L19 - Cancer Screening & Prevention

Question 1

What is screening?

Answer 1

• The investigation of asymptomatic people in order to classify them as likely or unlikely to have a disease
• People who appear likely to have to disease are investigated further to arrive at a final diagnosis
• Those with the disease can be treated

Question 2

What is lead time bias?

Answer 2

• Successful screening will detect disease in its preclinical detectable phase
• Lead time bias is therefore the bias that arises from observing the process of disease for longer without actually increasing the length of time that the patient would have survived had the disease been detected at the onset of symptoms
• Due to lead time bias, the period between detection and death could be longer, but the time between contracting the disease and death could be the same

Question 3

What is length bias?

Answer 3

• Length bias is the bias that arises from the fact that disease which lend themselves to be identified by screening are more likely to be less aggressive conditions
• This is because more aggressive disease is less likely to be detected by screening because it is more likely to develop fully between routine screening points, therefore the preclinical detectable period is more likely to be missed
• Therefore, survival following screen-detected disease may be lengthened by the relatively less aggressive nature of the disease

Question 4

How can length bias be identified?

Answer 4

Length bias can be identified by comparing the aggressiveness of disease detected clinically with disease detected by screening

Question 5

Define sensitivity and specificity.

Answer 5

• Sensitivity is the proportion of screened patients with the condition that test positive
• Specificity is the proportion of screened patients without the condition that test negative

Question 6

Define positive predictive value and negative predictive value.

Answer 6

• PPV is the proportion of screened patients that test positive that have the condition
• NPV is the proportion of screened patients that test negative that don’t have the condition

Question 7

What is DCIS?

Answer 7

• DCIS is ductal carcinoma in-situ, a form of breast cancer that accounts for 20% of all screen-detected cancers
• There is concern that identifying DCIS is overdiagnosis of breast cancer, as these lesion may never progress or threaten a woman’s life

Question 8

List 5 problems with PSA testing.

Answer 8

1 - 2/3 of men with elevated PSA do not have prostate cancer (bad PPV)

2 - 15% of men with normal PSA have prostate cancer (bad NPV)

3 - The PSA test is unreliable when undertaken in different laboratories

4 - It is not currently possible to differentiate between aggressive and indolent tumours

5 - A substantial proportion of patients will receive unnecessary treatment

Question 9

Which cancer screening programmes are in place in England?

Answer 9

1 - Breast cancer

2 - Cervical cancer

3 - Bowel cancer