Medical Matters: The news that the forthcoming national cancer strategy will recommend the extension of breast cancer screening to older women as well as introduce national screening programmes for bowel cancer and cervical cancer may well stimulate a debate on the pros and cons of screening.
Screening for any disease is a form of prevention. Most screening carried out by health professionals is aimed at catching the disease at an early stage, when there are no symptoms but when treatment is likely to be curative. Nor does the screening test provide a firm diagnosis; people with positive test findings are then referred on for full assessment and treatment.
Before deciding on the benefits or otherwise of screening for a particular cancer, experts must consider a number of questions about both the disease and the screening test. Is the disease being screened for serious or potentially life-threatening? Can it be treated or controlled? Will early diagnosis lead to a better prognosis for the patient? Is the disease reasonably common in the population? The test should be relatively inexpensive, acceptable to the patient (a test that is either dangerous or excessively painful is unlikely to be successful), reliable and the results consistently reproducible. The test should be valid to the point where it has been confirmed by a "gold standard".
One of the most important aspects of a test is that it have a low rate of false positive results (those found to be positive by the test but who do not have the disease). Equally false negatives must be kept to a minimum (people who are found to be negative by the test but who do have the disease). False positive tests can lead to unnecessary emotional distress as well as the risk of unnecessary treatment. Also, some doctors worry that screening can detect a cancer that, because of its nature and the patient's age, would never have become a problem in the person's lifetime.
The decision to extend the age group for breast cancer screening by five years to 69 is unlikely to be controversial. Most other countries screen women from 50 to 69, with a number of studies finding a reduction in breast cancer deaths attributable to screening. And, while breast cancer does occur in women over 70, it tends to be a slower-growing, less-aggressive disease.
Cervical cancer screening programmes are also well-established internationally. The cancer is a slow-growing one, with simple treatment available for early in situ tumours.
The Pap test, although a little uncomfortable, seems acceptable to most women. However, cervical cancer is a relatively rare disease.
Screening for colorectal cancer is likely to have been the subject of more intense debate by the cancer strategy group. Factors in favour include the fact that it is the second most-common cancer in the Republic. It's a slow-growing tumour that starts life as a benign polyp, and fewer than 10 per cent of these polyps progress to cancer within a decade. This allows for longer intervals between screening tests, with a subsequent reduction in cost.
But the major challenge is to identify a suitable screening test and to agree who should be screened. A paper published in the British Medical Journal earlier this month may help clarify the issue. The authors followed up 1,678 people with a family history of bowel cancer for 16 years.
The participants underwent colonoscopy (a flexible scope capable of examining the entire colon and taking a biopsy) at five-yearly intervals. The results showed that surveillance using colonoscopy is effective in preventing colorectal cancer in individuals with a history of bowel cancer in a first degree relative. The authors added that "the benefit of screening (with five-yearly colonoscopy) seems minimal below the age of 45".
So the creation of an "at-risk" population using preliminary data would seem to make sense in the case of colorectal cancer screening.
I would have given long odds against lung cancer screening joining the pantheon of national screening programmes any time soon. But that was before hearing a lecture by Dr Frank Sullivan, medical director of the department of radiation oncology at University College Hospital Galway recently.
He presented figures from the US which showed good results from the selective screening for lung cancer (using a low dose CT scan) of those aged over 50 who smoked at least one packet of cigarettes a day for 20 years.
The decision to introduce national screening programmes is rightly made at a population level. But because each individual must weigh up the personal pros and cons of taking a screening test, there will always be an element of uncertainty surrounding the process.
And that is probably how it should be in a free society.
• Dr Muiris Houston is pleased to hear from readers at mhouston@irish-times.ie but regrets he cannot answer individual queries.
