Screening

What is screening?

Screening involves checking a patient for a disease without them having any symptoms or signs of the disease.

This check is performed with a screening test. To screen for a disease, we must have some accurate way of testing if a patient has it or not. Ideally, this test should not be negative in someone who actually does have the disease (false negative). This screening test should also not be positive in someone who doesn’t have the disease (false positive). Unfortunately no test is perfect and both of these falsities exist. This means that although a screening test may suggest that someone has a disease, further investigation is usually necessary to confirm this. Additionally, even though a screening test is negative, a patient may still have the disease (although this is less likely than before the screening test).

Some examples of screening and screening tests include:

  • Colon cancer screening. The screening test for this is a fecal occult blood test (FOBT) or a colonoscopy.
  • Cervical cancer screening. The screening test for this is a Papanicolaou (Pap) smear.
  • Breast cancer screening. The screening test for this is with mammography.
  • Genetic disease screening. Ontario newborns have a blood test to screen for several genetic diseases at birth (link).

Suggesting that a patient undergo screening is usually based on risk factors. These may include age or family history of the disease. It doesn’t make much sense to screen babies for colon cancer, a disease generally uncommon before adulthood. Conversely, it doesn’t make sense to check healthy adults for certain genetic diseases that causes someone to become sick in childhood.

A final important aspect of screening is that there should be a treatment for the disease screened for. Colon, cervical and breast cancer all have effective treatments if caught early and have a higher mortality rate if caught later.

Why should you be screened for prostate cancer?

Screening theoretically decreases morbidity and mortality from prostate cancer because it is detected at an earlier and more-treatable stage.

There have been two large trials on prostate cancer screening. The first, the American PLCO trial, randomized 76,693 men to PSA screening vs. usual care. This trial suggested that there was no significant reduction in prostate cancer deaths with PSA screening. This trial had a high contamination rate, with 52% of the control arm getting screened. It is uncertain if this high contamination rate explains the lack of reduction in PSA mortality. Due to flaws in this trial’s methodology, the findings of this trial are not widely accepted by the urologic community.

The second large trial–the European ERSPC trial–is a trial where 162,243 men between 55-69 were randomized to PSA screening vs. usual care. The rate of contamination was significantly less in the ERSPC trial–estimated to be around 24%. At its most recent update with 13 years of follow-up, this trial suggested that:

  • 781 men would need to be invited for prostate cancer screening to prevent 1 prostate cancer death.
  • 27 cases of prostate cancer would need to be diagnosed to prevent 1 prostate cancer death

Both of these trials were conducted in the 1990’s when our approach to prostate cancer screening and treatment was significantly less developed than it is now. It is uncertain how these results would change with our modern approach which includes:

  • Use of a variety of calculations based on PSA including PSA velocity, PSA density, free PSA, age-specific PSA, and PSA nomograms.
  • Use of additional biomarkers like PCA3.
  • Use of advanced imaging technologies like MRI.
  • Increased number of biopsy cores.
  • Modifications in the prostate cancer grading system.
  • Increased uptake of active surveillance for patients at very low risk of prostate cancer complications
  • Advances in minimally invasive surgery
  • Advances in advanced prostate cancer management with radiation therapy, hormonal therapy, and chemotherapy

 

What does prostate cancer screening involve?

Prostate cancer screening initially involves a digital rectal examination and a blood test called PSA.

1. The digital rectal examination involves your physician inserting a finger into your anus to feel your prostate (Figure 1). This allows for a determination of whether:

  • There are any areas of your prostate that feel abnormal. These should prompt investigation for prostate cancer.
  • Whether you might have a prostate infection (prostatitis) that could elevate your PSA should it be tested.
  • The approximate size of your prostate, which may also elevate your PSA.
Figure 1 - A digital rectal examination image. NCI public domain image.
Figure 1 – A digital rectal examination image. NCI public domain image.

2. PSA refers to a protein that the prostate produces. Its full name is prostate-specific antigen. This protein’s normal function is to liquefy semen after it has been deposited–although this does not to relate to its role in prostate cancer screening. PSA can be detected with a blood test. Although every man has some PSA in their blood, readings can be elevated in almost any process involving the prostate. This means that men with a high PSA should be investigated further, but having a high PSA does not necessarily mean prostate cancer. PSA can be elevated because of:

  • Prostate cancer
  • Enlarged prostates (BPH, benign prostatic hyperplasia)
  • Infection of the prostate (prostatitis)
  • Recent ejaculation preceding test
  • Trauma to the prostate including  recent surgery or urethral catheterization
  • Prostate massage or vigorous rectal examination

3. An additional screening test, the PSA free to total ratio (F/T), may add further information in equivocal cases. Prostate cancer cells secrete a slightly different version of PSA than normal prostate cells. The level of this can be measured with the PSA F/T ratio.

4. In patients where PSA is being checked routinely, a rising PSA without an explanation should also be investigated (PSA Velocity)

Who should be screened? When should they be screened?

The Canadian Urological Association guidelines suggest the following:

  • Prostate cancer screening be offered to men 50 years of age with a life expectancy of 10 years or greater.
  • Men should be screened at regular intervals from 1-4 years.
  • Screening may be offered at 40 years of age to those with risk factors, including family history of prostate cancer and African heritage.
  • Screening should be stopped at around age 75.
  • A baseline PSA for men in their 40’s may predict future prostate cancer risk.

 

When is a screening test considered positive?

A variety of triggers are generally used to consider a screening test positive. At the end of the day, these triggers all suggest an increased risk of prostate cancer. There is no set cut-off point and a decision to move forwards in diagnosing prostate cancer involves a discussion of risks and benefits in each individual situation. Prostate cancer risk can be estimated with calculators, described later. A positive screening test is suggested when there is:

  • An abnormal digital rectal examination.
  • An elevated age-specific PSA (Table 1).
  • An increased PSA density or PSA velocity.
  • A reduced PSA free/total ratio
  • A lack of PSA decline on finasteride or dutasteride, or a rising PSA on these medications
Table 1 – Age specific PSA cutoffs
Age Normal PSA
40-49 <2.5
50-59 <3.5
60-69 <4.5
70-79 <6.5

What are the next steps once a screening test is considered positive?

Please proceed to the Diagnosis of Prostate Cancer section.

How likely is it that I have prostate cancer?

The following is a calculator that allows you to calculate your prostate cancer risk based on screening information (link). Risk is estimated based on known risk factors analyzed in a large number of men.

Please see the disclaimer here (link).