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Routine breast cancer screening leads to overdiagnosis


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Routine breast cancer screening for women of average risk has been controversial for many years because some believe that the benefits do not outweigh the risks. Recent headlines covering a study in Denmark suggests that routine breast cancer screening leads to “overdiagnosis” of breast cancer. (4/4/17)


STUDY AT A GLANCE

This study is about:

The potential for breast cancer overdiagnosis and the types of breast tumors when routine screening for women of average breast cancer risk was implemented in Denmark.  

Why is this study important?

A goal of breast cancer screening programs is to reduce the number of advanced cancers, hopefully providing less invasive treatment for patients, decreasing disease burden (including financial cost and general quality of life), and resulting in fewer deaths from the disease. However, screening programs may be leading to “overdiagnosis,” defined as identifying tumors that would not have caused symptoms or spread had they not been detected. The risk associated with overdiagnosis is that women may undergo treatment that they may not need and would not have if the screening program had not uncovered the tumor.  As there are physical, emotional, and financial effects of treatment, the risk of overdiagnosis and overtreatment is considered when weighing the risks and benefits of breast screening programs.

Study findings: 

  1. More nonadvanced tumors were identified when routine screening programs were introduced in Denmark.
  2. The number of “overdiagnosed" tumors (including both invasive tumors and DCIS) were calculated as the difference between the number of tumors before and after screening programs were implemented.  The rate of overdiagnosis ranged from about 16% to about 48%, depending on the specifics of women included in the estimate (age range, where they lived, etc.) and the type of tumor (invasive cancer or DCIS).

What does this mean for me?

This study suggests that more breast cancer patients were overdiagnosed after routine screening was implemented in Denmark. However, the results from this study are not conclusive and more work needs to be done. The study authors considered a tumor to be overdiagnosed if it was a nonadvanced cancer (a tumor of 20 mm or smaller), with the assumption that it would not progress further. Although they associated advanced tumors with mortality, they did not have data to back up this assumption.

It is important to remember that this study looked at routine screening, and did not include screening that was triggered by a lump or other breast cancer symptom.  Also critical to remember is that this study included women with varying risk for breast cancer; it did not focus only on screening of women at increased risk of breast cancer due to family history and/or mutations in BRCA or other genes associated with increased cancer risk. Patients and their health care providers should work together to determine a patient’s optimal breast cancer screening interval based on her personal breast cancer risk factors, including family and personal history of cancer and/or genetic mutation associated with increased cancer risk.

Questions to ask your health care provider:

  • What is my breast cancer risk?
  • What risk factors for breast cancer do I have?
  • When should I start breast cancer screening?
  • How can I lower my breast cancer risk?
  • I have a family history of breast cancer; should I start screening mammograms before age 50?
  • I have a mutation in a gene associated with increased breast cancer risk; what other breast screening should I consider?
  • Are there tests available to help determine the best treatment options for my early stage cancer? 

IN DEPTH REVIEW OF RESEARCH

Study background:

Breast cancer screening is a controversial topic—health care providers and researchers are not sure when women at average risk should start screening or how often they should be screened. One reason for the controversy is that health care providers and researchers are still trying to determine the benefit of screening: does it prevent more advanced-stage breast tumors, and will doing that lead to fewer deaths due to breast cancer? Or does screening catch smaller, nonadvanced tumors that may never lead to advanced disease or death?

Karsten Jørgensen and colleagues from The Nordic Cochrane Centre and other institutions published work in the Annals of Internal Medicine in March 2017, using data collected through two Danish cancer registries to determine whether overdiagnosis of breast tumors was occurring.  

Researchers of this study wanted to know:

What types of breast tumors are found through routine breast cancer screening and what rate of overdiagnosed breast cancer occurred after screening was implemented in Denmark?     

Population(s) looked at in the study:

The researchers collected data from women in two registries: the Danish Breast Cancer Group (DBCG) and the Danish Cancer Registry (DCR). These women were between the ages of 35 and 84 and had tumors that were either nonadvanced (20 mm or smaller) or advanced (greater than 20 mm).  Diagnoses of both invasive cancer and ductal carcinoma in situ (DCIS) were considered in this study. Women in the study received screening every other year.

Study findings: 

  1. When routine screening programs were introduced in Denmark, more nonadvanced tumors were identified.
  2. The number of overdiagnosed tumors (including both invasive tumors and DCIS) were calculated as the difference between the number of tumors before and after screening programs were implemented in Denmark.  The rate of overdiagnosis ranged from about 16% to about 48%, depending on the specifics of the women included in the estimate (age range, where they lived, etc.) and the type of tumor (invasive cancer or DCIS).

Limitations:

This research study looked at the number of nonadvanced and advanced breast tumors, but it did not include data on breast cancer mortality. This means that the study authors can claim incidence of either type of tumors increasing or decreasing, but it cannot connect that data to mortality, because they do not know how many of the nonadvanced tumors would have advanced and caused death. Additionally, this study was done in Denmark, and may not directly translate to the United States. Also, this was a retrospective study; because the authors did not collect their own data, some of it  may have been incomplete, possibly excluding or including factors that may have affected the findings and control for them. Finally, this study included women with or without a family/personal history of breast cancer, and women with or without genetic mutations that put them at higher risk for developing breast cancer. Because of this, it is not known whether the study findings would have been different if the study included only women at higher risk of breast cancer.

Also of importance is the fact that the published study results rely on the authors’ definition of “overdiagnosis” and does not consider the benefits to women whose cancers were caught by routine screening before they had symptoms. This study also excluded women whose tumors were tested with prognostic tests that are now widely used to help guide treatment decisions.                 

Conclusions:

This study suggests that overdiagnosis occurs after breast cancer screening. However, more work needs to be done to determine which women benefit or don’t benefit from screening, and when and how often to screen. In an accompanying editorial, Dr. Otis Brawley of the American Cancer Society reviewed other data showing overdiagnosis of breast cancer, and argues that “a significant minority” of breast cancers detected through routine screening do not threaten the health or life of the women.  Identifying which of these tumors do not require aggressive treatment is an area of active research, and as Dr. Brawley writes, “…we must carefully examine screening, realize its limitations, maximize its effectiveness, and try to improve it.”

Not all women have the same risk for breast cancer, and a “one-size-fits-all” screening guideline for women does not exist. Current guidelines already call for distinct screening for women who are known to be at high risk for breast cancer due to mutations in BRCA or other genes associated with increased cancer risk, a strong family history of breast cancer, and/or history of radiation treatment to the chest.  In some cases, guidelines recommend combining magnetic resonance imaging (MRI) with mammography.

Current breast cancer screening guidelines for women at average risk of breast cancer are controversial and differ among reputable organizations. The United States Preventative Services Task Force (USPSTF) recommends a screening mammogram every other year for women ages 50-74. Guidelines of numerous other organizations, including the American Cancer Society (ACS), the National Comprehensive Cancer Network (NCCN), the American Medical Association (AMA), the American College of Radiology (ACR), and the American Congress of Obstetricians and Gynecologists (ACOG) recommend annual screening mammograms beginning at younger ages—the ACS recommends age 45, while the other organizations recommend age 40. Recently, the NCCN added that women and their doctors consider using 3D mammography (tomosynthesis), which is not addressed in this study. Patients should work with their health care providers to determine when and how often they are screened, and to be sure to discuss all of their concerns.

Posted 4/4/17

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References:

Brawley OW. “Accepting the existence of breast cancer overdiagnosis.” Annals of Internal Medicine. Published online first on January 10, 2017. 

Jørgensen KJ, Gøtzsche PC, Kalager M, et al. “Breast cancer screening in Denmark: A cohort study of tumor size and overdiagnosis.” Annals of Internal Medicine. 2017; 166(5): 313-324.  

Siu, AL and the U.S. Preventive Services Task Force. “Screening for Breast Cancer: U.S. Preventive Services Task Force Recommendation Statement.” Annals of Internal Medicine. 2016; 164(4): 279-296.  

American Cancer Society. American Cancer Society recommendations for early breast cancer detection in women without breast symptoms. 2015. 

American College of Radiology. “ACR and SBI Continue to Recommend Regular Mammography Starting at Age 40.” 2015.

The American Congress of Obstetricians and Gynecologists. “ACOG Statement on Breast Cancer Screening Guidelines.” 2016. 

National Comprehensive Cancer Network. “NCCN Framework for Resource Stratification of NCCN Guidelines (NCCN Framework™).” 2016.  

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