
| Abdomen x-ray (AP) |
0.7 mSv |
3 months |
| KUB x-ray (2 views) |
1.2 mSv |
4 months |
| CT Abdomen |
8 mSv |
2 1/2 - 3 years |
| CT Abdomen / Pelvis (combined) |
10 - 15 mSv |
3 - 5 years |
| CT, Body |
2 - 10 mSv |
|
| CT / Colonography |
10 mSv |
3 years |
| IVP (Intravenous pyelography) |
3 mSv |
1 year |
| Ureteric stent insertion |
4.7 mSv |
20 months |
| Voiding Cystourethrogram |
1.6 mSv (5-10 years old)
0.8 mSv (infant) |
6 months
3 months |
| Skeletal |
|
|
| Spine x-ray |
1.5 mSv |
6 months |
| Extremity x-ray |
0.001 mSv |
Less than 1 day |
| CT - Spine |
6 mSv |
2 years |
| Myelography |
4 mSv |
16 months |
DEXA (Bone densitometry)
Male or Female |
0.001 mSv |
Less than 1 day |
| Women's Imaging |
|
|
| Galactography |
0.7 mSv |
3 months |
| Hysterosalpingography |
1 mSv |
4 months |
| Mammography |
0.4 - 0.5 mSv |
2 months |
| Others |
|
|
| CT - Whole Body |
2 - 10 mSv |
6 months to 3 years |
| |
| Note: Some use 3.6 mSv as the amount of naturally-occurring background radiation exposure. |
| Note:The estimation of radiation dosage is an inexact science, so these numbers listed should be considered as best approximations. |
| Note: Radiation exposures vary depending upon the equipment and patient. |
| UNDERESTIMATION OF CT SCAN RADIATION / RISKS |
| • Study participants had poor understanding of the risks associated with CT scanning. The majority underestimated the XR radiation from CT scans and cancer risk comprehension was poor. |
| • Many were unaware that 2 or 3 abdominal CT scans exposed a person to the same amount of radiation exposure as experienced by Hiroshima survivors who lived near the atomic blast. |
| • The radiation dosage from one scan typically ranges from a few mSv (comparable to a year's worth of background radiation) to ten of mSv. |
| • A CT scan that delivers 5 mSv of radiation is equivalent to 50 chest xrays. A four-phase abdominal CT gives the same radiation as 300 chest x-rays. |
| • Hiroshima survivors living a few miles from the blast received between 5 to 100 mSv. There is little difference between beamed hospital procedure radiation and mix of xrays and gamma radiation from a nuclear explosion. A difference is that atomic bomb survivors got whole-body radiation compared to the direct CT radiation exposure. |
| • In the U.S. the use of CT scans has skyrocketed in the past 3 decades, from 3 million scans in the 80s to 70 million scans in 2007. More than 4-7 million children are getting CT scans, a number that increases by 10% a year. The increase radiation exposure estimates 29,000 new cases of cancers in the coming years. |
| • A retrospective cross-sectional study describing radiation dose associated with 11 of the most common types of diagnostic CT procedures concludes: Radiation doses from the commonly performed diagnostic CT examinations are higher and more variable than generally quoted. |
| HOW MUCH IS TOO MUCH? |
| • Experts, scientists, and radiologists agree radiation in large doses causes cancer. The controversy and debate centers on what is the "acceptable" dose. |
| • Some experts espouse the linear-no-threshold (LNT) theory for radiation cancer risk, that any radiation dose, no matter how small, can cause cancer, i.e., the risk is never zero. |
| • Some experts disagree, stating cancer risk with low exposure is overstated. Using rem unit, no risk of adverse health conditions has been established for exposures to 5,000 milli-rem (mrem) or less. |
| • The Health Physics Society recommends against quantitative estimation of health risk for individual exposures of 5,000 mrem (milli-rem) . The threshold of radiation above which cancer risks begins to increase is proposed to be about 10,000 mrem. |
| • A radiation dose of 50 mSv starts concerns regarding human health: 100 mSv and above raise the risk for developing solid cancers and leukemia. |
| • Recent estimates suggest 2% of cancers could be attributed to CT scan radiation. Although single exposures may be minimal, cumulative exposures increase the risk. |
| WHAT PATIENTS SHOULD DO |
| • Keep a record of your radiation exposure history: Xrays, CT scans, nuclear imaging, barium studies; especially if you're changing physicians or seeing different doctors, specialists, and consultants. |
| • Avoid repetitive tests. |
| • Most doctors are not aware of the cumulative dose of radiation their patients have received. |
| • Although it is your doctor's responsibility to weight the benefits of imaging against risks, be interactive in the decision making needed with your work-up. Try to be informed. Discuss alternative workup procedures that deliver less radiation: limited scanning, ultrasounds, MRI. Some doctors are just too busy or more concerned with the ka-ching aspects of their practice; some might have vested interests on procedures and scanners. If so, look for another doctor. And, second opinions won't hurt. |
| • Go to facilities committed to radiation safety. Check out the age of the facility's equipment. Old CT scanners deliver a lot more radiation than the newer ones. Radiation dosages differ significantly between sites/centers and CT systems. |
| • Search out facilities using new software technology that can significantly reduced the required radiation dose. |
| • A study using dose-reduction strategy, performed a 64-slice CT angiogram with a measured radiation exposure of 2.1 mSv. In comparison, other centers performed similar CT angiograms using 21 mSv, 10 times the level of radiation. |
| • Newer scanners - Electron Beam CR , multidetector computed tomography (MDCT), low-dose hi-def CT scanners, are in use or in research-and-development, delivering only a fraction of radiation while achieving high image quality. |
| Sources and Suggested Readings |
| Are Full-Body Airport Scanners Safe? / Medscape |
Cancer Risk Overestimated With Radiation From CT Scans / Medscape Medical News |
| Patient Perceptions of Computed Tomographic Imaging and Their Understanding of Radiation Risk and Exposure / Ann of Emerg Med / Abstract |
| Radiation Exposure Tied to Lymphoma Risk in Men / Am J Epidemiol 2009;169:969-976 / ACR |
| Radiation Dosage of EBT Procedures / prepared by Dr. Alan Boyer |
| Radiation Dose in Computed Tomography of the Heart: Morin RL, Gerber TC, MD & McCollough CH; Circulation 2003;107:917-922. |
| Reduced Radiation Exposure with the Use of an Air Retrograde Pyelogram During Fluoroscopic Access for Percutaneous Nephrolithotomy / Lipkin ME, Mancini JG, Ziberman DE et al / Journal of Endourology (Mar 2011) |
| Radiation Protection in Urology / |
| Radiation During Cardiovascular Imaging / Ariel Roguin and Prashant Nair / From British Journal of Cardiology / Medscape |
Radiation Dose Associated With Common Computed Tomography Examinations and the Associated
Lifetime Attributable Risk of Cancer / Rebecca Smith-Bindman, MD; Jafi Lipson, MD, Ralph Marcus, BA, Kwang-Pyo Kim, PhD et al / Arch Intern Med. 2009;169(22):2078-2086 |
| Radiation exposure: a quick guide to what each level means / DataBlog |
| Sievert / Wikipedia |
| CT Scans of the Heart Can Be Done with Low Radiation Dose /New Study Shows Wide Variation in Centers; Calls for Increased Education and Training to Achieve Optimum Results / Angioplasty.Org/Imaging News |
| Ultra-Low Radiation Dose Renal CT Examinations / Janet Cochrane Miller, D Phil., Author / Radiology Rounds, June 2011, Vol 9, Issue 6 |
| Long-Term Cancer Risks From Cardiac Imaging Radiation Remain Unknown / Marlene Busko / Medscape Internal Medicine News / |
| Beir VII: Health Risks from Exposure to Low Levels of Ionizing Radiation / Report in Brief / Expert Consensus Report |
| Multiple CT Scans Add Minimal Additional Cancer Risk / Daniel M. Keller, PhD / Medscape Internal Medicine News |
