Tuesday December 22 2009
Women with lower levels of education have higher cancer risk
"Girls who have sex in their teens are at greater risk of developing cervical cancer,” the Daily Mail reported. It said a study into why poorer women have a higher risk of the disease found that they tended to have sex four years earlier than more affluent women.
This study investigated how socioeconomic status affects the risk of HPV infection, a sexually transmitted virus that causes almost all cases of cervical cancer. It was not aimed at determining whether the age that a woman first has sex is a risk factor for cervical cancer. However, based on what is already known, it makes sense that the sooner a woman first has sex, the greater her risk of being infected by HPV, and for a longer period of time.
This research was carried out mainly in developing countries that did not have widespread high-quality screening programmes in place, and before cervical cancer vaccination was available. Therefore, these results may not apply to the UK.
Where did the story come from?
The research was carried out by Dr Silvia Franceschi and colleagues from the International Agency for Research on Cancer (IARC) study groups. It was funded by the Bill and Melinda Gates Foundation, and published in the peer-reviewed British Journal of Cancer.
The BBC and Daily Mail covered the story. Both reports concentrated on the increased risk of cervical cancer associated with intercourse and pregnancy at an early age, rather than focusing on the main factor that was assessed in this study: level of education (as an indicator of socioeconomic status).
What kind of research was this?
Almost all cases of cervical cancer are caused by certain strains of human papillomavirus (HPV), which is spread through sexual contact. This research analysed data from two sets of studies looking at risk factors for cervical cancer. One study used a case control design (the IARC Multicentric Case–Control Study) and the other study used a cross-sectional design (the IARC HPV Prevalence Surveys).
The researchers say that cervical cancer risk is associated with low socioeconomic status, but the reasons for the association are not fully understood. This analysis evaluated the association between education as a measure of socioeconomic status, and the risk of HPV infection and cervical cancer.
One limitation to this approach is that education level may not fully capture a woman’s socioeconomic status. Also, due to the non-randomised observational nature of these studies, other factors may have influenced the results. This possibility can be reduced by taking other factors into account in the analyses.
What did the research involve?
The studies asked women how long they were in education, then examined whether this was related to their risk of HPV infection or cervical cancer.
The IARC case control study compared 2,446 women with invasive cervical cancer (cases) with 2,390 women of a similar age and without cervical cancer (controls). The study was carried out between 1985 and 1999. The IARC surveys included 15,051 women aged 15 and above, who were mostly married (94%) and had had sexual intercourse. These surveys were cross-sectional studies and carried out between 1993 and 2006.
Both sets of studies asked the women about their education, sexual and reproductive history, and smoking, and tested them for the HPV virus. Education level was classified into four groups (0 years, 1–5 years, 6–10 years, or more than 10 years). Owing to small numbers, the last two groups were pooled in the case–control studies. The studies were carried out in countries across the world, mainly in Africa, Asia, and Central and South America. Most of these countries did not have cervical cancer screening programmes in place at the time of the studies.
The researchers then compared the level of education in women with cervical cancer to the educational levels of control women in the case-control study. They also investigated whether level of education affected a woman’s risk of HPV infection in either the case control or the cross-sectional studies.
The researchers took factors that could affect the results into account, including age, where a woman lived, number of sexual partners, age at first sexual intercourse, whether their husband had extramarital sexual relationships, number of pregnancies, age at first pregnancy, use of contraceptives, smoking, and history of cervical screening (Pap smear).
Taking these factors into account is important, as they may not be balanced between women with different levels of education, and could affect results if analyses did not adjust for them.
What were the basic results?
In the case control study, 82% of cases reported having only up to five years of education, compared to 66% of the controls. Statistical analysis showed that the less education a woman had, the greater her risk of developing cervical cancer. When the researchers took into account the women’s age when they first had sexual intercourse and age at their first pregnancy, this weakened the link between educational level and cervical cancer risk. Taking into account how many children the women had, and whether they had had cervical screening, also reduced the strength of this link.
Other factors had less of an impact in this analysis. After taking these factors into account, women who had received five years or less of formal education were at 41% greater odds of developing cervical cancer than those who had more than five years of education (odds ratio [OR] 1.41, 95% confidence interval [CI] 1.11 to 1.79).
There was no association between education level and risk of HPV infection in either the case control study or the surveys.
How did the researchers interpret the results?
The researchers conclude that the higher rates of cervical cancer in women with low socioeconomic status do not seem to be explained by a higher rate of HPV infection, but “by early events in a woman’s sexually active life that may modify the cancer-causing potential of HPV infection”.
They suggest that this could be because women who have sexual intercourse earlier and contract an HPV infection will have that infection for longer than women who contract the infection later in life.
This research aimed to investigate why women with low socioeconomic status are at greater risk of cervical cancer. There are a number of points to note:
- The number of years of education was used as an indicator of socioeconomic status. However, this relatively simple measure may not be fully representative of a person’s socioeconomic status. Other indicators, such as income and house ownership, were collected in some countries, but not in others, therefore could not be used in this analysis.
- It wouldn’t be possible to use a randomised controlled trial to look at the effects of education or socioeconomic status on risk of cervical cancer or HPV infection. Observational studies such as this are therefore the best way to look at this question. However, these studies may be affected by confounding, i.e. where factors other than those of interest are causing the links seen. The researchers tried to take some of these factors into account, but say that confounding factors are still likely to account for some of the link seen between education level and cervical cancer risk.
- The women reported their own sexual histories. This may lead to inaccuracies due to inability or unwillingness to accurately recall these details.
- This study looked mainly at women in developing countries, and the results may not be directly applicable to more developed countries. In particular, these countries did not have widespread high-quality cervical screening programmes in place at the time of the studies. Such screening programmes could have an impact on the links seen, as could vaccination programmes against HPV that have recently begun in the UK.