Wednesday, 7 March 2007

Cervical Screening: When to begin?

Dr Hazel Lewis, Clinical Leader, National Cervical Screening Programme

Since the introduction of the National Cervical Screening Programme (NCSP) in 1990 there has been a dramatic decrease in cervical cancer incidence (approximately 40 percent) and mortality (approximately 60 percent).  Much of this reduction has been due to widespread use of the cervical smear test – sometimes called the Pap test – which has enabled clinicians to detect precancerous abnormalities of the cervix and treat these before they progress to cervical cancer.

Routine cervical screening for prevention of cervical cancer is recommended for all women who are or who have ever been sexually active.  Questions still remain however, about when to begin screening and how often to screen.

NCSP policy on screening age and interval was last reviewed in 1997 and has remained unchanged for the past decade. The policy is that all women who have ever had sexual intercourse should be offered a three-yearly cervical smear test from age 20 to age 69.

Our understanding of the natural history of cervical cancer, and in particular of the role of human papillomavirus (HPV) infection in the pathogenesis of this cancer, has advanced considerably since 1997.  This has led several countries, as well as the World Health Organization (WHO), to change their policies or recommendations on when to begin and when to stop cervical screening. 

In other developed countries with organised cervical screening programmes, such as the United Kingdom, it is now recommended that women should not be screened before 25 years, should then continue to be screened three-yearly until 50 years of age; and thereafter should be screened five-yearly until 65 years.  In the Netherlands and Finland, cervical screening targets women aged 30 to 60 years, with five-yearly screening; women in these countries are not screened after 60 years of age.  A recent review comparing the Australian cervical screening programme policies of two-yearly screening with the three- to five-yearly screening policy in the United Kingdom shows similar reductions in incidence and mortality and recommends a review of the Australian policy of two-yearly screening.

A recent World Health Organisation guide on cervical cancer control recommends:

• new programmes should start screening women aged 30 years or more
• existing programmes should not include women less than 25 years of age
• a five-year screening interval is appropriate for women over 50 years
• a three-year interval is considered appropriate in the age group 25-49 years
• annual screening is not recommended at any age
• screening is not necessary for women over 65 years provided the last two previous smears were negative.

European guidelines recommend screening for cervical abnormalities “starting at the age of 30 and definitely not before the age of 20”.

In New Zealand every year approximately 180 women continue to be diagnosed with cervical cancer and about 60 women die from this largely preventable disease, despite the availability of an organised screening programme.   Incidence of cervical cancer is twice as high, and mortality four times higher in Maori than in non-Maori women.  Over half the women who develop or die from cervical cancer have never been screened, and many of the remaining women have been screened only irregularly and infrequently.  Overall programme coverage is currently only approximately 75 percent for the total population and 50 percent for Maori women.

Yet every year approximately 14,000 smears are taken annually from women under the age of 20 years, despite policy to the contrary.  This unnecessary screening wastes precious resources, diverts attention from women who could genuinely benefit from screening, and is unlikely to be of any benefit to these young women – in fact early and unnecessary screening can potentially cause them serious harm.

The appropriate age at which to start cervical screening and the most appropriate frequency of screening will depend on the age-related risk of cervical cancer in the population, and must take into account the risk of harms of treatment of cervical abnormalities detected through screening, as well as the potential benefits.

Over the past ten years, our understanding of the pathogenesis of cervical cancer has greatly improved.  We now know that infection with certain strains of HPV, acquired through sexual activity, results in cellular changes to the cells lining the surface of the cervix.  While the majority of HPV infections occur in young women (less than 30 years of age), most clear spontaneously.  Very infrequently does infection become persistent and give rise to abnormal cells that progress to invasive cervical cancer.  Even then, the progression to cancer is generally very slow, taking ten years or longer.  There is therefore a long ‘window of opportunity’ in which to detect and treat abnormal precancerous cervical lesions by screening women, without causing undue harm through unnecessary screening and treatment.

The NCSP will shortly review the New Zealand data to update current policy regarding the age at first screen and the screening interval, in the light of this new understanding and recent policy changes in other countries, as well as revised recommendations from the WHO and other intergovernmental organisations.