cancer screening

Cancer Screening

Screening is generally not diagnostic. It is the application of a test to an asymptomatic population to determine who is likely to have the disease and who is not likely to have the disease.

Screening is therefore a two phase process since those who have a positive screening test must undergo a diagnostic procedure to determine if they actually have the disease.

Cancer Screening

Synonymous with secondary prevention.

Earlier therapeutic intervention is possible through screening an asymptomatic population to identify cancer at an earlier stage than it would have been diagnosed in the absence of screening.

Expectation is that early diagnosis and treatment lead to a reduction in mortality from the disease and/or a reduction in the severity of the disease.

Distinguished from case detection or case finding, which occurs when the patient presents to a physician with symptomatic disease.

Cancer Screening

There are circumstances in which screening can contribute to primary prevention, for example, when colorectal cancer screening leads to the identification and removal of an adenoma, which subsequently reduces the incidence of colorectal cancer

History of Cancer Screening

Screening for cancer began with the Pap smear, a test developed by George Papanicolaou who in 1941 published his seminal paper “The diagnostic value of vaginal smears in carcinoma of the uterus” in the American Journal of �Obstetrics and Gynecology.

He initially presented his research at the Third Race Betterment Conference in Battle Creek, Michigan, in 1928, but his colleagues were sufficiently discouraging of his ideas about detecting cancer through exfoliative cytology that Papanicolaou abandoned the work for many years.

He returned to it more than a decade later and eventually established the correlation between cells scraped from the surface of the cervix and the detection of cervical carcinoma, which he published in 1941. In 1942, he published “Diagnosis of uterine cancer by the vaginal smear”

History of Cancer Screening

Cervical cytology followed by biopsy of a positive test is still the principal method for cervical cancer screening in the world.

Principles of Cancer Screening

Although most cancers have a better prognosis if diagnosed earlier in their natural history, this basic observation is not in itself sufficient to justify screening an asymptomatic population for cancer.

A number of criteria should be met before initiating cancer screening, these having been first outlined by Wilson and Junger in 1968 for the World Health Organization.


The disease should be an important public health problem in terms of its frequency and/or severity.

Today some of the cancer sites considered for screening are not particularly common diseases, but, nevertheless, early detection and subsequent reduction of mortality can result in a significant benefit in life-years saved.


The natural history of the disease presents a window of opportunity for early detection.

For cancer this generally refers to a detectable preclinical phase (DPCP), and it represents the interface between characteristics of the disease and the screening technology. It is during this period that screening is considered optimal to detect the disease early and prior to the development of symptoms.

For screening to be effective, the recommended screening interval must be shorter than the estimate of the DPCP.


An effective treatment should be available that favorably alters the natural history of the disease. Usually for cancer this means a reduction in cause-specific mortality.

The treatment should be more effective if initiated during the presymptomatic (or earlier) stage than during the symptomatic (or later) stage; that is, if treating early (presymptomatic) has no advantage over treating late (symptomatic) then the cost and the risk of screening cannot be justified.

A suitable screening test should be available, that is, one that is accurate, acceptable to the population, fairly easy to administer, safe, and relatively inexpensive.

Basis of screening guidelines

The screening guidelines should be based on good scientific evidence (usually based on results of a RCT) and economically feasible.

Screening program should have the following characteristics:

Should high rates of participation from the eligible population. Should take into account specific resources available for

screening, diagnosis, and treatment so that countries can focus on optimal recommendations based on available resources.

Should be sensitive to patient and provider concerns. Should ensure prompt follow-up of positive tests with a diagnostic

examination and prompt treatment of cases. Screening programs should be cost-effective. Screening programs should be monitored and regularly evaluated.

Screening Test Validity

The accuracy or validity of a screening test, that is, its ability to distinguish between diseased and non-diseased people, is measured by sensitivity and specificity

Sensitivity Refers to the ability of the screening test to correctly identify

people with the disease among the screened population. Defined as the number of people screened with a positive test

divided by those who actually have the disease. Specificity

Refers to the ability of the test to correctly identify people without the disease among the screened population

Defined as the number of people with a negative test divided by the number of people who do not have the disease.


Ideally, sensitivity and specificity would be 100% accurate, but unfortunately there is no cancer screening test that performs this well. Some people with a positive screening test will eventually be

found not to have the disease following the diagnostic work-up (false positives)

Some with the disease will be labeled negative by the screening test and thus are missed cases or false negatives.

Sensitivity and specificity are inversely related, ultimately increasing one results in a decrease in the other after a certain threshold of accuracy is reached.


Balancing Sensitivity and Specificity:

For a quantitative test (e.g., a quantitative immunochemical test for colorectal cancer), the cutoff level to designate a test positive can be adjusted to the extent that all potential cases can be identified (sensitivity equals 100%). However, to do so would sacrifice specificity, resulting in a large number of individuals who would be unnecessarily subjected to a costly and sometimes risky diagnostic procedure.

Thus, balancing sensitivity and specificity (when possible) is important in determining the outcome of a screening program.

Test Validity

Diseased Not Diseased TotalPositivescreening test

True positives(a)

False positives(b)

Test positives(a + b)

Negativescreening test

False negatives(c)

True negatives(d)

Test negatives(c + d)

Total Total withdisease (a + c)

Total withoutdisease (b + d)

Total screened(a + b + c + d)

Sensitivity = a/(a + c)Specificity = d/(b + d)Positive predictive value = a/(a + b)Negative predictive value = d/(c + d)

Positive predictive value

Positive predictive value (PPV): Which is the proportion of individuals with a positive screening

test who actually have the disease. The PPV can only be computed after the diagnostic

examinations of those who test positive had been completed. A PPV of 10% means that only one in ten of the test positives

truly had the disease. The PPV is influenced by three factors: sensitivity and

specificity of the test and the prevalence of disease. Specificity has a bigger effect on PPV since most people who are screened for cancer do not have the disease. If the specificity is increased to improve PPV, the sensitivity will likely decrease (since sensitivity and specificity are inversely related) and the number of false negatives will increase.

Biases

Lead time bias: Diagnosing the disease earlier however having the same mortality as without screening

Length time bias: Slow growing tumors have the better prognoses than fast growing tumors, and screening tests more likely to detect these tumors that are more treatable anyway

Selection bias: If patients with higher risk of disease are more likely to be screened, screening test results will look worse than they are

Over-diagnosis bias: Test may diagnose abnormalities that would never cause a problem in a person’s lifetime (i.e. prostate cancer)

Avoid bias by using Randomized Control Trials (RCTs)

Breast Cancer

Breast Cancer poses a serious health risk for women throughout the world.

More than one million women world wide are diagnosed with breast cancer every year.

It is estimated that 1 in 9 Pakistani women will develop breast cancer at some stage of their life.

In developed countries there are national cancer registries where every newly diagnosed cancer patient is registered. This is very helpful to understand the epidemiology and causative factors of cancer.

In Pakistan we do not have such a registry at national level. Several studies and reports suggest that among Asian population, Pakistani women have the highest risk of breast cancer (after non-Arab Israeli women).

Breast Cancer

With the improving knowledge of molecular pathophysiology of breast cancer, therapies are coming onto the market, which have given a new lease of life to these patients.

Since the early 1990’s death rates from breast cancer have decreased by approximately 25% in the USA and Europe.

This has been possible mostly due to screening mammography and continuously improving treatment strategies utilizing chemotherapy, hormone therapy and more recently targeted therapy.

Screening recommendation for Breast Cancer

Yearly mammograms are recommended starting at age 40 and continuing for as long as a woman is in good health

Clinical breast exam (CBE) about every 3 years for women in their 20s and 30s and every year for women 40 and over

Women should know how their breasts normally look and feel and report any breast change promptly to their health care provider. Breast self-exam (BSE) is an option for women starting in their 20s.

The American Cancer Society recommends that some women -- because of their family history, a genetic tendency, or certain other factors -- be screened with MRI in addition to mammograms. (The number of women who fall into this category is small: less than 2% of all the women

Breast Self Examination

Screening recommendation for Colorectal Cancer

Beginning at age 50, both men and women should follow one of these testing schedules: Tests that find polyps and cancer

Flexible sigmoidoscopy every 5 years*, or Colonoscopy every 10 years, or Double-contrast barium enema every 5 years*, or CT colonography (virtual colonoscopy) every 5 years*

Tests that primarily find cancer Yearly fecal occult blood test (gFOBT)**, or Yearly fecal immunochemical test (FIT) every year**, or Stool DNA test (sDNA), interval uncertain**

* If the test is positive, a colonoscopy should be done.** The multiple stool take-home test should be used. One test done by the

doctor in the office is not adequate for testing. A colonoscopy should be done if the test is positive. The tests that are designed to find both early cancer and polyps are preferred

if these tests are available and the individual is willing to have one of these more invasive tests.

Screening recommendation for Cervical Cancer

All women should begin cervical cancer screening about 3 years after they begin having vaginal intercourse, but no later than 21 years old. Screening should be done every year with the regular Pap test or every 2 years using the newer liquid-based Pap test.

Beginning at age 30, women who have had 3 normal Pap test results in a row may get screened every 2 to 3 years. Women older than 30 may also get screened every 3 years with either the conventional or liquid-based Pap test, plus the human papilloma virus (HPV) test.

Screening recommendation for Cervical Cancer

Women 70 years of age or older who have had 3 or more normal Pap tests in a row and no abnormal Pap test results in the last 10 years may choose to stop having Pap tests.

Women who have had a total hysterectomy (removal of the uterus and cervix) may also choose to stop having Pap tests, unless the surgery was done as a treatment for cervical cancer or pre-cancer. Women who have had a hysterectomy without removal of the cervix should continue to have Pap tests.

Screening recommendation for Prostate Cancer

The American Cancer Society recommends that men make an informed decision with their doctor about whether to be tested for prostate cancer.

Research has not yet proven that the potential benefits of

testing outweigh the harms of testing and treatment.

Starting at age 50, after the pros and cons of testing have been explained, individual can decide if testing is the right choice.

If the individual decides to be tested, he should have the PSA blood test with or without a rectal exam.

Take control of your health and reduce your cancer risk

Stay away from tobacco. Stay at a healthy weight. Get moving with regular physical activity. Eat healthy with plenty of fruits and vegetables. Limit how much alcohol you drink (if you drink at all). Protect your skin. Know yourself, your family history, and your risks. Have regular check-ups and cancer screening tests.

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