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OVERVIEW

OVERVIEW OF CANCER SCREENING

WHAT CONSTITUTES CANCER SCREENING?
  • Methods employed to detect cancer or abnormal cells that may turn cancerous when people do not have symptoms are considered as screening methods.

It is easier to treat an early cancer (WITHOUT SYMPTOMS) when it has not spread than when it has become symptomatic (MAY HAVE SPREAD).
Screening tests help in detecting cancer early and help in reducing the probability of death due to cancer.
Doctor advising screening test does not necessarily mean that the person has cancer but is an attempt to pick one before it has symptoms.
Includes the following:

  • Physical Examination & History – Patient habits, symptoms, past illnesses and treatments along with complete examination to look for any abnormalities.
  • Laboratory Investigations – Samples of blood, urine, tissue and appropriate substances are advised.
  • Radiological Tests –To visualize the disease if present within the body.
  • Gene Analysis – To identify certain pre existing genetic/chromosomal abnormalities.

DRAWBACKS OF CANCER SCREENING:

  • FALSE POSITIVE RESULTS – Test indicates cancer is present when it may not be present. They cause anxiety and are usually followed by additional tests and procedures with their associated potential harms.
  • FALSE NEGATIVE RESULTS – Test indicates cancer is not present when It is present. Unfortunately gives false assurance and delays the diagnosis and a prompt treatment when it could have been cured.
  • OVER DIAGNOSIS – Screening test picks up a slow growing harmless cancer; the treatment of such tumors does not alter the lifespan of the individual even with the resultant additional treatment of the cancer diagnosed.
ROLE OF INFORMED AND SHARED DECISION MAKING?

It is essential to understand the potential benefits and associated harm associated with screening tests and thereafter make an informed decision regarding the necessity of screening test for you.
The process of discussing the benefits and risks of undergoing screening tests with your doctor/health care provider and arriving at the best possible screening options constitutes INFORMED AND SHARED DECISION MAKING.
The possibility of false test results, over diagnosis and over treatment against the advantage of diagnosing the cancer early will be explained to you by the health care provider in the form of booklet, video or website.
Once the decision is made then the health care provider will document your decision, is written in your medical record and order the screening test.

GOALS OF SCREENING
  • To identify the cancer before the symptoms appear.
  • Screen for early cancer so that it is easier to treat and cure.
  • To have minimal false positive/false negative results.
  • To reduce the risk of dying from cancer.

Screening tests are not meant to diagnose cancer.

If screening test result shows abnormal finding then further investigations (diagnostic) are needed to confirm the diagnosis of cancer.

WHO SHOULD BE SCREENED?

INDIVIDUALS AT HIGH RISK FOR CANCER –  Any condition that increases the probability of cancer is labeled as risk factor. Presence of risk factor does not necessary mean that the individual will develop cancer and the absence of risk factor does not mean you might not get it.

Persons with high need to be screened from a younger age and more frequently than non risk individuals.

Some screening tests are for high risk individuals like:

  • History of cancer present – screening for recurrence.
  • Family history of cancer.
  • Genetic mutations in the family.
  • Exposure to cancer causing substances like tobacco & workplace exposure to harmful substances.
  • Older age.

CANCER SCREENING RESEARCH –  Scientists evaluating the probable causes of cancer and the type of cancer screening and the frequency of testing.

SEER (Surveillance, Epidemiology, and End Results) program of national cancer institute has been collecting data on cancer and is used to study potential candidates at risk of developing cancer

MEASUREMENT OF CANCER RISK

The various methods by which risk can be calculated are

ABSOLUTE RISK : Researchers calculate the number of people in a certain population who can develop a disease over a stipulated period of time.

RELATIVE RISK : It is used to assess if a certain trait or factor can actually be linked to increased risk of cancer.

It is calculated as percentage of people who develop the disease amongst exposed  divided by percentage of people  who develop the disease amongst the unexposed.

It can be interpreted as:

  • Larger than 1 – trait is associated with increase in risk.
  • Equal to 1 – trait is not associated with risk.
  • Less than 1 – trait is associated with decrease in risk.

ODDS RATIO

It gives an estimate of relative risk.

It is usually used in case control study .

Odds  is determined as  the frequency of trait occurrence divided by the frequency of trait non occurrence.

Odds ratio is obtained by dividing the odds of one group divided by the other group.

ODDSS RATIO SCENARIOS can be like

  • Larger than 1 – trait is associated with increase in risk.
  • Equal to 1 – trait is not associated with risk.
  • Less than 1 – trait is associated with decrease in risk.

Knowledge of increased risk can help the treating physician to determine the screening modality.

WILL SCREENING ADD TO PEOPLES LIFE SPAN?

KEY POINTS

  • Diagnosing at early stage helps in decreasing the chance of dying from these cancers.
  • Screening tests to determine whether deaths from cancer can be reduced on screening .

EARLY DIAGNOSIS TO REDUCE THE CHANCE OF DEATH DUE TO CANCER

Cancers that are diagnosed early stage have a higher chance of getting cured.

Chance of recovery and cure are dependent on the stage of disease in cancer patients.

Certain screening tests have been shown to be useful in detecting cancers early and also reducing the chance of dying from these cancers.

Mammograms for breast cancer and colonoscopy and fecal occult blood testing for colorectal cancer are examples for the above scenarios.

DETERMINATION IF DEATHS CAN BE REDUCED ON SCREENING

Studies define survival as percentage of alive individuals  5 years after diagnosis ; often used to determine how well cancer treatments work.

Data suggesting that cancer screening tests work shall include

  • Increased Early stage cancer detection.
  • Decreased late cancer detection.
  • Decreased number of cancer deaths.

The number of deaths from cancer is lower today than ever before, probably either because of earlier detection or onset of better treatment modalities.

Factors that improve survival rates but in reality do not include LEADTIME BIAS & OVERDIAGNOSIS.

LEADTIME BIAS

Survival time is usually calculated from the day of diagnosis until the day of death.

Cancer is diagnosed only on the onset of symptoms.

Screening test helps in diagnosing the cancer before the onset of symptoms and hence the survival time is prolonged; it is called lead time bias.

The reason the survival time appears longer is because of an earlier date of diagnosis  whereas the actual time of death is not altered by screening test in comparison to the unscreened individuals

WHEN DO SCREENING TESTS BECOME STANDARD TESTS?

KEY POINTS:

  • Research studies demonstrating that screening tests produce consistent results can then be considered as standard tests.

The following tests are performed to assess cancer screening tests  :

  • Non randomized controlled trials.
  • Randomised controlled trials.
  • Cohort studies.
  • Case control studies.
  • Ecological studies.
  • Expert opinions.

Examples of cancer screening tests which were once studied and have become standard otests are

  • Colonscopy for colorectal cancers.
  • Mammograms for breast cancer.
  • Pap smears for cervical cancer.

Cancer screening trials assess new ways of finding cancer in people before they have symptoms.

Screening trials are designed to expect the potential benefits and harms associated with cancer screening tests.

Strongest evidence of benefits of screening comes from clinical trials.

RANDOMISED CONTROLLED TRIALS

Trials give the highest level of evidence  regarding the safety, accuracy and efficacy of cancer screening tests.

People are randomly allotedto the study group where new  screening tests are employed and to standard group where routine screening is employed, the test results are compared to see if the new screening tests is better than the standard test.

NONRANDOMISED CONTROLLED TRIALS

Evidence is not as strong as that in randomized control trials.

Volunteers choose which group they eant to be a part of or the study head assigns them.

COHORT STUDIES

Study follows a large volume of people over time.

They are divided into groups called cohorts.

Information is collected and evaluated after a predefined outcome.

CASE CONTROL STUDIES

Shorter follow up compared to cohort studies.

Evidence from case control studies is inferior to that of clinical trials/ cohort studies.

Information is collected from individuals who already have the disease and compared with people who do not have the disease.

For example, in a population susceptible for melanoma, asking the group of people with melanoma and with out melanoma to observe their skin for abnormal growths and the frequency of checking might be a effective screening tool to decrease the number of melanoma cases and deaths from melanoma.

ECOLOGICAL STUDIES

Information is collected for an entire population like city/county.

These studies are weak  when compared to other research studies.

Might give information about utility of screening tool in the entire population.