Tumour markers

Question 1

What are tumour markers?

Answer 1

Used in cancer diagnosis

Usually measured in body fluids or tissues

More than 20 available

Usually a protein

Question 2

Where do tumour markers come from?

Answer 2

Either produced

• directly by cancer cells
• by the body in response to the presence of cancer cells

Question 3

What are examples of tumour markers?

Answer 3

CEA

HER2/ neu

AFP

PSA

Question 4

What is HER2/ neu?

Answer 4

Expressed in certain cancers of the breast, stomach and oesophagus

Detected via biopsy using immunohistochemical staining

Used to determine prognosis and guide treatment

HER2/neu-positive cancers tend to grow/spread faster than other cancers

More likely to respond to treatment with drugs that work against the HER2 receptor on cancer cells

Question 5

What is AFP?

Answer 5

Major serum protein in the developing embryo

Expression quickly declines in the months after birth

Can become elevated in certain cancers of the liver, ovaries and testes

Measured using a blood test

Monitoring helps diagnose, monitor treatment and detect recurrence

Question 6

What is PSA?

Answer 6

Measured via blood test

Used to screen, helps diagnose, monitor treatment and detect recurrence of prostate cancer

Used to screen men at high risk due to family history

Not specific to prostate cancer: may also be elevated in benign prostatic hyperplasia, prostatis and increasing age

Question 7

What is CEA?

Answer 7

Carcinoembryonic antigen

Normally produced in the gastrointestinal tissue in the developing embryo - believed roles in cell adhesion

Group of related gycoproteins

Found in fetal colon

Found in very low levels in normal population, raised in patients with colorectal cancer

Question 8

Who first discovered CEA?

Answer 8

Gold and Freedman

In 1965

Antigen present in foetal colon and colon adenocarcinoma

Appeared to be absent from healthy adult colon

Question 9

Which cells produce CEA in normal population?

Answer 9

Colon my mature columnar cells

Released into the gut lumen and disappears with the faeces

Not high levels are found in the blood

Question 10

Which cells produce CEA in colon cancer cases?

Answer 10

Malignant cells have no basal lamina and have lost their polarity

CEA is distributed around the cell surface membrane

Continually exfoliated away in plasma membrane-derived vesicles which end up in the blood via draining lymph and blood vessels

Question 11

Can CEA be used to diagnose all cases of colon cancer?

Answer 11

Less likely to produce CEA in poorly differentiated tumour

Cells no longer look like colon cells

CEA not useful as tumour maker

Question 12

What does the histology of normal colon look like?

Answer 12

Mucosa contains deep crypts bordered by a single-layer of columnar epithelium

Epithelial cells are constantly sloughed off into the lumen and are replaced by new cells

Question 13

What does the histology of colon adenocarcinoma look like?

Answer 13

Muscularis mucosa is infiltrated by abnormal neoplastic glandular tissue

Question 14

What are the limits of CEA?

Answer 14

Not useful in mass screening programme

The aim of the mass screening programme is to detect colorectal cancers early

Large proportion of early stage colorectal cancer would not be detected by CEA concentration in the blood

Lead to unacceptably high numbers of false negative results

Due to low sensitivity

Question 15

What is the sensitivity and specificity of CEA?

Answer 15

Using an upper limit of normal of 2.5 microg/L

CEA has

• sensitivity: 36%
• specificity: 87%

Question 16

Which factors could give false positive results in CEA?

Answer 16

Smokers

Irritable bowel syndrome

Others cancers

• lung
• breast
• liver
• pancreas
• thyroid
• stomach
• ovaries

Peptic ulcers

Ulcerative colitis

Rectal polyps

Benign breast disease

Emphysema

Question 17

What is sensitivity?

Answer 17

Measure of the proportion of positive results gained by people who do have the condition

How good is the test at identifying those with the condition?

Question 18

What is specificity?

Answer 18

Measure the proportion of negative results gained by people who do not have the condition in question

How good is the test at identifying those without the condition?

Question 19

What is a false negative?

Answer 19

People testing negative who do in fact have the condition

Question 20

What is false positive?

Answer 20

People testing positive who do not have the condition

Question 21

What are the two uses of measuring CEA in colorectal cancer?

Answer 21

CEA measured to see if the cancer has come back

CEA measured to see if the cancer is responding to chemotherapy

Question 22

How can CEA be used to see if the cancer has come back?

Answer 22

CEA is measured before resection of the colorectal cancer

The aim of CEA monitoring after curative resection of colorectal cancer is to detect recurrent disease at an early and treatable stage

• before the patient notices it has come back
• give treatment for the early stages of the disease = more successful

Question 23

How can CEA be used to see if the cancer is responding to chemotherapy?

Answer 23

Patients who exhibited a decrease in CEA while on chemotherapy had better overall survival

Increases in CEA while on chemotherapy predicts progressive disease - cancer is still growing

Clue before the scans show any change to the cancer shape/size

Question 24

What sample is used to measure CEA levels?

Answer 24

Plasma or serum

Occasionally another body fluid like peritoneal, pleural or cerebrospinal is collected for testing

Presence of CEA in other body fluids may indicate metastasis to that system

Question 25

What department analyses CEA levels?

Answer 25

Clinical biochemistry

Question 26

What assay is used to measure CEA levels?

Answer 26

Chemiluminescent Magnetic Immunoassay

Question 27

What machine measures CEA levels?

Answer 27

Automated modular analyser

Question 28

What assay is used to measure CEA levels?

Answer 28

Chemiluminescent magnetic immunoassay

Question 29

What are the three components of CMIA?

Answer 29

Utilises

• the specificity of immune-complex formation
• the detectability of chemiluminescence
• the ease of separation possible with magnetism

Question 30

What is used in CMIA?

Answer 30

An immune complex sandwich is made from two types of antibody

Using CEA protein from the sample (50 microl) as the filling

The two antibodies that bind to the sample to form the sandwhich:

• ruthenium-labeled antibody
• biotin-labeled antibody

Streptavidin-coated magnetic microparticles are added

These bind to sandwich complex via biotin

Question 31

How do we measure CEA levels with CMIA?

Answer 31

The mixture is tranderred to the measuring cell

Complexes are magnetically captured onto the surface of an electrode via streptavidin

Application of voltage through the electrode generates an unstable excited electron state in the ruthenium complex

Upon relaxation to the lower-level state photons are released

The amount of light generated is proportional to the amount of CEA in the sample

Measured by a photomultiplier

The CEA concentration is calculated using a calibration curve

Question 32

What happens to the sandwich complex when voltage is applied?

Answer 32

Application of voltage stimulates and electrochemiluminescent reaction between ruthenium and a substance in the buffer called TPA

Results in the emission of light

Question 33

What is the turnaround of CEA concentration measurement?

Answer 33

1 day

Question 34

What is the reference range of CEA levels?

Answer 34

< 2.5 microg/L

< 5.0 microg/L in smokers