Breast cancer screening

Breast cancer screening

From Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Breast_cancer_screening (2010-12-13)

Breast cancer screening refers to testing otherwise-healthy women for breast cancer in an attempt to achieve an earlier diagnosis. The assumption is that early detection will improve outcomes. A number of screening test have been employed including: clinical and self breast exams, mammography, genetic screening, ultrasound, and magnetic resonance imaging.

A clinical or self breast exam involves feeling the breast for lumps or other abnormalities. Evidence however does not support its use.[1] Mammographic screening for breast cancer is also controversial. The Cochrane collaboration in 2009 concluded that it is unclear whether screening does more good than harm.[2] Many national organizations still however recommend it. If mammography is decided up it should only be done every two years in women between the ages of 50 and 74.[3] Several tools are available to help target breast cancer screening to older women with longer life expectancies.[4]

Abnormal findings on screening are further investigated by surgically removing a piece of the suspicious lumps (biopsy) to examine them under the microscope. Ultrasound may be used to guide the biopsy needle during the procedure. Magnetic resonance imaging is used to guide treatment, but is not an established screening method for healthy women.

Women with a family history of breast and ovarian cancer have a higher risk of mutations of the BRCA1 and BRCA2 genes. These mutations result in a higher risk of breast cancer. Testing for these genes is expensive and not done routinely. However those with this mutation should be screened more aggressively: starting at an earlier age, with greater frequency, and possibly with magnetic resonance imaging.

Contents
1 Breast exam
2 Mammography
2.1 Enhancements
2.2 Health programs
2.3 Criticisms
3 Ultrasonography
4 Breast MRI
5 BRCA testing
6 References
7 External links

Breast exam
See also: Breast self-examination

An pictorial example of breast self-examination in six steps. Steps 1-3 involve inspection of the breast with the arms hanging next to the body, behind the head and in the side. Step 4 is palpation of the breast. Step 5 is palpation of the nipple. Step 6 is palpation of the breast while lying down.Breast examination ( either clinical breast exams (CBE) by a health care provider or by self exams ) were once widely recommended. They however are not supported by evidence and may contribute to harm. Their use in women without symptoms and at low risk is thus controversial.[5]

A 2003 Cochrane review found no benefit in terms of mortality from screening by breast self-examination or by clinical exam, but rather that they do possible harm in terms of increased numbers of benign lesions identified and an increased number of biopsies performed. They conclude "screening by breast self-examination or physical examination cannot be recommended." [1]

Mammography
Main article: Mammography

Normal (left) versus cancerous (right) mammography image.Mammography is a common screening method, since it is relatively fast and widely available in developed countries. When detected by mammography breast cancers are usually smaller (in an earlier stage) than those detected by patients or doctors as a breast lump, and presumably treatment in an earlier stage will improve outcome. This assertion however has been challenged by recent reviews which have found the significance of these benefits to be questionable.

A 2009 Cochrane review estimated that mammography in women between 50 and 75 years old results in a relative risk reduction of death from breast cancer of 15% or an absolute risk reduction of 0.05%. Those who have mammograms however end up with increased surgeries, chemotherapy, radiotherapy and other potentially procedures resulting from the over-detection of harmless lumps. The authors also note that many women will experience important psychological distress for many months because of false positive findings.[2] Consequently, the value of routine mammography in women at low or average risk is controversial.[2] With unnecessary treatment of ten women for every one woman whose live was prolonged, the authors concluded that routine mammography may do more harm than good.[2].

A 2009 U.S. Preventive Services Task Force analysis come to similar conclusions: for women in their 40s 2000 would need to be screened for 10 years, resulting in 1000 people with false results and 250 unnecessary biopsies to prevent 1 breast cancer death, for women in their 50s 1339 would need to be screened for 10 years to prevent a single breast cancer related death, and for women in their 60s 377 women would need to be screened for 10 years.[6]

An analysis of Norwegians published in 2010 found a 10% reduction in breast cancer mortality (2.4 deaths per 100,000 person-years) attributable to screening but this difference was non significant.[7]

For places that recommend screening the age at which this should begin and how frequently women differ around the world. In the UK, all women are invited for screening once every three years beginning at age 50. As of 2009 the US Preventive Services Task Force recommends that women over the age of 50 receive mammography once every two years.[8]

Women at higher risk may benefit from earlier or more frequent screening. Women with one or more first-degree relatives (mother, sister, daughter) with premenopausal breast cancer often begin screening at an earlier age, perhaps at an age 10 years younger than the age when the relative was diagnosed with breast cancer.

Mammography is not generally considered as an effective screening technique for women less than 50 years old. A systematic review by the American College of Physicians concluded that, for women 40 to 49 years of age, the risks of mammography outweighed the benefits,[9] and the US Preventive Services Task Force says that the evidence in favor of routine screening of women under the age of 50 is "weak".[8] Part of the difficulty in interpreting mammograms in younger women stems from breast density. Radiographically, a dense breast has a preponderance of glandular tissue, and younger age or estrogen hormone replacement therapy contribute to mammographic breast density. After menopause, the breast glandular tissue gradually is replaced by fatty tissue, making mammographic interpretation much more accurate. Some authors speculate that part of the contribution of estrogen Hormone replacement therapy to breast cancer mortality arises from the issue of increased mammographic breast density.

Enhancements
In general, digital mammography and computer-aided mammography have increased the sensitivity of mammograms, but this may come at the cost of more numerous false positive results.[citation needed].

Computer-aided diagnosis(CAD) Systems may help radiologists to evaluate X-ray images to detect breast cancer in an early stage.[citation needed] CAD is especially established in US and the Netherlands. It is used in addition to the human evaluation of the diagnostician.

Health programs
In 2005, 67.9% of all U.S. women age 40–64 had a mammogram in the past two years (74.5% of women with private health insurance, 56.1% of women with Medicaid insurance, 38.1% of currently uninsured women, and 32.9% of women uninsured for > 12 months).[10] All U.S. states (except Utah) mandate that private health insurance plans and Medicaid provide some coverage for breast cancer screening.[11] Section 4101 of the Balanced Budget Act of 1997 required that Medicare (available to those aged 65 or older or who have been on Social Security Disability Insurance for over 2 years), effective January 1, 1998, cover and waive the Part B deductible for annual screening mammography in women aged 40 or older.

All organized breast cancer screening programs in Canada offer clinical breast examinations for women aged 40 and over and screening mammography every two years for women aged 50–69.[12] In 2003, about 61% of women aged 50–69 in Canada reported having had a mammogram within the past two years.[13]

The NHS Breast Screening Programme, the first of its kind in the world, began in 1988 and achieved national coverage in the mid-1990s, provides free breast cancer screening mammography every three years for all women in the UK aged 50 and over.[14] As of March 31, 2006, 75.9% of women aged 53–64 resident in England had been screened at least once in the previous three years.[15]

The Australian national breast screening program, BreastScreen Australia, was commenced in the early 1990s and invites women aged 50–69 to screening every 2 years. No routine clinical examination is performed, and the cost of screening is free to the point of diagnosis.

The Singapore national breast screening program, BreastScreen Singapore, is the only publicly funded national breast screening program in Asia, and enrols women aged 50–64 for screening every two years. Like the Australian system, no clinical examination is performed routinely. Unlike most national screening systems however, clients have to pay half of the cost of the screening mammogram; this is in line with the Singapore health system's core principle of co-payment for all health services.

Criticisms
Some scientific groups however have expressed concern about the public's perceptions of the benefits of breast screening.[16]

Data reported in the UK Million Woman Study indicates that if 134 mammograms are performed, 20 women will be called back for suspicious findings, and four biopsies will be necessary, to diagnose one cancer. Recall rates are higher in the U.S. than in the UK.[17] The contribution of mammography to the early diagnosis of cancer is controversial, and for those found with benign lesions, mammography can create a high psychological and financial cost. For those diagnosed with cancer, mammography can be the difference between a lumpectomy versus metastatic disease.

Screening leads to false positive results and subsequent invasive procedures.[18]

Nevertheless, surveys have shown that most women participating in mammography screening programs accept the risk of false positive recall and the majority do not find this highly distressing. The majority of women recalled will undergo additional imaging only, without any further intervention. There is some debate over how harmful such noninvasive recall assessment truly is.

A major effect of routine breast screening is to greatly increase the rate of early breast cancer detection, in particular for preinvasive ductal carcinoma in situ (DCIS), which is almost always impalpable and which cannot, for the most part, be detected reliably by any other test. While this ability to detect such very early breast malignancies is at the heart of claims that screening mammography can improve survival from breast cancer, it is also controversial. This is because a large percentage of such cases will almost certainly not progress to kill the patient, and thus mammography cannot be genuinely claimed to have been beneficial in such cases; in fact, it would lead to increased morbidity and unnecessary surgery for such patients.

It has thus been claimed that finding and treating many cases of DCIS represents "overdiagnosis" and "overtreatment". However, it is not possible to accurately predict which patients with DCIS will have an indolent nonfatal course, and which will inevitably progress to invasive cancer and death if left untreated. Consequently, all patients with DCIS are treated in much the same way, with at least wide local excision, and sometimes mastectomy if the DCIS is very extensive. The cure rate for DCIS if treated appropriately is extremely high. Thus, it can be argued that some women with DCIS detected by screening mammography do in fact benefit from screening, even if others do not. Any refinement of this therapeutic approach to breast malignancy requires further research and the development of methods that accurately predict the future cellular fate and biological behaviour of early cancers.

It is salient to note that the phenomenon of finding preinvasive malignancy or nonmalignant benign disease is commonplace in all forms of cancer screening, including pap smears for cervical cancer, fecal occult blood testing for colon cancer, and prostate-specific antigen testing for prostate cancer. All of these tests have the potential to detect very early malignancy before it becomes symptomatic, and to potentially lead to long term cure. All of them have false positives, and can lead to invasive procedures that may not benefit the patient.

Given current limitations in knowledge and technology, the only way to avoid "overdiagnosis" and "overtreatment", and to eliminate all harms inherent in breast cancer screening is not to screen at all, and thus to eliminate any benefit to at least some of the screened population that screening entails. In the case of breast cancer, cessation of screening would lead to an increase in the size and stage of breast cancers at diagnosis, since most cancers would only be detected when they become symptomatic.

Ultrasonography
Medical ultrasonography (Ultrasound) is a diagnostic aid to mammography.

Breast MRI
Magnetic resonance imaging (MRI) has been shown to detect cancers not visible on mammograms. The chief strength of breast MRI is its very high negative predictive value. A negative MRI can rule out the presence of cancer to a high degree of certainty, making it an excellent tool for screening in patients at high genetic risk or radiographically dense breasts, and for pre-treatment staging where the extent of disease is difficult to determine on mammography and ultrasound. MRI can diagnose benign proliferative change, fibroadenomas, and other common benign findings at a glance, often eliminating the need for costly and unnecessary biopsies or surgical procedures. The spatial and temporal resolution of breast MRI has increased markedly in recent years, making it possible to detect or rule out the presence of small in situ cancers, including ductal carcinoma in situ.

However, breast MRI has long been regarded to have disadvantages. For example, although it is 27–36% more sensitive, it has been claimed to be less specific than mammography.[19]. As a result, MRI studies may have more false positives (up to 30%), which may have undesirable financial and psychological costs. It is also a relatively expensive procedure, and one which requires the intravenous injection of gadolinium, which has been implicated in a rare reaction called nephrogenic systemic fibrosis. Although NSF is extremely uncommon, patients with a history of renal disease may not be able to undergo breast MRI. Further, an MRI may not be used for screening patients with a pacemaker or breast reconstruction patients with a tissue expander due to the presence of metal.

Proposed indications for using MRI for screening include:[20]

Strong family history of breast cancer
Patients with BRCA-1 or BRCA-2 oncogene mutations
Evaluation of women with breast implants
History of previous lumpectomy or breast biopsy surgeries
Axillary metastasis with an unknown primary tumor
Very dense or scarred breast tissue
In addition, breast MRI may be helpful for screening in women who have had breast augmentation procedures involving intramammary injections of various foreign substances that may mask the appearances of breast cancer on mammography and/or ultrasound. These substances include:

Silicone oil
Polyacrylamide gel
Two studies published in 2007 demonstrated the strengths of MRI-based screening:

In March 2007, an article published in the New England Journal of Medicine demonstrated that in 3.1% of patients with breast cancer, whose contralateral breast was clinically and mammographically tumor-free, MRI could detect breast cancer. Sensitivity for detection of breast cancer in this study was 91%, specificity 88%.[21]
In August 2007, an article published in The Lancet compared MRI breast cancer screening to conventional mammographic screening in 7,319 women. MRI screening was highly more sensitive (97% in the MRI group vs. 56% in the mammography group) in recognizing early high-grade Ductal Carcinoma in situ (DCIS), the most important precursor of invasive carcinoma. Despite the high sensitivity, MRI screening had a positive predictive value of 52%, which is totally accepted for cancer screening tests.[22] The author of a comment published in the same issue of The Lancet concludes that "MRI outperforms mammography in tumour detection and diagnosis."[23]
Based on this evidence, and the lack of effective alternative methods for screening in young women of very high genetic risk (either an extremely strong first degree family history or proven BRCA1 or BRCA2 oncogene mutation carrier status) for breast cancer, the Australian federal government decided to routinely reimburse annual breast MRI scans for such women under the age of 50 from January 2009 onwards.

BRCA testing
A clinical practice guideline by the US Preventive Services Task Force :[24]

"recommends against routine referral for genetic counseling or routine breast cancer susceptibility gene (BRCA) testing for women whose family history is not associated with an increased risk for deleterious mutations in breast cancer susceptibility gene 1 (BRCA1) or breast cancer susceptibility gene 2 (BRCA2)" The Task Force gave a grade D recommendation.[25][verification needed]
"recommends that women whose family history is associated with an increased risk for deleterious mutations in BRCA1 or BRCA2 genes be referred for genetic counseling and evaluation for BRCA testing." The Task Force gave a grade B recommendation.[25][verification needed]
The Task Force noted that about 2% of women have family histories that indicate increased risk as defined by:

For non–Ashkenazi Jewish women, any of the following:
"2 first-degree relatives with breast cancer, 1 of whom received the diagnosis at age 50 years or younger"
"3 or more first- or second-degree relatives with breast cancer regardless of age at diagnosis"
"both breast and ovarian cancer among first- and second- degree relatives"
"a first-degree relative with bilateral breast cancer"
"a combination of 2 or more first- or second-degree relatives with ovarian cancer regardless of age at diagnosis"
"a first- or second-degree relative with both breast and ovarian cancer at any age"
"a history of breast cancer in a male relative."
"For women of Ashkenazi Jewish heritage, an increased-risk family history includes any first-degree relative (or 2 second-degree relatives on the same side of the family) with breast or ovarian cancer."
References
Gilberto Schwartsmann (2001) "Breast Cancer in South America: Challenges to improve early detection and medical management of a public health problem." J Clin Oncol 19 118-124
1.^ a b Kösters JP, Gøtzsche PC (2003). "Regular self-examination or clinical examination for early detection of breast cancer". Cochrane Database Syst Rev (2): CD003373. doi:10.1002/14651858.CD003373. PMID 12804462.
2.^ a b c d Gøtzsche PC, Nielsen M (2009). "Screening for breast cancer with mammography". Cochrane Database Syst Rev (4): CD001877. doi:10.1002/14651858.CD001877.pub3. PMID 19821284.
3.^ "Breast Cancer: Screening". United States Preventive Services Task Force. http://www.ahrq.gov/clinic/USpstf/uspsbrca.htm.
4.^ Schonberg M. Breast cancer screening: at what age to stop? Consultant. 2010;50(May):196-205.
5.^ Saslow D, Hannan J, Osuch J, et al. (2004). "Clinical breast examination: practical recommendations for optimizing performance and reporting". CA Cancer J Clin 54 (6): 327–44. doi:10.3322/canjclin.54.6.327. PMID 15537576.
6.^ Nelson HD, Tyne K, Naik A, Bougatsos C, Chan BK, Humphrey L (November 2009). "Screening for breast cancer: an update for the U.S. Preventive Services Task Force". Ann. Intern. Med. 151 (10): 727–37, W237–42. doi:10.1059/0003-4819-151-10-200911170-00009. PMID 19920273.
7.^ Kalager M, Zelen M, Langmark F, Adami HO (September 2010). "Effect of screening mammography on breast-cancer mortality in Norway". N. Engl. J. Med. 363 (13): 1203–10. doi:10.1056/NEJMoa1000727. PMID 20860502.
8.^ a b US Preventive Services Task Force (November 2009). "Screening for breast cancer: U.S. Preventive Services Task Force recommendation statement". Ann. Intern. Med. 151 (10): 716–26, W–236. doi:10.1059/0003-4819-151-10-200911170-00008. PMID 19920272.
9.^ Armstrong K, Moye E, Williams S, Berlin JA, Reynolds EE (2007). "Screening mammography in women 40 to 49 years of age: a systematic review for the American College of Physicians". Ann. Intern. Med. 146 (7): 516–26. PMID 17404354.
10.^ Ward E, Halpern M, Schrag N, Cokkinides V, DeSantis C, Bandi P, Siegel R, Stewart A, Jemal A (Jan-February 2008). "Association of insurance with cancer care utilization and outcomes". CA Cancer J Clin 58 (1): 9. doi:10.3322/CA.2007.0011. PMID 18096863. http://caonline.amcancersoc.org/cgi/content/full/CA.2007.0011v1.
11.^ Kaiser Family Foundation (December 31, 2006). "State Mandated Benefits: Cancer Screening for Women, 2006". http://www.statehealthfactsonline.org/comparemaptable.jsp?ind=488&cat=10&yr=17&typ=5.
12.^ Canadian Cancer Society (August 10, 2007). "Breast cancer screening in your 40s". http://www.cancer.ca/ccs/internet/standard/0,3182,3172_573785695_2026817819_langId-en,00.html.
13.^ Canadian Cancer Society (April 2006). "Canadian Cancer Statistics, 2006" (PDF). http://www.cancer.ca/vgn/images/portal/cit_86751114/31/21/935505792cw_2006stats_en.pdf.pdf.
14.^ NHS Cancer Screening Programmes (2007). "NHS Breast Screening Programme". http://cancerscreening.org.uk/breastscreen/.
15.^ The Information Centre (NHS) (March 23, 2007). "Breast Screening Programme 2005/06". http://www.ic.nhs.uk/statistics-and-data-collections/screening/breast-cancer/breast-screening-programme-2005-06-%5Bns%5D?.
16.^ "Women 'misjudge screening benefits'". BBC. 15 October 2001. http://news.bbc.co.uk/1/hi/health/1601267.stm. Retrieved 2007-04-04.
17.^ Smith-Bindman R, Ballard-Barbash R, Miglioretti DL, Patnick J, Kerlikowske K (2005). "Comparing the performance of mammography screening in the USA and the UK". Journal of medical screening 12 (1): 50–4. doi:10.1258/0969141053279130. PMID 15814020.
18.^ Croswell JM, Kramer BS, Kreimer AR, et al. (2009). "Cumulative incidence of false-positive results in repeated, multimodal cancer screening". Ann Fam Med 7 (3): 212–22. doi:10.1370/afm.942. PMID 19433838.
19.^ Hrung J, Sonnad S, Schwartz J, Langlotz C (1999). "Accuracy of MR imaging in the work-up of suspicious breast lesions: a diagnostic meta-analysis.". Acad Radiol 6 (7): 387–97. doi:10.1016/S1076-6332(99)80189-5. PMID 10410164.
20.^ Morrow M (2004). "Magnetic resonance imaging in breast cancer: one step forward, two steps back?". JAMA 292 (22): 2779–80. doi:10.1001/jama.292.22.2779. PMID 15585740.
21.^ Lehman CD, Gatsonis C, Kuhl CK, Hendrick RE, Pisano ED, Hanna L, Peacock S, Smazal SF, Maki DD, Julian TB, DePeri ER, Bluemke DA, Schnall MD (2007). "MRI evaluation of the contralateral breast in women with recently diagnosed breast cancer.". N Engl J Med. 356 (13): 1295–1303. doi:10.1056/NEJMoa065447. PMID 17392300.
22.^ Kuhl CK, Schrading S, Bieling HB, Wardelmann E, Leutner CC, Koenig R, Kuhn W, Schild HH (2007). "MRI for diagnosis of pure ductal carcinoma in situ: a prospective observational study". The Lancet 370 (9586): 485–492. doi:10.1016/S0140-6736(07)61232-X.
23.^ Boetes C, Mann RM (2007). "Ductal carcinoma in situ and breast MRI". The Lancet 370 (9586): 459–460. doi:10.1016/S0140-6736(07)61207-0.
24.^ U.S. Preventive Services Task Force (2002). "Screening for breast cancer: recommendations and rationale". Ann. Intern. Med. 137 (5 Part 1): 344–6. PMID 12204019. http://www.annals.org/cgi/content/full/137/5_Part_1/344.
25.^ a b "Guide to Clinical Preventive Services, Third Edition: Periodic Updates, 2000-2003". Agency for Healthcare Research and Quality. US Preventive Services Task Force. http://www.ahrq.gov/clinic/3rduspstf/ratings.htm. Retrieved 2007-10-07.

Prospective vs. retrospective studies

Prospective vs. retrospective studies

 

Prospective

A prospective study watches for outcomes, such as the development of a disease, during the study period and relates this to other factors such as suspected risk or protection factor(s). The study usually involves taking a cohort of subjects and watching them over a long period. The outcome of interest should be common; otherwise, the number of outcomes observed will be too small to be statistically meaningful (indistinguishable from those that may have arisen by chance). All efforts should be made to avoid sources of bias such as the loss of individuals to follow up during the study. Prospective studies usually have fewer potential sources of bias and confounding than retrospective studies.

 

Retrospective

A retrospective study looks backwards and examines exposures to suspected risk or protection factors in relation to an outcome that is established at the start of the study. Many valuable case-control studies, such as Lane and Claypon's 1926 investigation of risk factors for breast cancer, were retrospective investigations. Most sources of error due to confounding and bias are more common in retrospective studies than in prospective studies. For this reason, retrospective investigations are often criticised. If the outcome of interest is uncommon, however, the size of prospective investigation required to estimate relative risk is often too large to be feasible. In retrospective studies the odds ratio provides an estimate of relative risk. You should take special care to avoid sources of bias and confounding in retrospective studies.

 

Prospective investigation is required to make precise estimates of either the incidence of an outcome or the relative risk of an outcome based on exposure.

 

Case-Control studies

Case-Control studies are usually but not exclusively retrospective, the opposite is true for cohort studies. The following notes relate case-control to cohort studies:

• outcome is measured before exposure

• controls are selected on the basis of not having the outcome

• good for rare outcomes

• relatively inexpensive

• smaller numbers required

• quicker to complete

• prone to selection bias

• prone to recall/retrospective bias

• related methods are risk (retrospective), chi-square 2 by 2 test, Fisher's exact test, exact confidence interval for odds ratio, odds ratio meta-analysis and conditional logistic regression.

 

Cohort studies

Cohort studies are usually but not exclusively prospective, the opposite is true for case-control studies. The following notes relate cohort to case-control studies:

• outcome is measured after exposure

• yields true incidence rates and relative risks

• may uncover unanticipated associations with outcome

• best for common outcomes

• expensive

• requires large numbers

• takes a long time to complete

• prone to attrition bias (compensate by using person-time methods)

• prone to the bias of change in methods over time

• related methods are risk (prospective), relative risk meta-analysis, risk difference meta-analysis and proportions

from:http://www.statsdirect.com/help/basics/prospective.htm

Prospective cohort study

From Wikipedia, the free encyclopedia

  (Redirected from Prospective study)

A prospective cohort study is a research effort that follows over time groups of individuals who are similar in some respects (e.g., all are working adults) but differ on certain other characteristics (e.g., some smoke and others do not) and compares them for a particular outcome (e.g., lung cancer).^[1] It should be emphasized that prospective studies begin with a sample whose members are free of the disease or disorder under study (e.g., free of lung cancer or free of major depression). Given the individual differences that exist in a sample (e.g, some people smoke, others do not), all the individuals in the sample are followed over time. The incidence rates for the disease under study are ascertained in key subgroups. For example, in a sample that was free of lung cancer at the outset of the study, after 20 years, one may anticipate that the nonsmokers have the lowest 20-year incidence rate of the disease, moderate smokers, the next highest rate, and the heavy smokers, the highest rate. The prospective study is important for research on the etiology of diseases and disorders in humans because for ethical reasons people cannot be deliberately exposed to suspected risk factors in controlled experiments.

It can be more expensive than a case–control study.^[2]

References

1. ^ "Definition of prospective cohort study - NCI Dictionary of Cancer Terms".
2. ^ Manolio TA, Bailey-Wilson JE, Collins FS (October 2006). "Genes, environment and the value of prospective cohort studies". Nat. Rev. Genet. 7 (10): 812–20. doi:10.1038/nrg1919. PMID 16983377.

This article includes text from the U.S. National Cancer Institute's public domain Dictionary of Cancer Terms

Cohort Study, also known as Panel Study, is a form of longitudinal study

Cohort study

From Wikipedia, the free encyclopedia

A cohort study or panel study is a form of longitudinal study used in medicine and social science. It is one type of study design and should be compared with a cross-sectional study.

A cohort is a group of people who share a common characteristic or experience within a defined period (e.g., are born, leave school, lose their job, are exposed to a drug or a vaccine, etc.). Thus a group of people who were born on a day or in a particular period, say 1948, form a birth cohort. The comparison group may be the general population from which the cohort is drawn, or it may be another cohort of persons thought to have had little or no exposure to the substance under investigation, but otherwise similar. Alternatively, subgroups within the cohort may be compared with each other.

Contents  [hide] 1 Application 2 Examples 3 Variations 3.1 Retrospective cohort 3.2 Nested case-control study 3.3 Household panel survey 4 See also 5 References 6 External links

Application

In medicine, a cohort study is often undertaken to obtain evidence to try to refute the existence of a suspected association between cause and disease; failure to refute a hypothesis strengthens confidence in it. Crucially, the cohort is identified before the appearance of the disease under investigation. The study groups, so defined, are observed over a period of time to determine the frequency of new incidence of the studied disease among them. The cohort cannot therefore be defined as a group of people who already have the disease. Distinguishing causality from mere correlation cannot usually be done with results of a cohort study alone.

The advantage of cohort study data is the longitudinal observation of the individual through time, and the collection of data at regular intervals, so recall error is reduced. However, cohort studies are expensive to conduct, are sensitive to attrition and take a long time to generate useful data.

Some cohort studies track groups of children from their birth, and record a wide range of information (exposures) about them. The value of a cohort study depends on the researchers' capacity to stay in touch with all members of the cohort. Some of these studies have continued for decades.

Examples

An example of an epidemiologic question that can be answered by the use of a cohort study is: does exposure to X (say, smoking) correlate with outcome Y (say, lung cancer)? Such a study would recruit a group of smokers and a group of non-smokers (the unexposed group) and follow them for a set period of time and note differences in the incidence of lung cancer between the groups at the end of this time. The groups are matched in terms of many other variables such as economic status and other health status so that the variable being assesed, the independent variable (in this case, smoking) can be isolated as the cause of the dependent variable (in this case, lung cancer).

In this example, a statistically significant increase in the incidence of lung cancer in the smoking group as compared to the non-smoking group is evidence in favor of the hypothesis. However, rare outcomes, such as lung cancer, are generally not studied with the use of a cohort study, but are rather studied with the use of a case-control study.

Shorter term studies are commonly used in medical research as a form of clinical trial, or means to test a particular hypothesis of clinical importance. Such studies typically follow two groups of patients for a period of time and compare an endpoint or outcome measure between the two groups.

Randomized controlled trials, or RCTs are a superior methodology in the hierarchy of evidence, because they limit the potential for bias by randomly assigning one patient pool to an intervention and another patient pool to non-intervention (or placebo). This minimises the chance that the incidence of confounding variables will differ between the two groups.

Nevertheless, it is sometimes not practical or ethical to perform RCTs to answer a clinical question. To take our example, if we already had reasonable evidence that smoking causes lung cancer then persuading a pool of non-smokers to take up smoking in order to test this hypothesis would generally be considered quite unethical.

An example of a cohort study that has been going on for more than 50 years is the Framingham Heart Study.

The largest cohort study in women is the Nurses' Health Study. Started in 1976, it is tracking over 120,000 nurses and has been analyzed for many different conditions and outcomes.

Variations

Retrospective cohort

A "prospective cohort" defines the groups before the study is done, while a "retrospective cohort" does the grouping after the data is collected. Whereas prospective cohorts should be summarized with the relative risk, retrospective cohorts should be summarized with the odds ratio. An example of a retrospective cohort is Long-Term Mortality after Gastric Bypass Surgery.^[1]

Nested case-control study

Main article: Nested case-control study

An example of a nested case-control study is Inflammatory markers and the risk of coronary heart disease in men and women, which was a case control analyses extracted from theFramingham Heart Study cohort.^[2]

Household panel survey

Household panel surveys are an important sub-type of cohort study. These draw representative samples of households and survey them, following all individuals through time on a usually annual basis. Examples include the US Panel Study on Income Dynamics (since 1968), the German Socio-Economic Panel (since 1984), the British Household Panel Survey (since 1991), the Household, Income and Labour Dynamics in Australia Survey (since 2001) and the European Community Household Panel (1994-2001).

See also

• Panel analysis

References

1. ^ Adams TD, Gress RE, Smith SC, et al (2007). "Long-term mortality after gastric bypass surgery". N. Engl. J. Med. 357 (8): 753–61. doi:10.1056/NEJMoa066603. PMID 17715409.
2. ^ Pai JK, Pischon T, Ma J, et al (2004). "Inflammatory markers and the risk of coronary heart disease in men and women". N. Engl. J. Med. 351 (25): 2599–610. doi:10.1056/NEJMoa040967. PMID 15602020.

External links

• Prospective cohorts
• Retrospective cohorts
• Born in Bradford - an example of a 2006 UK-based birth cohort study

Cross-sectional study - the simplest variety of descriptive epidemiology

A cross-sectional study is the simplest variety of descriptive or observational epidemiology that can be conducted on representative samples of a population. Simply put, it is a study that aims to describe the relationship between diseases (or other health-related states) and other factors of interest as they exist in a specified population at a particular time, without regard for what may have preceded or precipitated the health status found at the time of the study. For instance, a single cross-sectional study may include questions about smoking behavior, occupational exposure to dusts and fumes, respiratory symptoms (cough, breathlessness), and physical examinations of physical fitness—including simple tests of lung function. Such a study would throw some light on the relationship of both occupational exposures and smoking behavior to respiratory symptoms and respiratory function. However, it is impossible either to establish causal relationships or to get reliable perspectives on the natural history of respiratory disease from such a study.

Cross-sectional studies must be done on representative samples of the population if generalizations from the findings are to have any validity. These studies gather information about the prevalence of health-related states and conditions, but they cannot distinguish between newly occurring and long-established conditions. All they can do is measure the frequency (prevalence) of conditions and demonstrate associations. They cannot identify cause-and-effect relationships, though they do identify the existence of health problems.

Cross-sectional studies, also known as surveys, are a useful way to gather information on important health-related aspects of people's knowledge, attitudes, and practices (such studies are known as "KAP" surveys). In the area of reproductive health, such a survey might include questions such as: How much do girls and women in their reproductive years know about pregnancy and control over their own fertility? What are their beliefs, values, and attitudes towards making decisions about getting pregnant and about desired family size? How do they control their own fertility? KAP surveys are a good example of a tried and tested form of cross-sectional study. Many have been conducted serially to measure the efficacy of family-planning programs, anti-smoking measures, and other public health and health-promotion interventions. The distinction between a cohort study and a repeated cross-sectional study is that a cohort study is conducted with the same individuals who participate over a long period; repeated or serial cross-sectional studies, on the other hand, do not necessarily (or even usually) study the same individuals repeatedly.

The U.S. National Health Surveys are a form of cross-sectional study. Like many other cross-sectional studies, they can identify health problems and provide data from which many useful inferences can be made and hypotheses generated.

Cross-sectional studies are often used as a basis for health-policy decisions, and it is important to ensure that only current, rather than obsolete, information is used for this purpose. When health department resources are limited, information gathered in a cross-sectional study in the past can usually be refreshed with up-to-date facts from a small sub-sample; it would be necessary to repeat a large cross-sectional study only if the findings from a current sub-sample are seriously discrepant from earlier results.

JOHN M. LAST

(SEE ALSO: Case-Control Study; Cohort Study; Epidemiology; Survey Research Methods;Surveys)

BIBLIOGRAPHY

Abramson, J. H., and Abramson, Z. H. (2000). Survey Methods in Community Medicine, 5th edition. Edinburgh & London: Livingstone.

Kelsey, J. E.; Whittemore, A. S.; Evans, A. S.; and Thompson, D. (1996). Methods in Observational Epidemiology, 2nd edition. New York: Oxford University Press.

from: http://www.enotes.com/public-health-encyclopedia/cross-sectional-study