Women around the world attend cervical screening regularly during most of their adult life, but in most countries, they are not encouraged to continue once they reach their mid-60s. Why? Well hopefully because if you are screened regularly up to the age of 65 your risk of cervical cancer becomes so low that there is little point in continuing screening. That is the theory, but is it true? Surprisingly there is relatively little data on which to make a rationale decision regarding the upper age for cervical screening.
The risk of cervical cancer after the age of 65 in screened women
A few years ago, we looked at the risk of cervical cancer in women aged over 65 in England stratified by the age at which they were last screened. We found that the risk of cancer was extremely low within 5 years of a negative screen; remained very low 5-10 years after a negative screen; but by 15 years after the last negative screen the risk was nearly half that in women who were not screened at all between ages 50 and 64. In that paper we estimated that without screening about 9 women per 1000 would get cancer aged 55-84 in England. Five yearly screening up to age 65 would prevent about 7 of those 9 cancers; whereas stopping at 55 would prevent only 5 and continuing to age 75 would prevent all but one (8 out of 9). This paper was based on screening using cytology (“the smear test”).
In America they use both cytology and HPV testing: is it safer?
Colleagues in America wanted to who whether similar results would apply there. In Northern California women (enrolled in Kaiser Permanente) have been screened by both cytology and HPV testing – called co-testing – since 2003. We wanted to know whether it was even safer to exit screening after a series of negative co-tests.
For the American study researchers had detailed data on screening tests of all women in Kaiser Permanente of Norther California (KPNC). We looked at the risk of having cervical “pre-cancer” detected on screening following one, two or three consecutive negative co-tests. There were nearly 175,000 women with follow-up after a negative co-test at ages 55-64.
Was there any point in having one more screen?
We looked at the likelihood of “one extra screen” detecting pre-cancer. Our reasoning was that screening prevents cervical cancer by detecting and then treating pre-cancer. So, if you don’t find any pre-cancer you haven’t changed the risk of cancer and the screen was not necessary.
We found about 25 cases of pre-cancer for every 100,000 women with a previous negative co-test, and this number was not very different after a single negative co-test or after three consecutive negative co-tests.
In most countries, women are also treated for pre-precancer (known as CIN2). So, we also looked at the chance of the screening resulting in treatment. Here there was a trend with a lower chance of treatment after more negative co-tests. For every 100,000 women, 85 were treated on a screen following a single negative co-test; 44 after two negative co-tests; and 25 after three negative co-tests.
What does all this mean?
Previous research suggests that about a third of pre-cancer progresses to cancer over 15 years or so. That would mean that screening women after a negative co-test aged 55-64 prevents perhaps one cancer for every 10,000 women screened. If a quarter of pre-cancer also progresses to cancer, then exiting women in this age-group after a single co-test might result in an extra cancer for every 4,000 women over the rest of their lives.
These results suggest that it is very safe for women to stop being screened after a negative co-test aged 55-64. But it should be noted that most women in this study will have been screened regularly for at least 20 years before the study started and the results cannot be generalised to women whose only ever screen was a co-test at age 55. Additionally, the interval between the negative co-test and the next screen was about 3 years in most of these women. The value of a final screen 10 years after a negative co-test is less certain.
A recent Canadian study modelled the lifetime risk of cervical cancer in women screened to different ages. It predicted that the risk from age 70 was more than threefold lower following a negative HPV test (or a negative co-test) at age 70 than they were when the last negative test was at age 55. They suggest that offering a final screening test at age 70 maybe worth considering. However, the magnitude of the risks is tiny. According to their model a woman who has participated in screening until age 55 and has a negative HPV test at age 55, has about a 0.05% chance of getting cervical cancer after the age of 55. That compares to a risk of about 1.7% from age 55 onward in unscreened women which really does reinforce the value of screening women aged 25-55.
There is still some uncertainty regarding the optimal age to stop cervical screening, but studies considering the occurrence of cervical cancer in older women, the chance of finding significant disease on one last screen, and computer simulation of the development and prevention of cervical cancer all suggest that not inviting well-screened women over the age of 65 is safe.
The views expressed are those of the author. Posting of the blog does not signify that the Cancer Prevention Group endorse those views or opinions.
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