Thursday July 30 2009
Cervical smears can detect pre-cancerous cells in the uterus
The British Medical Journal recently published research that looked at the effectiveness of smear tests across different age groups of women. This large and well-designed case-control study analysed the impact of cervical screening on the risk of cancer in more than 4,000 diagnosed cases and nearly 8,000 age-matched controls without cancer.
It found that screening reduced the risk of developing cervical cancer in all age groups except the youngest. As women got older, the more their risk was reduced in the five years after screening. Screening women between the ages of 20 and 24 had no detectable impact on cervical cancer rates at ages 25 to 29. These are important findings, which support the NHS approach of only inviting women for cervical screening once they reach the age of 25.
Where did the story come from?
The research was carried out by Peter Sasieni, Alejandra Castanon and Jack Cuzick of Bart’s and the London School of Medicine. The study was funded by Cancer Research UK and the NHS cervical screening programme. It was published in the British Medical Journal.
What kind of scientific study was this?
The aim of this case-control study was to investigate the effect of cervical screening on the incidence of cervical cancer in different age groups. It focused particularly on the number of new cancer cases in women under 25 years who are screened.
The cases were 4,012 women aged 20–69 with a histological diagnosis of invasive cervical cancer made between January 1990 and April 2008. The cases were matched by age to two women registered with the same NHS GP (and therefore had a record in the national cervical screening/recall system). This resulted in 7,889 controls. All cases and controls had records on all screening tests carried out in the UK after 1998.
The researchers used statistical methods to look at the association between having an adequate smear test in a particular three-year age band (e.g. 22–24), and the incidence of cervical cancer in the subsequent five-year band (e.g. 25–29). They then calculated the risk of cancer development for women who were screened, and those who were not screened.
What were the results of the study?
Screening reduced the risk of developing cervical cancer in all age groups but the youngest. As women got older, the more their risk was reduced by screening. In detail, screening:
- had no effect on cancer development at 25–29 years if screening was done at 20–24 years (odds ratio for cancer risk with screening at 22–24, 1.11, 95% confidence interval 0.83 to 1.50)
- decreased cancer risk by 45% in 35–39 year-olds if screened at 32–34 years of age (non-significant association if screened specifically at ages 30 or 31)
- decreased cancer risk by 63% in 45–49-year-olds if screened at 42–44 years of age (60% decreased risk if screened specifically at ages 40 or 41)
- decreased cancer risk by 74% in 55–59-year-olds if screened at 52–54 years of age (73% decreased risk if screened specifically at ages 50 or 51)
Risk reduction was greatest with screening in the oldest age groups: there was an 80% reduced risk among women who were screened at age 64. Screening was particularly effective in preventing advanced stage cancers, which had a particularly low incidence among screened women.
What interpretations did the researchers draw from these results?
The authors conclude that cervical screening in women under the age of 25 has little or no impact on rates of invasive cervical cancer up to age 30. By contrast, screening older women leads to a substantial reduction in incidence of and mortality from cervical cancer. They say, “on average, participation in the UK cervical screening programme by a woman aged between 35 and 64 years reduces her risk of cervical cancer over the next five years by 60-80%”.
What does the NHS Knowledge Service make of this study?
This is a large and well-designed study. It assessed the age-specific effects of cervical screening on risk of cancer development in 4,012 diagnosed cases and 7,889 age-matched controls without cancer. The researchers found that the association between cervical screening and subsequent decrease in cervical cancer varies with age, and that screening 20 – 24-year-old women has no detectable impact on cervical cancer rates at ages 25 – 29.
With increasing age, screening reduced the risk of developing cervical cancer for the next five years. These are important findings, as the policy to only invite women for cervical screening once they reach the age of 25 has often been a point of contention.
It is possible that unidentified confounding factors may lie behind the observed associations, such as differences in the health and lifestyles of women who attend for screening and those who do not. The matching of cases with controls at the same GP surgery may have accounted for some of this potential bias.
Observational studies provide the best evidence for assessing the effectiveness of screening programmes. However, although inferences can be made on the subsequent testing and invasive treatment that follows a positive screening result, this particular research has not analysed the effect of screening results on treatment options and their harms or benefits to women of different age groups.
As the authors say, their data should help policy makers balance the impact of screening on cancer rates against its harms, which mainly involves the overtreatment of lesions that are unlikely to lead to invasive cancer.