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 Looking for Clarity: Medical Imaging, Night Shift Work and Breast Cancer Risk
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Looking for Clarity: Medical Imaging, Night Shift Work and Breast Cancer Risk - Tuesday, November 12, 2013 11:00 AM

Looking for Clarity: Medical Imaging, Night Shift Work and Breast Cancer Risk
Few diseases have been studied as much as breast cancer.  And in part, this is what can make breast cancer such a frustrating topic to many women.  While there are countless scientific findings, all this work has identified only a handful of risk factors that women can actually control.    

Yet, a number of breast cancer risk factors fall into a middle ground– somewhat under a woman’s control, somewhat not.  Two less well-known factors that fall into this category are radiation exposure from medical imaging tests and working night shifts.  The science behind each of these factors is still developing, but each can have an impact on breast cancer risk.  According to Komen Scholar Dr. Melissa Bondy, Professor of Pediatric Hematology/Oncology at Baylor College of Medicine and Associate Director for Cancer Prevention and Population Sciences at the Dan L Duncan Cancer Center, “Like some other risk factors, radiation exposure from medical imaging and night shift work are not completely under a woman’s control. However, identifying these risk factors has increased our understanding of breast cancer and may lead to interventions to lessen their impact on risk.”  

Medical imaging
Does the radiation from medical imaging tests increase the risk of breast cancer? It’s been known for a long time that large doses of radiation – such as from an atomic bomb explosion – can greatly increase the risk of cancer.  Less well understood are the specific risks related to lower amounts of radiation women might encounter in everyday life.  Some medical imaging tests, however, are sources of radiation that have important breast cancer risks.  

Do X-rays and mammograms increase risk?
There seems to be no completely risk-free amount [font="inherit; font-style: inherit; margin: 0px; padding: 0px; border: none; vertical-align: baseline"]of exposure to radiation. However, the most common imaging test women will have throughout their lives – X-rays – carry little breast cancer risk at the frequency most women get them. X-rays may be used to find a broken bone, cavities in teeth or early signs of breast cancer (mammography).  Studies looking specifically at mammography have found that the very small risks that come with the low-level radiation exposure from these tests are outweighed by the potential benefits of finding breast cancer at early stages.1  One study projected that if one million women were screened with mammography, nearly 19,000 lives could be saved from breast cancer, yet only a little over 21 deaths would be caused from the low-level radiation exposure over time.1  And, with doses of radiation from mammography continuing to decline as the technology improves, any risks from this test are likely to become even smaller.[font="inherit; font-style: inherit"] 
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Do computed tomography (CT) scans increase risk?
There is growing evidence that CT scans can increase cancer risk.  CT scans provide detailed images of the body, but to do this they also deliver much greater amounts of radiation than standard X-rays.  Though it can vary depending on the machine and the test, CT scans of the chest can deliver 100 to 500 times the amount of radiation from an average chest X-ray.2-3  In addition, they’re being used more often. The rate of use of CT scans is about five times higher than it was 20 years ago.2  The result is that today women are more likely to get higher lifetime radiation doses from these tests.  A report from the Institute of Medicine (IOM) projected that 2,800 excess breast cancers are caused each year by medical imaging tests, with the large majority of these linked to CT scans.2-3  The younger women are when exposed to the radiation from CT scans, the greater the lifetime risk of cancer.2-3   

Are there ways to lower the risk from imaging tests?
It’s important that every woman get the medical tests that are important for her health. Mammograms save lives and have little, if any, impact on breast cancer risk.   

CT scans are also important but are sometimes being ordered for reasons other than medical need.  This is often simply because the scans are so easy to do and provide such clear images.2 Concerned patients can also request them, looking for reassurance.  Yet, because they’re not risk-free, patients should feel empowered to ask their doctors about other options and together decide if a CT scan is really needed for their current situation. This can be difficult for a patient – but it is something doctors are used to discussing. The American Society of Clinical Oncology’s Choosing Wisely initiative can help with this discussion by providing lists of tests that may or may not be needed for certain situations. Patients should always feel free to seek a second opinion if the situation allows.   

CT scans can provide life-saving information, so no woman should endanger her health by refusing a scan simply to lower her exposure to radiation. However, with a few thoughtful questions, it’s possible to help cut down on exposure and avoid excess breast cancer risk.  

Learn more about  the American Society of Clinical Oncology’s Choosing Wisely recommendations.  

Night shift work
Does working the night shift increase the risk of breast cancer?
Over the past decade, good evidence has shown that regularly working night shifts over many years raises the risk of breast cancer.  One analysis that combined the results from 13 studies found that long-term night shift workers had a 50 percent greater risk of developing breast cancer than women who didn’t work through the night.4  Based on such findings, in 2007, the International Agency for Research on Cancer classified night shift work as “probably carcinogenic.”5  

Find a summary table of studies on shift work and breast cancer risk

How does working night shifts increase risk?
Although the exact reasons aren’t known, working regularly through the night may increase breast cancer risk by lowering blood levels of melatonin. Melatonin is a hormone that helps maintain normal waking and sleeping cycles. It is commonly called “the sleep hormone.” Melatonin is normally produced at night, but exposure to light at night suppresses melatonin production.  Several studies have found that women with lower melatonin levels may have higher breast cancer risk. 6-7  
Can anything limit the effects of night shift work on risk?  
It’s currently unclear whether women can do anything to limit the effect that working at night may have on their breast cancer risk.  Trying to cut back on night shift hours will likely help, but may not be an option for all workers.  Taking melatonin supplements has been proposed by some as a possible approach, but currently there is no evidence that it would provide any benefit.  One small, short-term study found that melatonin supplements had no effect on certain breast cancer risk markers.8  Larger, longer-term studies are needed, though, before ruling out any benefit for melatonin supplementation. 
There’s good evidence that certain medical imaging tests and long-term night shift work can increase the risk of breast cancer. These factors aren’t often in a woman’s control, yet there are promising steps being taken to help lessen their impact.  Changes in approaches to medical imaging are being developed to limit CT scans to situations where they are truly medically necessary, and technology advances should help reduce the amount of radiation used during such tests.  Further research should also help identify steps that could limit the increased breast cancer risk linked to long-term night shift work – whether it’s melatonin supplementation, changing work schedules or some other approach entirely.    

What is Komen doing?
Since 1982, Komen has provided more than $2.6 million to support research related to breast cancer risk from medical tests or shift work. Examples of such research projects include:   
  • Understanding the biological effects of ionizing radiation and the way it contributes to breast cancer development or progression 
  • Studying specific gene mutations that may increase the chance of getting radiation-induced breast cancer  
  • Developing a new method to more accurately measure the amount of radiation a person receives during routine medical tests   
  • Identifying genes and pathways involved in regulating the cancer-causing effect of disruptions in the circadian rhythm (sleep/wake cycles), such as working the night shift 
  1. Feig SA. Adverse effects of screening mammography. Radiol Clin North Am. 42(5):807-19, 2004. 
  2. Smith-Bindman R. Environmental causes of breast cancer and radiation from medical imaging: findings from the Institute of Medicine report. Arch Intern Med. 172(13):1023-7, 2012. 
  3. Smith-Bindman R, Lipson J, Marcus R, Kim KP, Mahesh M, Gould R, et al. Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer. Arch Intern Med. 169(22):2078-86, 2009. 
  4. Megdal SP, Kroenke CH, Laden F, Pukkala E, Schernhammer ES. Night work and breast cancer risk: a systematic review and meta-analysis. Eur J Cancer. 41(13):2023-32, 2005. 
  5. International Agency for Research on Cancer (IARC). IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, Volume 98:  Painting, Firefighting and shiftwork. Lyon, France. IARC, 2007.  
  6. Schernhammer ES, Berrino F, Krogh V, et al. Urinary 6-sulfatoxymelatonin levels and risk of breast cancer in postmenopausal women. J Natl Cancer Inst. 100(12):898-905, 2008. 
  7. Schernhammer ES, Hankinson SE. Urinary melatonin levels and postmenopausal breast cancer risk in the Nurses' Health Study cohort. Cancer Epidemiol Biomarkers Prev. 18(1):74-9, 2009. 
  8. Schernhammer ES, Giobbie-Hurder A, Gantman K, et al. A randomized controlled trial of oral melatonin supplementation and breast cancer biomarkers. Cancer Causes Control. 23(4):609-16, 2012. 

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